Forces AQA Physics topic 5
5.1 Forces and their Interactions
Vector vs. scalar Scalar quantities have size ( magnitude ) only and no direction. Vector quantities have both size and direction. Scalar or vector??? Scalar 8. Power 2. Distance 1. Mass 6. Energy 7. Time 3. Acceleration 4. Speed 9. Force 5. Velocity Vector
Vectors Here s a man walking 10km north and then 10km east. Notice that we can replace his two movements with a displacement vector. Note the length and direction of this vector: 10km The same can be applied to velocity vectors: 10km 14.1km 5ms -1 100ms -1 100.1ms -1
Introduction to Forces A force is a push or a pull. What forces do these pictures represent? Which of these forces would you describe as contact forces and which ones are noncontact forces?
Contact or non-contact forces? Contact Non-contact 1. Friction 2. Air resistance 3. Gravitational forces 4. Tension 5. Electrostatic forces 6. Reaction 7. Magnetic forces
Weight vs. Mass Earth s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) W You need to learn this equation!! M g 1) What is the weight on Earth of a book with mass 2kg? 2) What is the weight on Earth of an apple with mass 100g? 3) Charles weighs 700N on the Earth. What is his mass? 4) On the moon the gravitational field strength is 1.6N/kg. What will Charles weigh if he stands on the moon? 20N 1N 70kg 112N
More information about Weight 1) How much does 1kg weigh on the Earth? 2) How much does 2kg weigh? 3) How much does 3kg weigh? 4) What are you noticing about your answers? Whatever mass goes up by, weight goes up by the same ratio. For example, if you double mass you double weight. This is called proportionality :
Centre of Mass The centre of mass is defined as the point at which an object s mass is centred on. Where is the centre of mass for these objects?
Resultant Force A resultant force is a single force that can replace all of the other forces acting on something. Calculate and draw the resultant force of the following: 500N 100N 700N 600N 50N 700N 700N 800N 800N 200N 100N
Higher Tier Resultant Force 1. Draw the resultant force for these people and describe where the person will go: 50N 500N 100N 200N 700N 600N 2. If you were going to describe these resultant forces as the resultant of two other forces, what forces would you draw? 700N 800N
Higher Tier Drawing Resultant Force Here are two forces acting on a person. How can we work out the resultant force and its direction? 200N We can represent these forces as a vector diagram 500N
Higher Tier - Vector Diagrams 200N 500N You can now use a ruler and protractor to measure the size and angle of this resultant force
Higher Tier Example Questions Use squared or graph paper to find the resultant vector for these forces: 4N 300N 8N 800N Magnitude = 8.9N Angle to vertical = 63 O Magnitude = 854N Angle to (upwards) vertical = 159 O
5.2 Work Done and Energy Transfer
Work done When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x distance moved in J in N in m W You need to learn this equation!! F s
Example questions 1. Amy pushes a book 5m along the table with a force of 5N. She gets tired and decides to call it a day. How much work did she do? 25J 2. Jodie lifts a laptop 2m into the air with a force of 10N. How much work does she do? 3. Ronnie does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? What type of energy did the wheelbarrow gain? 4. Julian cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use? 5. Travis drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do? 20J 4m, heat and kinetic energy 50N 2MJ
Recap questions on Weight and work done 1) Matt weighs 600N on the Earth. What is his mass in kg? 60kg 2) Chris pushes Gabriel with a force of 20N. If Gabriel moves 2m how much work did Chris do on him? 40J 3) Matt weighs 120N on the moon, where g=1.6n/kg. What is his mass and what would he weigh on the Earth? 75kg, 750N 4) Rebecca does 100J of work by pushing her pencil case across the table. If she applied a force of 5N how far did she push it? 20m 5) If you push a book with a force of 1N a distance of 1m, how much work did you do? In other words, what is 1 Joule in terms of Newton metres? 1J = 1Nm
5.3 Forces and Elasticity
Force and Extension Consider a mass on a spring: What happens when a mass is added? When a force is applied to this spring it will change shape and extend.
Investigating Force and Extension Task: Find an expression that relates extension to the amount of weight added. Weight added (N) 1 2 3 4 5 6 Extension (cm) Force = Spring constant x extension Q. What is the spring constant for your spring? F = ke You need to learn this equation!!
Force-Extension Graph for a spring Force/N The limit of proportionality. Force is proportional to extension as long as you don t go past the limit of proportionality. There is a linear relationship up to this point. Extension/mm
Force/N Elastic and Inelastic Deformation If you don t use too much force on the spring you can take the force off and the spring returns to it s original shape this is elastic deformation. If you put too much force on the spring it stretches in other words, when you remove the force the spring does not go back to its original length. This is inelastic deformation. Extension/mm
Force and compression Consider some springs: The force-compression graphs for objects like these can be determined and plotted. Example questions: 1) A stiff spring has a spring constant of 10N/m. How much will it compress by if a force of 20N is applied to it? 2) Another spring compresses by 2cm when a force of 50N is applied. What is its spring constant? 2m 2500N/m
Elastic Potential Energy Consider a mass on a spring: What happens when a mass is added? When a force is applied to this spring it will change shape and extend. The spring will have stored elastic potential energy
Elastic Potential Energy Elastic potential energy is the energy stored in a system when work is done to change its shape, e.g: Describe the energy changes when the mass is: 1) At the top of it s movement 2) In the middle 3) At the bottom
Elastic Potential Energy Task: Calculate how much stored EPE there is in your springs Stored EPE = ½ke 2 F = ke Weight added (N) 1 2 3 4 5 6 Extension (m) Stored EPE (J)
5.4 Moments, levers and gears (PHYSICS ONLY)
Balanced or unbalanced?
Turning Moments A moment is a turning force, e.g. trying to open or close a door or using a spanner. The size of the moment is given by: Moment (in Nm) = force (in N) x PERPENDICULAR distance from pivot (in m) Calculate the following turning moments: 5 metres You need to learn this equation!! 100 Newtons 2 metres 200 Newtons
Turning Moments 2 metres 2 metres 200 Newtons 100 Newtons Total ANTI-CLOCKWISE turning moment = 200x2 = 400Nm Total CLOCKWISE turning moment = 100x2 = 200Nm The anti-clockwise moment is bigger so the seesaw will turn anti-clockwise
An example question 5 metres? metres 2000 Newtons 800 Newtons
Calculate the missing quantity The following are all balanced: 2N??N 4m 2m 5N 3N 2m??m 5N 5N 15N 4m??m 2m
A hard question Consider a man walking along a plank of wood on a cliff. ow far can he walk over the cliff before the plank tips over? Aaarrgghh Man s weight = 800N 3m 1m Plank s weight = 200N
A recap question Calculate the mass of man in the example given below: 30kg 0.4m 1.2m
How do Levers work? Consider a simple lever the nutcracker: Pivot Effort Load Notice how the distance between the effort and the pivot is much larger than the distance between the load and the pivot. Larger distance = less force needed
How do Gears work? Can you explain how gears work using turning moments? If the smaller wheel is turned, it turns the larger one slower but with more force (i.e. the distance to the pivot has been increased).
5.5 Pressure and Pressure Differences in Fluids (PHYSICS ONLY)
Pressure in Gases and Liquids Particles in a liquid or a gas ( fluids ) move around randomly, a little like this: Every time the particles hit the side of the container the particles exert a force at right angles on the container this is called pressure.
Pressure Pressure depends on two things: 1) How much force is applied, and 2) How big (or small) the area on which this force is applied is. Pressure can be calculated using the equation: Pressure (in N/m 2 ) = Force (in N) Area (in m 2 ) F OR in cm 2 and N/cm 2 You need to learn this equation!! P A
Some pressure questions 1) Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 2m 2. 500 Pa 2) A brick is rested on a surface. The brick has an area of 20cm 2. Its weight is 10N. Calculate the pressure. 3) A woman exerts a pressure of 100N/cm 2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on? 4) (Hard!) The pressure due to the atmosphere is 100,000N/m 2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body? 0.5 N/cm 2 5cm 2 Around 1kg per cm 2!
Consider a column of fluid: Pressure in Fluids Area A The pressure at the base of this column would be given by: Pressure = ρhg Density ρ h where ρ = the density of the liquid, h = the height of the container and g = gravitational field strength. You DON T need to learn this equation!!
Example questions 1) Calculate the pressure at the bottom of a 2 litre bottle of water of height 40cm (density of water = 1000kg/m 3 and g = 10N/kg). 4000Pa 2) What is the pressure at the bottom of a can of coke if the density of coke is 1000kg/m 3 and the can is 15cm tall? 1500Pa 3) If the density of seawater is 1027kg/m 3 what depth would you need to be at to experience a pressure of 50,000Pa? 4.87m
Pressure vs. Depth What does this demonstration tell you? Pressure increases with. This is because the water at the of this container is pushed on by the of the water further up, which causes it to be under higher. Words pressure, bottom, weight, depth
Why do objects float? Whether or not an object will float depends on its DENSITY. For example: The metal block will because it is dense than water The wooden block will because it is dense than water
Floating in more detail Consider a floating object: How does the pressure at the bottom of this object compare to the pressure at the top of the object? This difference in pressure causes the force called upthrust. If weight equals upthrust the object will. This is because the object displaces a weight of fluid to its own weight. If weight is greater than upthrust the object will. This is because the object is to displace a weight of liquid equal to its own weight. Words equal, unable, sink, float
Atmospheric Pressure Recall our earlier explanation of how collisions cause air pressure: Every time the particles hit the side of the container the particles exert a force at right angles on the container this is called pressure.
Atmospheric Pressure Why does atmospheric pressure decrease when you go up a mountain? There are less air molecules up here than down here Less air molecules = fewer collisions = less pressure!
5.6 - Forces and Motion
Distance vs Displacement Distance is how far you have gone, displacement is how far you are from a point and can be positive or negative: Distance = Displacement = Start Distance = Displacement = -1 metre 1 metre Distance = = Displacement = = Which one is a scalar quantity and which one is a vector quantity?
Some questions on Displacement 1) A man walks 10km north and then 10km west. a) What distance has he covered? b) How would you measure his displacement? c) What angle would his displacement be at compared to north? 10km 10km 2) A car drives around in a circle. What is its displacement after it has completed one circle?
Speed Speed is a scalar quantity. What does this mean? Typical values for speed: Walking 1.5m/s Running 3m/s Cycling 6m/s What about cars? Aeroplanes?
Speed of sound The speed of sound in air is around 330m/s. Notice that the speed can vary as well: Speed of sound (in m/s) 5000 4000 3000 2000 1000 0 Air Water Brick Iron Material Conclusion the denser the material, the faster sound travels through it. Q. Would sound travel faster or slower at the top of a mountain?
Distance, Speed and Time Speed = distance (in metres) s time (in seconds) You need to learn this equation!! v t 1) Oli walks 200 metres in 40 seconds. What is his speed? 2) Ella covers 2km in 1,000 seconds. What is her speed? 3) How long would it take Grace to run 100 metres if she runs at 10m/s? 4) Alex runs to the shop to buy the new Fallout game and travels at 50m/s for 20s. How far does he go? 5) Jasmine drives her car at 85mph (about 40m/s). How long does it take her to drive 20km? 5m/s 2m/s 10s 1000m 500s
Distance, Speed and Time (higher) D Speed = distance (in metres) time (in seconds) S T 1) Matilda walks 2000m in 50 minutes. What is her speed in m/s? 2) James tries to walk the same distance at a speed of 5m/s. How long does he take? 3) Greg drives at 60mph (about 100km/h) for 3 hours. How far has he gone? 4) The speed of sound in air is 330m/s. Evie shouts at a mountain and hears the echo 3 seconds later. How far away is the mountain? (Careful!) 0.67m/s 400s 300km 495m
Speed vs. Velocity Speed (a SCALAR quantity) is simply how fast you are travelling This car is travelling at a speed of 20m/s Velocity (a VECTOR quantity) is speed in a given direction This car is travelling at a velocity of 20m/s east
Speed vs Velocity 1) Is this car travelling at constant speed? 2) Is this car travelling at constant velocity?
Distance-time graphs 2) Horizontal line = 40 4) Diagonal line downwards = Distance (metres) 30 20 10 Time/s 0 20 40 60 80 100 1) Diagonal line = 3) Steeper diagonal line =
40 Distance (metres) 30 20 10 0 20 40 60 80 100 Time/s 1) What is the speed during the first 20 seconds? 2) How far is the object from the start after 60 seconds? 3) What is the speed during the last 40 seconds? 4) When was the object travelling the fastest? 0.5m/s 40m 1m/s 40-60s
Distance-Time graphs Task: Produce a distance-time graph for the following journey: 1) Charlie walks 50m in 20 seconds. 2) She then stands still for 10 seconds 3) She then runs away from Harry and covers 100m in 30 seconds. 4) She then stands still and catches her breath for 20 seconds. 5) She then walks back to the start and covers the total 150m in 50 seconds.
Distance 40 30 G B N (metres) 20 10 0 20 40 60 80 100 Y Time/s 1) Who was travelling the fastest? 2) Who was travelling the slowest (but still moving)? 3) Who didn t move?
40 Distance (metres) 30 20 10 0 20 40 60 80 100 Time/s 1) What was the velocity in the first 20 seconds? 2) What was the velocity between 20 and 40 seconds? 3) When was this person travelling the fastest? 4) What was the average speed for the first 40 seconds? 1.5m/s 0.5m/s 80-100s 1m/s
Understanding Velocity (Higher tier) 40 30 Displacement (metres) 20 10 0 20 40 60 80 100 Time/s 1) What s the average velocity? 2) What s the velocity at 60s? 0.4m/s 0.5m/s
Acceleration Acceleration = change in speed (in m/s) (in m/s 2 ) time taken (in s) You need to learn this equation!! A V T 1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? 2) A ball is dropped and accelerates downwards at a rate of 10m/s 2 for 5 seconds. How fast will it be going? 3) A car accelerates from 0 to 20m/s with an acceleration of 2m/s 2. How long did this take? 4) A rocket accelerates from 0m/s to 5,000m/s in 2 seconds. What is its acceleration? 2m/s 2 50m/s 10s 2500m/s 2
Acceleration (harder) V Acceleration = change in velocity (in m/s) (in m/s 2 ) time taken (in s) A T 1) A cyclist slows down from 10 to 0m/s in 5 seconds. What is her acceleration? 2) A ball is dropped and accelerates downwards at a rate of 10m/s 2 for 12 seconds. How much will the ball s velocity change by? 3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s 2. How long did this take? 4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration? -2m/s 2 120m/s 5s 2000m/s 2
Acceleration (harder) V Acceleration = change in velocity (in m/s) (in m/s 2 ) time taken (in s) A T 1) Mikey accelerates from standstill to 50m/s in 25 seconds. What is his acceleration? 2) Jack accelerates at 5m/s 2 for 5 seconds. He started at 10m/s. What is his new speed? 3) Rob is in trouble with the police. He is driving up the A29 and sees a police car and brakes from 50m/s to a standstill. His deceleration was 10m/s 2. How long did he brake for? 4) Another boy racer brakes at the same deceleration but only for 3 seconds. What speed did he slow down to? 2m/s 2 35m/s 5s 20m/s
Velocity-time graphs 1) Upwards line = 80 4) Downward line = Velocity m/s 60 40 20 0 10 20 30 40 50 2) Horizontal line = 3) Upwards line = T/s
80 Velocity m/s 60 40 20 0 10 20 30 40 50 1) How fast was the object going after 10 seconds? 2) What is the acceleration from 20 to 30 seconds? 3) What was the deceleration from 30 to 50s? 4) How far did the object travel altogether? T/s 40m/s 2m/s 2 3m/s 2 1700m
80 Velocity m/s 60 40 20 0 10 20 30 40 50 1) How fast was the object going after 10 seconds? 2) What is the acceleration from 20 to 30 seconds? 3) What was the deceleration from 40 to 50s? 4) How far did the object travel altogether? T/s 10m/s 4m/s 2 6m/s 2 1500m
80 Velocity m/s 60 40 20 0 10 20 30 40 50 T/s This velocity-time graph shows Mai s journey to school. How far away does she live? 2500m
80 Velocity m/s 60 40 20 0 10 20 30 40 50 T/s This velocity-time graph shows Kier s journey to school. How far away does he live? 2200m
Another equation of motion For a constantly-accelerating body, we can also use this equation: v 2 = u 2 + 2as You DON T need to learn this equation!! 1) An object starts from rest and accelerates at a rate of 2m/s 2 over a distance of 20m. What is its final velocity? 2) Steve drives up the M1 and covers 30km. He started at 2m/s and constantly accelerated during the whole journey at a rate of 0.001m/s 2. What was his final speed? 3) (Harder!) Sarah decelerates from 30 to 10m/s over a distance of 5m. What is her acceleration? 80m/s 64m/s -80m/s 2
Acceleration due to Gravity If I throw this ball upwards with a speed of 40m/s why does it come back down again? The ball is acted on by a force called gravity, which accelerates the ball downwards at a rate of 9.8m/s 2 near the Earth s surface. Extension question how far up would the ball go? 1) Take u = 40m/s and v = 0m/s (at the top of the throw) 2) Take a = 9.8m/s 2 3) Therefore s = 81.6m
Terminal Velocity Consider a ball falling through a liquid: Some questions to consider: 1) What forces are acting on the ball? 2) How do those forces change when the ball gets faster? 3) Will the ball keep getting faster? Explain your answer in terms of forces
Terminal Velocity Consider a skydiver: 1) At the start of his jump the air resistance is so he downwards. 2) As his speed increases his air resistance will 3) Eventually the air resistance will be big enough to the skydiver s weight. At this point the forces are balanced so his speed becomes - this is called TERMINAL VELOCITY Words increase, small, constant, balance, accelerates
Terminal Velocity Consider a skydiver: 4) When he opens his parachute the air resistance suddenly, causing him to start. 5) Because he is slowing down his air resistance will again until it balances his. The skydiver has now reached a new, lower. Words slowing down, decrease, increases, terminal velocity, weight
Velocity-time graph for terminal velocity (Physics only) Velocity Speed increases Terminal velocity reached Parachute opens diver slows down New, lower terminal velocity reached Time Diver hits the ground
Balanced and unbalanced forces Consider a camel standing on a road. What forces are acting on it? Reaction These two forces would be equal we say that they are BALANCED. The camel doesn t move anywhere. Weight
Balanced and unbalanced forces What would happen if we took the road away? Reaction The camel is acted on by an unbalanced force, which causes it to accelerate. This is called Newton s 1 st law of motion. Weight
Newton s 1 st Law of Motion Basically, a body will remain at rest or continue to move with constant velocity as long as the forces acting on it are balanced. and an unbalanced backwards force will make me slow down An unbalanced forwards force will make me accelerate Newton 1642-1727 Without an unbalanced force, Newton would carry on doing what he was doing. This is called Inertia.
Balanced and unbalanced forces Q. What will these cars do and why?
Balanced and unbalanced forces 1) This animal is either or moving with 2) This animal is getting 3) This animal is getting. 4) This animal is also either or moving with.. Words - Stationary, faster, slower or constant speed?
Summary of Newton s 1 st law Complete these sentences If an object is stationary and has NO resultant force on it the object will If an object is stationary and a resultant force acts on it the object will If an object is already moving and NO resultant force acts on it the object will If an object is already moving and a resultant force acts on it the object will accelerate in the direction of the resultant force continue to stay stationary continue to move at the same speed and the same direction accelerate in the direction of the resultant force
Newton s 2 nd Law of Motion The acceleration of a body is proportional to the resultant force causing its acceleration and is in the same direction. It is inversely proportional to the mass of the object. Newton 1642-1727 In other words force = mass x acceleration F You need to learn this equation!! M A
Force, mass and acceleration 1) A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? F 2) A force of 3000N acts on a car to make it accelerate by 1.5m/s 2. How heavy is the car? 3) A car accelerates at a rate of 5m/s 2. If it weighs 500kg how much driving force is the engine applying? 4) A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by? M A 2m/s 2 2000kg 2500N 0.5m/s 2
Inertial Mass (higher only) Inertial mass is a measure of how difficult it is to change the velocity of an object: Inertial mass = force / acceleration Determine the initial mass of the following: 1) A car that needs a force of 2000N to accelerate it by 1m/s 2. 2) A bus that accelerates at a rate of 0.5m/s 2 when 5 people push it, each with a force of 750N. Newton 1642-1727 2000kg 7500kg
Approximate Values Which approximate values of speed, acceleration and force would you put with these moving objects? Speed = 1.5m/s Speed = 30m/s Speed = 300m/s Acceleration = 1.5m/s Acceleration = 2m/s Acceleration = 3m/s Force = 70N Force = 3000N Force = 600,000N
Testing Newton s 2 nd Law For the experiment: 1) Draw a diagram of how you set it up 2) Describe your method 3) Describe what equipment you used to get the results and how you analysed them.
Newton s 3 rd Law of Motion When body A exerts a force on body B, body B exerts an equal and opposite force on body A. My third law says that if I push to the right I will move backwards as well. Newton 1642-1727
Newton s 3 rd Law of Motion What will happen if I push this satellite away from me?
Stopping a car What two things must the driver of the car do in order to stop in time?
Stopping a car Thinking distance (reaction time) Braking distance
Tiredness Too many drugs Stopping a car Thinking distance (reaction time) Too much alcohol Poor visibility Icy roads Tyres/brakes worn out Braking distance Wet roads Driving too fast Total Stopping Distance = Thinking Distance + Braking Distance
Representing Stopping Distance Graphically (Physics only)
Measuring Reaction Times For the experiment: 1) Describe your method (i.e. how will you measure reaction time?) 2) Describe what you are varying (your independent variable ) 3) Describe what equipment you used to get the results and how you analysed them. Typical reaction times are around 0.4 to 0.9s. How did you compare?
Stopping a car What happens inside the car when it stops? In order to stop this car the brakes must do work. This work is used to reduce the kinetic energy of the vehicle and the brakes will warm up. Greater speed = greater force needed to stop in a given distance = hotter brake pads!
Estimating Forces and Deceleration (higher only) Estimate rough values for the forces involved in decelerating these objects: A skydiver when he opens his parachute A car slowing down at traffic lights A formula 1 car about to take a sharp turn Taking u = 50m/s, v = 10m/s, t = 0.1s and m = 70kg we get Taking u = 20m/s, v = 0m/s, t = 2 s and m = 800kg we get Taking u = 100m/s, v = 20m/s, t = 2 s and m = 1500kg we get 28000N 8000N 60000N Q. What happens to the human body when these forces get TOO big?
5.7 Momentum (higher only)
Momentum Any object that has both mass and velocity has MOMENTUM. Momentum (symbol p ) is simply given by the formula: P Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s) You need to learn this equation!! What is the momentum of the following? 1) A 1kg football travelling at 10m/s 2) A 1000kg car travelling at 30m/s M V 10kgm/s 30,000kgm/s 3) A 0.02kg pen thrown across the room at 5m/s 4) A 70kg bungi-jumper falling at 40m/s 0.1kgm/s 2800kgm/s
Conservation of Momentum In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question: Two cars are racing around the M25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg Mass = 1800kg Speed =??m/s Momentum before = momentum after so 1000 x 50 + 800 x 20 = 1800 x V V = 36.7m/s
Momentum in different directions What happens if the bodies are moving in opposite directions? Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg Momentum is a VECTOR quantity, so the momentum of the second car is negative Total momentum = 1000 x 50 800 x 20 = 34000 kgm/s Speed after collision = 34000 kgm/s / 1800 = 18.9m/s
Another example Consider the nuclear decay of Americium-241: 237 93 Np 241 95 Am 4 2 α If the new neptunium atom moves away at a speed of 5x10 5 m/s what was the speed of the alpha particle? 2.96x10 7 m/s
More questions 1. A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? 2. A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? 3. A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have a mass of 1kg. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? 4. A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at? 5.6m/s 70m/s 3m/s 400m/s
Recap question on momentum 1. Bradley and Jack are racing against each other over 400m at Sports Day. Brad is running at 8m/s and catches up with Jack who is running at 6m/s. After the collision Brad stops and Jack moves slightly faster. If Brad s mass is 60kg and Jack s is 70kg calculate how fast Jack moves after the collision. 12.9m/s 2. Coryn is driving her 5kg toy car around. It is travelling at 10m/s when it hits the back of Shannon s (stationary) leg and sticks to it. Assuming Shannon s leg can move freely and has a mass of 10kg calculate how fast it will move after the collision. 3.3m/s
Change in Momentum and Force Instead of F=ma Newton actually said that the force acting on an object is that object s rate of change of momentum. In other words Force = Change in momentum (in N) Time (in s) (in kgm/s) mv You DON T need to learn this equation!! F T For example, Rob Stocker scores from a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate: 1) The change in momentum of the ball (its impulse), 2) The speed the ball moves away with
Example questions 1) Jack likes playing golf. He strikes a golf ball with a force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed. 2) Chad thinks it s funny to hit tennis balls at Illy. He strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball s change in momentum and its speed. 3) Oli takes a dropkick by kicking a 0.4kg rugby ball away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball. 4) Paddy strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for. 16kgm/s, 80m/s 4.5kgm/s, 18m/s 40N 0.2s