Chapter 4: Forces

From “Conceptual Physics for Everyone”, Paul G. Hewitt, Addison Wesley, 2002.

You cannot touch without being touched-

That’s Newton’s third law!Newton’s Third Law

Newton’s Third Law of MotionAny time a force is exerted on an object, that force is caused by another object.

Newton’s third law:

Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.

Newton’s Third Law:interaction pairs• Newton’s third Law: for every action, there is an

equal and opposite reaction.• We call these two equal and opposite forces

interaction partners.• Note: they operate on different objects• Note: they involve the same interaction :eg. Gravity,

contact…

Newton’s Third Law of Motion

A key to the correct application of the third law is that the forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object.

Newton’s Third Law of MotionRocket propulsion can also be explained using Newton’s third law: hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket.

Note that the rocket does not need anything to “push” against.

Newton’s Third Law of Motion

Conceptual Example 4-5: Third law clarification.

Michelangelo’s assistant has been assigned the task of moving a block of marble using a sled. He says to his boss, “When I exert a forward force on the sled, the sled exerts an equal and opposite force backward. So how can I ever start it moving? No matter how hard I pull, the backward reaction force always equals my forward force, so the net force must be zero. I’ll never be able to move this load.” Is he correct?

LawnmowerPicture

AutoRockClimber

Free Body Diagram Practice

Problem Solving Strategy1) Draw a FBD for the object (or objects). Labeling all forces with simple vector symbols.2) Pick a coordinate system3) Add up all of the forces in the x-direction. 4) Decide whether or not this sum is equal to zero or ma

5) Add up all of the forces in the y-direction6) Decide whether or not this sum is equal to zero or ma

7) Solve these two simultaneous equations for your unknown(s).

The inclined plane problem• A box of oranges with a weight of 100 N slides

down an incline of 25° above the horizontal with a constant velocity. What is the normal force? The friction force?

25°

Solving Problems with Newton’s Laws: Free-Body Diagrams

Example 4-16: Box slides down an incline.

A box of mass m is placed on a smooth incline that makes an angle θ with the horizontal. (a) Determine the normal force on the box. (b) Determine the box’s acceleration. (c) Evaluate for a mass m = 10 kg and an incline of θ = 30°.

Problem 10: A 20.0-kg box rests on a table. (a) What is the weight of the box and the normal force acting on it? (b) A 10.0-kg box is placed on top of the 20.0-kg box, as shown in Fig. 4–31. Determine the normal force that the table exerts on the 20.0-kg box and the normal force that the 20.0-kg box exerts on the 10.0-kg box.

Problem 27: A box weighing 77.0 N rests on a table. A rope tied to the box runs vertically upward over a pulley and a weight is hung from the other end .Determine the force that the table exerts on the box if the weight hanging on the other side of the pulley weighs (a) 30.0 N, (b) 60.0 N, and (c) 90.0 N.

Problem 51:The Figure below shows a block (mass ) on a smooth horizontal surface, connected by a thin cord that passes over a pulley to a second block which hangs vertically. (a) Draw a free-body diagram for each block, showing the force of gravity on each, the force (tension) exerted by the cord, and any normal force. (b) Apply Newton’s second law to find formulas for the acceleration of the system and for the tension in the cord. Ignore friction and the masses of the pulley and cord.