Introduction to Newton’s Laws
Forces, Motion and Gravity
A force is a push or a pull. Forces are causing movement all around us: your bicycle rolling downhill, a ball hitting a tennis racket, riding in a car. Whenever an object speeds up or slows down or starts moving in a different direction, it is because a force has acted on it.
Forces and Gravity
Gravity is the natural force of attraction exerted by a celestial body, such as the planet Earth, upon objects at or near its surface, tending to draw them toward the center of the body. When something rolls off a table, it falls to the floor due to the force of gravity pulling it down. Gravity can be measured using a spring scale. The spring stretches according to the amount of mass (weight) hung from it. The greater the force, the more the spring will stretch. The unit for measuring force is a newton. One newton (N) is the amount of force needed to cause a one-kilogram mass to accelerate at a rate of one meter per second for each second of motion. This is about the same as the force that a small mouse sitting on a table exerts on the table. You would write a newton as 1N = 1kg x 1 m / sec2.
Exerting a force on something does not always make it move. This is because there is nearly always more than one force acting on an object. If you are trying to move a large concrete block but it won’t move, it’s probably because of friction. Friction is the force created whenever two objects rub against one another. The heavy block is pressing strongly on the ground and creates strong friction. If the block was resting on ice, it would move more easily because ice is very smooth. If you were to drop something from a great height, it would gradually move faster until the force of friction from the air, which acts upward, equals the downward force of gravity. This is called terminal velocity. Dense objects with little surface area fall for several seconds before reaching terminal velocity. Less dense objects with a lot of surface area reach terminal velocity much faster.
Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force.
There are many forces around you. For example, when you sit in a chair, gravity pulls you toward the earth. Your body pushes outward with equal strength to the atmospheric pressure pushing in. The chair pushes up against the force of gravity to keep you from falling. The forces are balanced and you are at rest. You will remain at rest until some outside force moves it. You have inertia, or the tendency of an object to remain at rest or in motion until acted upon by an external force. You must exert some forces to get out of the chair.
The first law also tells us about objects in motion. If you are riding a bike and stop pedaling, the bike doesn’t stop. It stays in motion in the same direction until it is acted on by air resistance or friction, which causes it to stop.
Newton’s Second Law of Motion
The force of an object is equal to its mass times its acceleration.
You’re in the car with your family and it stalls. Your dad says he thinks he can start it if you push it. You are exerting a force on the car. You’re getting it moving pretty fast (acceleration) and your dad jumps in. The car slows considerably. This is because of the mass (weight) of what you are pushing. Acceleration is a change in velocity (speed) or the rate at which this change occurs. Newton’s second law tells us that force = mass x acceleration. It is also true that acceleration = force/mass.
A rolling marble can be stopped more easily than a rolling bowling ball when both are traveling at the same velocity (speed). The momentum of an object is related to its mass and its velocity. The larger the mass or the larger the velocity (or both) causes greater momentum. Momentum is the product of the mass and the velocity of an object. Momentum = mass x velocity.
Newton’s Third Law of Motion
For every action there is an equal and opposite reaction.
You can see Newton’s third law in action if you blow up a balloon and then release it.
Air shoots out of the neck of the balloon as it moves in the opposite direction. The force propelling the balloon is equal and opposite to the force of the air leaving the balloon.