2b: Newton’s Three Laws

Newton came up with several important laws of physics, three of which we may find useful for the physics of sound.

First Law: Objects continue at rest or in straight-line motion with constant velocity unless an unbalanced force acts on them. You might think that if no forces act the object is stationary. But this is only one case; the first law also says that if all the forces add to zero the object could still be moving at constant velocity. So why do you need to run the motor in your car if you are going down the road in the same direction at the same speed? The motor supplies enough force to overcome friction. Not more force than friction but a force exactly equal to friction so that the total force is exactly zero and you continue moving at a constant velocity. Similarly, a performer playing a wind instrument must continue to supply force on the air in the instrument to overcome friction as the air passes through the instrument. Inertia is the term used for the property of an object that causes it continue in straight line motion unless acted on by an unbalanced force.

Second Law: Forces cause accelerations (forces do not cause velocity but changes in velocity). The second law is also known as F = ma where F is the total force acting on mass m, and a is the acceleration of the mass. Note that as was stated in the first law, a net total force is not needed to maintain a constant velocity; the first law says once something is moving it will keep moving unless a force (for example friction) acts to stop it. When you accelerate your car there is a net force on it. But when you reach cruising speed the net force goes to zero (the motor force exactly cancels resistance forces). In this case (when the net force adds to zero) the object (your car) obeys Newton’s 1st law and you continue moving in a straight line at constant velocity.

Third Law: Anytime one object exerts a force on a second object, the second object exerts an equal force (but in the opposite direction) back on the first object. Forces always act in pairs on two different objects; an action force acting on one object and a reaction force acting on the second object. Another way to say this is you cannot touch something without it touching you back, and just as hard as you touched it. This seems simple if you are pushing on the wall; the wall obviously pushes back with the same force. But what about the force on a baseball as you throw it (before release)? Is the force pushing back by the baseball the same as the force you apply? YES! The laws of physics are always true with no exceptions. So how can you throw the baseball if the two forces are equal in magnitude but opposite in direction? The key is to realize the action and reaction forces act on different objects. Your force on the ball makes it go forward until it leaves your hand. The equal reaction force back from the ball acts on your hand (not the ball) so you can feel the ball as you throw it.

Video/audio examples:

The first law: car crash testing.  Watch the movement of the dummy’s body and head when the car suddenly stops. Because there is no force acting on the head, it keeps moving, according to Newton’s 1st law.

The first and second law: sky diving. If you understand this example you have a good idea of how Newton’s laws work.

The first, second, and third law: explanation with a bicycle.

The third law: examples on the space station.

A few more examples:
Simulation exercise 2A (turn in answers on a separate sheet of paper):
  1. Play this simulation on Force and Motion.
    1. There are four simulations, choose the third one, Friction. Use the slider to apply a force to the crate; the little figure will push on the crate. If you add enough force, the crate will move. Once the crate is moving, release the force and see what happens. Explain what is going on.
    2. Reset the original parameters (button on the right) and remove all friction (the slider labeled friction). Start the simulation again and have the little figure give a small push to the block and then let go. What is the difference in behavior of applying force with and without friction?
    3. Experiment with other values of friction and write a few sentences about what you discover.
    4. Add mass to the crate by dragging objects below onto the crate and experiment using the same value of friction. Write a few sentences about what you discover.

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