Now that we have ways to measure the motion of a hovercraft, we can begin to investigate why they move the way they do. A good start is to introduce three of the most fundamental laws of motion. These three laws have been named Newton’s Laws of Motion, after Sir Isaac Newton. Although they were formulated hundreds of years ago, they work so well that they are still widely used to calculate the motion of everything from pool balls colliding to planets traveling through space.
Sir Isaac Newton
1642 – 1727
Newton’s first law, often called the Law of Inertia states the following:
An object in motion tends to stay in motion, and an object at rest tends to stay at rest unless acted upon by an outside force.
- or -
An object with no net force acting on it will move at a constant velocity (which may be zero).
This means that when an object is moving, it should keep on moving at the same speed and in the same direction unless some outside force is acting on it. It also means that an object at rest ( not moving ) will stay at rest until some force causes it to start moving. If you’ve ever seen a video of astronauts in space, this law makes more sense. The astronauts can float without moving because there is no net force acting on them. If they push off the wall of the space ship, they will float away at a constant speed and direction and won’t stop until they collide or push off something else.
In order to understand how an object will move, you just need to know what forces are acting on it. Let’s describe some of the most common forces. The first is one that has been acting on you your entire life: gravity. Earth’s gravity is constantly pulling you down towards the center of the earth. When you jump into the air, you exert a force that pushes you up from the ground. Since this force is greater than gravity, you go up. Once you’re in the air, gravity becomes the only force acting on you and pulls you back down.
A second familiar force is called contact force. When two objects are touching each other, they are exerting a contact force. When you push or throw something, you exert a contact force on it. Go back to the jumping example. After you jump into the air and begin to fall back down due to gravity, the Law of Inertia says that you should continue that downward motion. If gravity had its way you’d keep falling towards the center of the earth, but you stop when you hit the ground because the ground exerts a contact force on you. When you’re standing on the ground, Earth’s gravitational force is pulling you down, but the ground is exerting a contact force pushing you up. These two forces have the same magnitude, or strength, but act in opposite directions, so they cancel each other out. Even though both of these forces continue to act on you, we say there is no net force because they cancel each other out. The term ‘net force’ refers to the directional sum of all the forces. When two forces act in the same direction, they add together. When they act in the opposite direction, one is subtracted from the other. If two forces that are equal in strength act in the opposite directions, they cancel each other out completely.
A third force you encounter all the time but may not realize it is friction. Friction is a force that opposes motion between two objects in contact. If you push a box across a floor and then stop pushing, it will slide for a while but soon come to a stop because friction opposes the motion between the box and the floor. If you go to a skating rink and push that box across the ice, it will slide further because there is less friction between the box and the ice. It will still eventually come to a stop. Hovercraft, in fact, were invented as a way of reducing this sliding friction! Another form of friction is called wind resistance. If you stick your hand out of the window of a fast moving car, you feel a force pushing your hand back. This is caused by friction between your hand and the air it’s moving through. If you stick your hand out of a moving hovercraft, you will also feel the wind resistance, but something interesting happens. Hovercraft are designed to have so little friction as they move that the small amount of wind resistance that occurs when you stick your hand out to the side of the hovercraft is enough to make the hovercraft turn in that direction!
We now have a better concept of what forces are, and we know from Newton’s first law that if there is no net force on an object, its velocity doesn’t change. Exactly how the velocity changes if there is a net force acting on the object is described by Newton’s Second Law. It is most easily stated with the following formula:
F = m · a
- or -
Force = Mass · Acceleration
This means that anytime a net force acts on an object, that object will accelerate. It also states that more massive objects require more force to achieve the same acceleration as less massive objects. This is why you have to push harder to move a big desk than a small chair.
Given what we know about velocity and acceleration, this second law really agrees with the first law. The equation for the second law states that a force leads to an acceleration, so if no net force is acting on an object, it will not be accelerated. No acceleration means that the velocity doesn’t change, so no force equals constant velocity… just like the first law states!
Knowing the relationship between force and acceleration means we can clear up one of the most common misconceptions in all of science: the relationship between mass and weight. It’s very common to believe that kilograms are just the SI (System International) equivalent of pounds and vice versa, but this isn’t true. The pound is a measure of weight in the Imperial unit system, and its equivalent in SI is the Newton, named after Sir Isaac Newton. Weight is actually a measure of the force that gravity exerts on an object. The kilogram is a unit of mass in SI. Although practically never used, the unit for mass in the Imperial system is called a slug. Refer to the unit conversion sheet to see exactly how these units are related.
The pound is therefore a measure of force exerted by gravity while the kilogram is a measure of the mass of an object. When you convert between pounds and kilograms, you’re essentially using Newton’s second law. The reason we can convert so easily is because of the way gravity acts. On earth, gravity acts to accelerate all objects the same, regardless of the mass. If you drop a bowling ball and a pebble from a bridge, they will hit the bottom at the same time because gravity accelerates them the same. If acceleration is constant for all masses and forces, then you can easily interchange between the two. Keep in mind, however, that mass and weight are different concepts. If you’ve seen a video of astronauts on the moon you would have noticed that they seem to weigh very little as they almost float around on the surface of the moon. This is because they really do weigh less on the moon than on the earth. Their mass, on the other hand, stays exactly the same.
We can now finish up with Newton’s Third Law, which states:
For every action there is an opposite and equal reaction.
This means that for every force that is exerted on an object, the object exerts back an equal force in the opposite direction. This can be demonstrated with two pool balls. If the cue ball runs into the eight ball, it will exert a force on the eight ball, causing it to accelerate away. At the exact same time, the eight ball will exert the exact same amount of force on the cue ball in the opposite direction, which causes the cue ball to decelerate. Essentially this means that all forces exist in pairs. You push against a wall, it pushes back. If you bang your head against a wall, you’re exerting a force on the wall. Keep doing this and the headache you’ll get will demonstrate the wall’s force exerting back on you! When a rifle is shot it forces the bullet forward, but the bullet, in turn, pushes back on the rifle. You can see this in the way the gun recoils after a shot, and you can feel it in your shoulder. It may be hard to believe, but this also works for gravity. The earth is pulling you with a force due to your mass, but you are also gravitationally pulling back on the earth with the same force. The earth doesn’t really notice this however, because its mass is so huge that the earth’s acceleration caused by you is very small according to Newton’s second law.