Newton’s three laws of motion do an excellent job of describing the world we experience daily. Many people learn to quote the laws in school, but never really grasp what they mean. If you find yourself in that position, here’s your chance to come to grips with that classical trio.
The first law tells us that if no net force act on an object, it will continue moving at the same speed and in the same direction. (Speed and direction together are known as velocity.) To say it slightly differently, neither speed nor direction of motion can change unless a net force acts on the object of interest. This applies to an object at rest (which is a speed of zero), an object moving slowly, or even something faster than a speeding bullet.
In our daily experiences, things left alone rarely seem to behave this way. A pen left laying on a slanted desk starts to roll, your bicycle slows to a stop if you stop pedaling and coast, and you can think of many other examples as well. In each case, however, the reason for the change in movement is one or more forces that you don’t necessarily think about at all times. Most people will realize that gravity is the force that starts the pen rolling. In the case of the bicycle, friction and air exert forces that slow it down. Indeed, in our universe, to be truly free of forces is rare, but they can be balanced out.
Only the Net Force (the total of all forces acting on an object) matters when determining changes in motion. It doesn’t matter if fifteen different things are pushing and pulling on an object. If they all balance out to a total of zero, that object isn’t going to change its behavior one bit. Imagine that your car breaks downs and four strong men come along to help push it to the shop. You could have two push it forwards and two push it backwards, and you’d find that the car stayed put. That’s because the forces they exerted balanced out, so the motion of the car didn’t change. If three pushed forwards and one pushed back, you’d see a net force forward, and the car would begin to accelerate (change its velocity) forward. If all four pushed forward, it would accelerate more rapidly, because the Net Force forward was greater.
People sometimes grow confused over the term “accelerate” because they think of the “accelerator” pedal in their car. You know that to push the pedal tells your car to move forward. When the car is speeding up, it is indeed accelerating, but when you get to the speed you want, you still have to hold that pedal down, or you slow down (a negative acceleration). The pedal doesn’t govern acceleration at all, but instead, the Force that your engine exerts on the car. Since your engine still exerts a force when you are moving at a constant speed (55 miles per hour, perhaps), that means that there is another force (or forces) acting to balance that force. In the case of your car, those are friction (from the ground and car parts) and air resistance. Again, because you don’t see those forces directly, it is hard to keep them in mind, but now you know. This is why car engineers work to keep cars aerodynamic – to reduce air resistance (the force air exerts against your car) and also to minimize friction in your car. By lessening these forces, your engine does not have to exert as much force to keep moving at a constant speed, and you use less gas, and save money too!
This is the math law. We just discussed that a net force causes acceleration. This law just sets it to math. It says:
(Net Force) = (mass of object) X (acceleration of object)
This is the easy one for most people. You usually just see it as F=ma, and sometimes that can mislead because then people think that any force will cause an acceleration, having forgotten that it is the Net Force that results in acceleration. It is a very good thing that this is the case too, otherwise every time you leaned against a wall (exerting a force on it) it would begin to move away from you, and we’d be knocking buildings down left and right. Other forces work to balance out the force you put on the wall, keeping the net force at zero, and it stays where it belongs. That is also why you need a large force (say, from a wrecking ball) to take out a wall – you’ve got to have a force big enough to make the net force more than zero.
People usually remember this one as “For every action there is an equal and opposite reaction.”
Sadly, that’s a bit misleading. In reality, this law tells us that forces always come in pairs. If you exert a force on a wall by pushing on it, it also exerts that same force back on you. It doesn’t cause you to accelerate, since your feet are planted on the ground, and friction keeps that force balanced, and unless it’s a poorly built wall, it doesn’t go anywhere either.
A slightly more animated example would be to drop a Superball to the ground. At first, there is the force of gravity at work. Gravity is an attractive force between all objects. The Earth exerts exactly the same force on the Superball that the Superball exerts on the Earth. (Now this is where someone usually objects – “NO WAY, the Earth doesn’t come rushing up to hit the Superball!”) This does not mean that the Earth comes rushing to meet the Superball. Remember that in Law #2, F=ma. The force exerted is equal, but the mass of the Earth is millions of times greater that that of the Superball. As a result, the acceleration of the Earth is much smaller than that of the Superball – too small to be noticeable during the short time it takes the ball to strike the ground. When the Superball and the Earth collide, they exert an equal force against one another, and bounce apart. Again, the acceleration is much greater for the Superball, so it speeds away while the Earth scoots the barest fraction of a micrometer.
Always remember, Law #3 only says that forces come in equal pairs. It says nothing about results being equal.