Some objects at rest require little force to move them. Other objects need a great deal of force to move. What do you know about the forces that objects exert on one another?
Imagine that you are an astronaut making a space walk outside your space station. In your excitement about your walk, you lose track of time and use up all the fuel in your jet pack. How do you get back to your station? Your jet pack is empty, but it can still get you back to your station if you throw it away. To understand how, you need to know Newton's Third Law of Motion, which states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.
Newton realized that forces are not "one sided." Whenever one object exerts a force on a second object, the second object exerts a force back on the first object. The force exerted by the second object is equal in strength and opposite in direction to the first force. Newton called one force the "action" and the other force the "reaction" force.
You may already be familiar with examples of Newton's Third Law of Motion. Perhaps you have watched figure skaters and have seen one skater push on the other. As a result, both skaters move. The skater who pushed is pushed back with an equal force, but in the opposite direction.
The speeds with which the two skaters move depends on their masses. If they have the same mass, they will move at the same rate of speed. If one skater has a greater mass than the other, they will move backward more slowly. Although the action and reaction forces will be equal and opposite, the same force acting on a greater mass results in a smaller acceleration. (This is Newton's Second Law of Motion).
Now can you figure out how to return from your space walk? In order to get a push back to the space station, you need to push on some object. You can remove your empty jet pack and push it away from you. In return, your jet pack will exert an equal force on you, sending you back to the safety of the space station.
13
Newton's third law is in action all around you. When you walk, you push the ground with your feet. The ground pushes back on your feet with an equal and opposite force. You go forward when you walk because the ground is pushing you! A bird flies forward by exerting a force on the air with its wings. The air pushes back on the wings with an equal force that propels the bird to fly.
-
One might wonder why action and reaction forces do not cancel each other. To answer this question, you have to consider the objects on which the forces are acting. When opposite and equal force is applied to one object, the forces cancel out. The object does not move. Newton's third law refers to forces on two different objects.
Momentum
When Newton presented his three laws of motion, he used two different words to describe moving objects. He used the word velocity, but he also wrote about something that he called the "quantity of motion." Today we call it momentum. The momentum of an object is the product of its mass and velocity.
Momentum = Mass X Velocity
What is the unit of measurement for momentum? Since mass is measured in kilograms and velocity is measured in meters per second, the unit for momentum is kilogram-meters per second (kg x m/s). Like velocity and acceleration, momentum is described by its direction as well as its quantity. The momentum of an object is in the same direction as its velocity.
The more momentum an object has, the harder it is to stop. You can catch a baseball moving at 20 m/s, for example, but you cannot stop a car moving at the same speed. Why does the car have more momentum than the ball? The car has more momentum because it has more mass.
A high velocity can also produce a large momentum, even when mass is small. A bullet shot from a rifle, for example, has a large momentum. Even thought it has a small mass, it travels at a high speed.
Conservation of Momentum
You know that if someone bumps into you from behind, you gain momentum in the forward direction. Momentum is useful for understanding what happens when an object collides with another object. When two objects collide, in the absence of friction, momentum is not lost. This fact is called the law of conservation of momentum. The Law of Conservation of Momentum states that the total momentum of the objects that interact does not change. The quantity of momentum is the same before an after they react. The total momentum of any group of objects remains the same unless outside forces act on the objects. Friction is an example of an outside force.