Welcome to Forces an anticipation guide A force is defined as a push or a pull When answering the following true or false statements, offer a

Transcription

1 Welcome to Forces an anticipation guide A force is defined as a push or a pull When answering the following true or false statements, offer a real-life example that justifies your answer. You haven t answered the question, unless you offer a physical event explaining your choice. I. If an object is moving, then a force must be acting upon it. II. If an object is still, then there cannot be a force acting on it. III. An object s natural tendency is, if moving, to stop moving eventually. IV. If an object is moving in a circle, then a force must be acting upon it. V. If an object is accelerating, then a force must be acting upon it. VI. If a force is exerted onto an object, that object will yield to the force without resistance. VII. If an object is moving, you must apply a force to stop it. VIII.The moon exerts the same amount of gravitational force on the earth as the earth exerts upon the moon.

2 IX. If force A results in a more dramatic motion than force B, than Force A must be greater than force B. X. If an object is moving in a straight line at a constant speed, then it is possible that no force is acting upon it.

3 The Laws that govern why and how things move aka, Newton s Three Laws of motion The events on the following screen, when analyzed correctly, can be used to understand all of Newton s Three Laws. When you go through the examples, you shouldn t simply predict what is going to happen, but you should also try to explain why it happens. If there was a change in motion, why did it change? If there was a force experienced, what did it cause? Even though these are things that you might experience every day, don t take them for granted, understand why what s happening is happening.

4 You re sitting shotgun in a car and you ve fallen asleep. The driver, your friend, is a bit sore because she s driving and you re sleeping. She feels it s your job to stay awake and keep her company. To teach you a lesson, she all of a sudden jerks left on the steering wheel. What does your head do and why? You re riding the subway and you decide you don t need to hold on, as the subway starts to leave the station, you find you have to brace your legs to keep from falling back. After a certain amount of time you find you can stand without needing to brace. Why did this happen? What was the train doing while you were bracing and what was it doing while you no longer needed to brace? During the same trip noted above, the tracks veer left. What does your body do and why? You re sitting shotgun in a car and you are foolish enough not to wear a seatbelt. As you re driving through a stoplight an oncoming car turns left in front of you. What does your body do and why? You re driving to your summer home, but doing so in the winter. This is your first trip in the winter you ve only driven this route during the dry, iceless and warm summer days. You are approaching a turn you always take at 60 mph. You decide to do the same again. What does your car do and why?

5 You re adventure plane has crash landed on a jungle island. For survival, you have no choice but to hunt the local native beasts massive rhinoceros. You fashion a sling out of some old leather and string. You place a rock on the leather sling and twirl when you let go, what does the rock do and why? You see a chair and sit down in it. Explain what happens and why? You approach a person three times your size on the street and you push them. Explain what happens and why?

6 Summary of what we ve Learned thus far I. For an object to experience a change in its motion, what must happen? II. So that we re clear, what does it mean to say that the motion of an object is changing (one of two things or both must be changing, what are those things)? III. Based on your answer to the above, if the sum of forces acting on an object is 0, can the object be moving, and if so, how? IV. How do the points we developed above provide reasoning for the following statement: If an object is accelerating, we know for a fact that the sum of the forces acting on the object is not 0. V. How do the points we developed above provide reasoning for the following statement: If an object is moving in a circle, we know for a fact that the sum of the forces acting on the object is not 0. VI. An object s natural tendency (in terms of its motion) can be manifested in two different types of motion. What are they? a. So if an object is doing anything other than the answer to V, to what definite conclusion can we come? VII.If you apply the same amount of force to two different objects, will both objects experience the same type of motion? Why? VIII.Based on your answer to the above, what factor influences how easy or how difficult it is to change the motion of an object? How is the difficulty or ease related to this factor? IX. Choose some real life example that illustrates the fact that if a force is exerted onto an object that object will exert an equal and opposite force in return.

7 What We ve Established thus far If an unbalanced force acts upon the object (an object experiencing a non-zero net force), the motion of that object will change. A change in motion is defined as either a change in the object s speed, direction or both (so if an object is accelerating, it is experiencing an unbalanced force; if an object is moving in a circle, it is experiencing an unbalanced force). If no force is acting, it is possible for the object to be at rest or moving in a straight line with a constant speed. These two types of motion can be referred to as a body s natural tendency. The greater the mass the harder it is to change its motion. If A exerts a force on B, B exerts an equal and opposite for on A.\ The above points sum up Newton s Three Laws of Motion.

8 Newton s Three Laws of Motion Newton s First Law: An object at rest will stay at rest and an object in motion will stay in motion (moving at a constant speed in a straight line) unless acted upon by an outside (unbalanced) force. An object s natural tendency is to resist changes to its motion. Newton s First Law is also known as the Law of Inertia Newton s Second Law: F net = ma or a = F net /m An object s acceleration as a result of the net force applied will be inversely proportional to its mass. Newton s Third Law: All forces are accompanied by an equal and opposite force. If object A exerts a force onto object B, object B will exert an equal and opposite force onto object A.

9 Inertia The tendency of anything with mass to resist changes to its motion. The more massive something, the more inertia it has. The more inertia something has, the harder it is to accelerate it. This explains why it is harder for you to stop the flying tackle of a sumo wrestler than it would be a high school physics teacher.

10 Forces We ve defined a force as a push or a pull. Based on the work we ve done thus far, we see that we can also define force by what it does: something that causes the motion of an object to change. All forces that exist can fall into one of four categories. Gravitational, electromagnetic, strong nuclear force and weak nuclear force These are the four fundamental forces of the universe. The other forces we mention in this unit all fall under one of the four fundamental forces (we mostly discuss forces that are gravitational forces or electromagnetic forces Tension force (F e ), weight (F g ), normal force (F e ), centripetal force (F e and F g ), elastic force (F e ) and friction force (F e ).

11 Weight Weight the pull that you feel on your body (or the pull on any mass) that is due to the gravitational force of attraction that exists between the mass of your body and the Earth. Weight is a force (and thus, being a vector, has magnitude and direction) it is not to be confused with mass, a scalar quantity (with only magnitude), that only describes the amount of matter. Your weight and your mass are two very different physical quantities. We can use Newton s 2nd Law (Fnet = ma) to derive an equation for weight. Newton s 2nd Law tells us that the product of an object s mass and the acceleration it is experiencing will give us the net force being exerted onto the object. That s how we get w = mg.

12 A mass weight is a force and it is always directed straight down. Since weight is determined by the acceleration due to gravity, your weight will not be the same on other planets or moons where the acceleration of gravity is different.

13 Tension Force The Tension Force is the force that develops in a wire, rope, cord, chain, or something of the like as it is being pulled or is pulling on an object. Tension Force is an electromagnetic force since it deals with the electrostatic bonds that binds matter. The Tension force is ALWAYS directed away from the object that it is pulling. The Tension force is evenly distributed along the length of the cord, rope, chain etc.

14 Free Body Diagram We will be analyzing a number of situations where numerous forces are acting on an object. Often, we will know the different types of forces that are acting, but not the magnitude of one of the forces. It will be very helpful in your understanding of the dynamics of the situation to draw a diagram depicting all the forces acting. A free body diagram depicts all the forces acting on an object. It does not depict the forces the object may be exerting. 10 kg T mg In this example to the left, you have a mass hanging from a string attached to the ceiling. The Tension force from the string is pulling up on the mass. Gravity (the mass weight) is pulling down on the mass. If the mass is at rest, what can we say about the net force acting on the mass?

15 Tension Problems 10 kg I. A 10 kg mass is hanging from a string attached to the ceiling. The mass is at rest. Determine the tension force in the string. II. A 10 kg mass is moving upward with a constant velocity of 2 m/s. Determine the tension force in the string. III. A 10 kg mass is accelerating downward with an acceleration of -2 m/s 2. Determine the tension force in the string. From our work in class you should have an understanding of the following: During what situation/s will the tension force be equal to the weight? During what situation/s will the tension force be greater than the weight? During what situation/s will the tension force be less than the weight?

16 The Normal Force (and elevator problems) The Normal Force, F N, defined in class yesterday, is the equal and opposite force that arises when two bodies are in contact. The example we ve returned to in class many times is that of a mass sitting on a table top. m F N In this case, since the mass is at rest, we know that F N is equal to mg. If F N mg, what would the mass be doing? mg

17 F N F A - additional force being applied downward on the mass. Maybe you re pressing down on the mass with your hand. m It should be clear in this example that F N is not going to be equal to mg. What would happen in this example if F N was equal to mg? What is F N equal to? mg

18 Elevator Problems Remember, as stated in class yesterday, the Normal Force will be equal to the reading on the scale. You re standing on a scale in the elevator I. In what situations will the reading on the scale be equal to your actual weight? II. When will the reading on the scale be greater than your weight? Less than your weight? (you should be able to use your own life experience to answer this question) III. The elevator is accelerating downwards at -2 m/s 2. What is the reading on the scale (your perceived weight)? Your mass is 75 kg IV. The elevator is accelerating downwards at m/s 2. What is the reading on the scale (your perceived weight)? Your mass is 75 kg

19 Normal Force (cont.) Before we close the book on our discussion of Normal force, we need to mention one more thing. We ve only examined situations where a mass is at rest on a level plane. In this situation, when the Normal Force and the weight are the only forces acting (and F = 0 N), F net N will be equal and opposite to mg. So here s the question: Will F N always be equal and opposite to mg when F N and mg are the only forces acting and F = 0 N? net Hint: Why is this force referred to as the Normal Force?

20 The term Normal, as just mentioned, comes from the fact that F N is always directed perpendicular to the surface. This happens because the normal force is the equal and opposite force due to contact. The mass presses into the plane with its body, therefore, the plane presses back into the mass with the Normal Force. Use this understanding to draw a Free Body Diagram for a mass on a frictionless inclined plane. Remember, when drawing a FBD, you draw the forces acting on the mass. m

21 Free Body Diagram for a Mass on a Frictionless Inclined Plane F N m mg As we saw when one of you stood on the inclined scale, F N mg in magnitude when the plane is inclined. We also see that F N is not opposite to mg when the plane is inclined. As we ll find out when we study inclined planes in more detail next week, F N is equal to the perpendicular component of the mass weight.

22 Friction Force The principles of friction force (as stated at the start of class): The friction force always opposes the motion of an object. The friction force is proportional to F N. The friction force between different materials is different (sand paper and wood as compared to steel and ice). The equation for the force of friction is the following: F f = μ F N μ is defined as the coefficient of friction. It is a number that offers an indication of how much friction exists between two materials. The larger the number, the larger the force of friction. What do you think has a greater coefficient of friction, asphalt and rubber or wet asphalt and rubber?

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24 Practice Friction Problems I. The force of kinetic friction between a block and a surface is 25 N. If the box has a mass of 17 kg, what might the block and the surface be composed of? II. A 20 N force is being applied in the horizontal direction towards the right onto a 15 N crate. The crate is moving with a constant velocity. a. What is the magnitude and direction of the friction force acting on the crate? b. What is the coefficient of friction between the crate and the floor. III. A force of 60 N is applied to a rope to pull a sled across a horizontal surface at a constant velocity. The rope is at an angle of 30 degrees above the horizontal. a. Calculate the magnitude of the component of the 60 N force that is parallel to the horizontal surface. b. Determine the magnitude of the friction force acting on the sled.

25 IV. You are applying a force of 70 N to a 10 kg rubber crate and it accelerates to the right with an acceleration of m/s2. a. What is the force of friction acting against the crate? b. What surface is the crate sliding across?

26 Kinetic Friction and Static Friction There are two types of friction forces that exist, one that is acting when there is no relative motion between the two surfaces (static friction) and one where there is relative motion between the two surfaces (kinetic friction). Static Friction the force of friction that keeps an object in place. If the object is not moving and a force is being applied, it is the force of static friction that is acting in the opposite direction and keeping the object from beginning to move. The force of static friction between any two surfaces is always greater than the force of kinetic friction. That is why on the table of coefficients the static coefficient is always greater than the kinetic coefficient. Kinetic (sliding) friction the force of friction that is acting when relative motion exists between the two surfaces