Teacher Reference Page

1. Teacher Reference Page • This powerpoint is a presentation of Forces and Newton’s Laws. It probably goes through more than is necessary for students to know on the proficiency test, but is not as detailed as what would be discussed in a physics class. • I made no effort to make this super-fancy because I just don’t know how. Sorry!! • There are links to the RPDP site at the end of this presentation. There are also a few links to sites that show animations of various principles described in this unit. • Please use your physics peers for help in understanding, presentation, or resources. That’s what we’re here for.

2. Forces and Newton’s Laws of Motion

3. What is a Force? • A force is defined as a push or a pull. • A force is something that CHANGES the motion of an object. This could mean that the object changes its speed by slowing down or speeding up. It might also mean that the object changes its direction of travel after a force has been applied.

4. Can Objects Move Without a Force? • Yes, a force only CHANGES the way an object moves. Therefore, if an object is already moving, it will continue to move at the same speed in the same direction. • Changes in speed or direction are known as ACCELERATIONS • Therefore, FORCES cause objects to ACCELERATE.

5. Newton’s First Law • An object at rest will stay at rest, and an object in motion will stay in motion in a straight line, UNLESS acted upon by an outside force. • Again, it takes a force to get an object moving (change in motion) or a force to change the speed or direction of motion (also a change in motion)

6. Why don’t objects keep moving when I throw them on Earth? • Because of Newton’s 1st Law. Gravity is a force (a push or pull acting between all objects with mass). If you throw an object through the air, gravity is a force that can change of the motion of the object.

7. Why do rolling objects slow down? • Because of Newton’s 1st Law. Friction is a force that impedes the motion of objects when two surfaces are in contact with one another. • If we lived in a world without friction, then objects would actually keep moving at the same speed. Because friction is a force, it changes the motion of the object.

8. More on Friction • Friction always acts in a direction opposite to motion. • Objects that weigh more have a greater frictional force acting on them. • Friction even acts when objects are at rest. To move an object, you must provide enough force to overcome friction.

9. Mass and Inertia • Mass is the amount of matter in an object. • Inertia is the mass of an object. • Inertia is also a measure of an object’s resistance to CHANGING its motion.

10. More Mass and Inertia • If something has a lot of mass, it has a lot of resistance to changing its motion – it has a lot of inertia. • Therefore, heavy objects are hard to get moving if they are at rest, OR they are hard to stop moving if they are already in motion.

11. Newton’s 2nd Law • The net force (all of the forces acting on an object) cause the object to have an acceleration. • F = ma • F = total force • m = mass of the object • a = acceleration

12. Newton’s 2nd Law • It may be easier to explain Newton’s 2nd Law in terms of acceleration (the CHANGE in the object’s motion) • a = F/m

13. a = F/m • Because acceleration and Force are directly related in the equation, a big force results in a big acceleration. Or a small force results in a small acceleration. • This means a big push causes a big CHANGE in motion like a drastic speed increase or decrease, or a sudden change in direction.

14. a = F/m • Because acceleration and mass are indirectly related, applying a force to a SMALL mass will result in a LARGE acceleration. Applying a force to a LARGE mass will result in a SMALL acceleration. • The units for ANY type of Force is the Newton.

15. Example • If I apply a 1 Newton force to a 1 kilogram object, what is its acceleration? a = F/m = 1/1 = 1 m/s2 This is not a very large acceleration and would result in only a minor change of motion.

16. Another Example • If a 500 N force is applied to a 1 kilogram object, what is the acceleration of the object? a = F/m = 500/1 = 500 m/s2 This is a very large acceleration which means that the object will have a drastic increase or decrease in motion.

17. USE COMMON SENSE • Does it take big forces to move big objects? OF COURSE!!! • Does a bigger force (push or pull) cause a greater change in motion? YES!!

18. Use Diagrams • A free body diagram shows all of the forces acting on an object. • An object always accelerates in the direction of the net force.

19. Example

20. What the picture says… • The object clearly moves to the right because that is the direction of the bigger force. In fact the net sideways force would be 13 N to the right because we would be subtracting the forces in opposite directions.

21. The picture also says… • The downward force of the weight is equal to the upward force from the table. By subtracting two equal forces, we get a net force of zero. • REMEMBER, a zero net force means no acceleration – no CHANGE of motion. This is why the object moves sideways, but not up and down.

22. But Just as Important… • An object may have two equal forces acting on it and still be moving. It just isn’t CHANGING the way it is moving if these equal forces act from opposite directions.

23. For Example • In cruise control, your engine puts out a certain force to make your car go forward. But if your speed is constant, friction and air resistance MUST BE putting an equal force back on your car to prevent any CHANGE of motion.

24. REAL LIFE • Why do you keep pedaling a bike when an object in motion wants to stay in motion. • Because….friction is a force trying to slow you down, so you need a force to keep you motion going.

25. Newton’s Third Law • “If object A exerts a force on object B, then object B exerts an equal force on object A in the opposite direction.” • This is more commonly known as, “for every action there is an equal and opposite reaction”.

26. What does the 3rd Law Mean? • Does that mean that if I punch a wall with 300 Newtons of force, the wall punches me back with 300 N of force? YES – that’s why your hand hurts. The wall is exerting a force to change the motion of your hand.

27. Classic Question • If a bee flies straight into a moving car, who experiences the greater force? The bee or the car? ANSWER – they both experience the same force. That’s what Newton’s Third Law states.

28. How does the 3rd Law Make Sense • How can both the bee and the car experience the same force? It doesn’t make sense. Answer – Remember that forces cause CHANGES in the motion of an object. Also remember that the more mass an object has – the less the change will be. (Continue on next slide)

29. Let’s say the bee and the car each experience a force of 10 N during the collision. How would a 10 N force affect a bee whose mass is 0.002 kg? a = F/m = 10 N/.002kg a = 5000 m/s2 This is a HUGE change in motion for the bee. But think about it. The bee DOES undergo a huge change in motion after striking the car, so this makes sense.

30. Now apply the same 10 Newton force to the car whose mass is 2000 kilograms. a = F/m = 10 N/2000 kg a = .005 m/s2 This is a very SMALL acceleration. In other words, the car’s motion CHANGES very little. This makes sense too. Why would a bee striking a car really impact the way the car is moving?

31. AVOID CONFUSION • It is a common misconception to think that the bigger force comes from the bigger object..- NOT TRUE! • The FORCES must be equal when two objects interact. What is not equal are the accelerations, or the effect of the forces on motion, because of the difference in mass between the two objects.

32. Common Misconceptions • Students believe that objects in motion have an “impetus” force. They think that if an object is moving, it is retaining some of the force that initially set the object in motion, and that’s why it continues to move. They don’t recognize that once the object is in motion, it while stay that way UNTIL an outside force makes its motion change.

33. Common Misconceptions • Big objects always exert the bigger force when they strike a small object. Truth – the forces are the same. It is the “effect” of those forces that are different. The larger object has a smaller acceleration, and therefore, a small CHANGE in its motion after the force is applied.

34. Common Misconceptions • Forces deal with velocity. Truth – forces deal with accelerations, which are changes in speed or direction. Therefore, an object can have a constant velocity without a net force acting on it. This doesn’t mean no forces act on the object, it means that the forces act equally from opposite directions to cancel each other. So an object with no acceleration can either be moving at a constant speed or not moving at all.

35. The following links go directly to the RPDP Tips Site and other Web Sites with Physics Animations and Examples. • http://rpdp.net/sciencetips_v2/P12B1.htm • http://nzip.rsnz.org/es/applets/index.htm • http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/ • http://phet.colorado.edu/new/simulations/index.php?cat=Motion • Please don’t hesitate to see Scott or Kris for other resources if it helps.