Marr College Science Forces Learning Outcomes and Summary Notes
Learning Intentions By the end of this unit I will be able to 1. Describe a force as a push or a pull. 2. Describe the effects of forces in terms of change in speed, change in direction and change in shape. 3. Provide examples where forces produce the above changes. 4. State that the unit of force is the newton (N). 5. Describe ways in which increasing friction can be useful. 6. Describe ways in which decreasing friction can be useful. 7. State that air resistance is the name given to friction caused by air molecules. 8. State that air resistance can be reduced by streamlining. 9. Provide examples of streamlined objects. 10. State that if two objects are dropped from the same height, they will hit the ground at the same time, as long as there is no air resistance. 11. Describe situations where apparent weightlessness occurs. 12. State that objects appear to be weightless if they are in free-fall. 13. State that mass is a measure of how heavy an object is. 14. State that the unit of mass is the kilogram (kg). 15. State that an object s weight is the size of the force of gravity on the object. 16. State that the unit of weight is the newton (N). 17. State that gravitational field strength is the size of gravity force on every kilogram. 18. Carry out calculations using the relationship between weight, mass and gravitational field strength. 19. State that forces which are equal in size but act in opposite 1
directions on an object are called balanced forces. 20. State that balanced forces have the same effect as no force at all. 21. State that if the forces on a stationary object are balanced, then the object will remain stationary. 22. State that if the forces acting on a moving object are balanced, then the object will continue to move at a constant speed in a straight line. 23. Identify on a force diagram whether the forces are balanced or unbalanced. 24. Solve problems involving balanced forces. 25. State that the upwards force provided by water is called the buoyancy force. 26. State that an object floats because the buoyancy force and the object s weight are balanced. 27. Draw diagrams and label the forces acting on objects using arrows (e.g. weight, buoyancy force, air resistance, lifting force, engine force, etc.) 28. Calculate the resultant force on an object if more than one force is acting. Forces and their effects A force can be described as a push or a pull. We can t see forces, but we can see the effect that forces have. Forces can: o Change the speed of a moving object o Change the direction of a moving object o Change the shape of an object Friction 2
Friction Friction is the force produced when two surfaces rub against each other. Friction can oppose the motion of a moving object. The force of friction causes moving objects to slow down and come to a stop. Friction also acts when moving objects collide with particles of air. The force of friction produced by the air is known as air resistance. Increasing friction You need to increase friction to: o Slow a moving object down, or o Provide more grip Car tyres are made of rubber and have treads to provide grip to the road so that the car can change direction. Parachutes increase air resistance and slow you down. Brake pads on a bike increase friction by rubbing against the wheels and slow the bike down. 3
Decreasing friction You need to decrease friction to: o Help a moving object go faster o Allow surfaces to slide over each other more easily A racing cyclist gets down low using the handlebars rather than sitting upright, to reduce friction and go faster Oiling the hinges of a door allows the door to open and close more easily, without creaking. Streamlining Streamlining involves changing the shape of an object to make the force of friction as small as possible. Air will flow very easily over a streamlined object and will not affect its motion much. Examples of streamlined objects include: o Formula 1 race cars o Racing cyclist helmets 4
Gravity Free-fall Two objects dropped from the same height will hit the ground at the same time, as long as there is no air resistance acting. An object in free-fall appears to be weightless o So astronauts inside a spacecraft orbiting around the Earth appear to be weightless Mass, weight and gravitational field strength Mass and weight mean different things. Mass is a measure of how heavy an object is. o The unit of mass is the kilogram (kg). Weight is the size of the force of gravity on an object. o The unit of weight is the newton (N) Gravitational field strength is the size of the gravity on every kilogram of mass. o The unit of gravitational field strength is newtons per kilogram (N/kg) The gravitational field strength on Earth is 10 N/kg 5
The weight of an object can be calculated using the formula: Example 1. An astronaut has a mass of 65 kg. (a) Calculate the weight of the astronaut on Earth. (b) Calculate the weight of the astronaut on the Moon where the gravitational field strength is 1 6 N/kg. Solution (a) W =? m=65 kg g=10 N/kg W = mg W= 65 10 W = 650 N 6
(b) W =? m = 65 kg g = 1.6 N/kg W = mg W= 65 1 6 W = 104 N 7
Balanced forces Two forces of the same size acting in opposite directions are called balanced forces. 150 N 150 N Balanced forces have the same effect as no force at all! If the forces acting on an object are balanced then: o A stationary object will remain stationary o A moving object will continue to move at a constant speed (it won t speed up or slow down) Buoyancy Force Buoyancy force is the upwards force provided by a liquid (or gas) An object floats because the buoyancy force balances the object s weight buoyancy force weight 8
Resultant forces 10 N 15 N 20 N Total force to the right = 15 +20 = 35 N Total force to the left = 10 N Resultant force = 35-10 = 25 N to the right 9
Word list force A push or a pull. newton friction The unit of force. Named after Sir Isaac Newton. e.g. a car pulls a caravan with a force of 4000 newtons (4000 N). The force that opposes (acts against) motion. air resistance Friction that is caused by air molecules. streamlining acceleration Changing the shape of an object to reduce air resistance. Getting faster, speeding up. Newton balance mass A device that measures the size of a force. A measure of the amount of matter that an object is made of. kilogram The unit of mass. e.g. the mass of a student is 55 kilograms (55 kg). gravity The force that pulls you to the ground. weight The size of the force that gravity pulls on an object with. e.g. the weight of a student is 550 N. 10
gravitational field strength free-fall The amount of force that gravity exerts on every kilogram of an object. Falling due to gravity apparent weightlessness balanced forces Feeling weightless because you are falling towards the Earth. When two forces have the same size and act in opposite directions, they are said to be balanced. Balanced forces have the same effect as no force at all. buoyancy force The upwards force provided by water. (and other liquids and gases) An object floats when the buoyancy force balances the object s weight. resultant force The overall force acting on an object (after you add up and take away all the single forces). 11