kinetic/potential | The Happy Scientist

This week we will investigate Newton's laws of motion. While Galileo laid the foundations for them, Newton was the one that put them into the form that we know them today. You will need:

- a bathroom scale
- an object that weighs at least a few pounds

Start by placing the scales on a flat, hard floor. Step onto the scale and look at your weight. Aaarrrrrggghhhh! OK, so I have done a few too many experiments involving cookies and ice cream. Now pick up the object you selected. Notice the new reading on the scales. It should be a bit higher for the total of you and the object. So far, everything is just as you would expect.

Now we will change things a bit. As you watch the reading on the scale, quickly lift the weight up over your head. Do this as quickly as you can. Did the reading on the scale change? Yes, it should have gone several pounds higher and then quickly back down. Next, bring the weight back down as fast as you can, again watching the scale. This time, the reading goes down by several pounds and then pops back up.

Why does this happen? We can explain it all with Newton's three laws of motion. They are:

1. A force is required to set an object in motion. It will continue to move in a straight line in a constant velocity unless another force act on it. Newton's first law is often called as the law of inertia. Inertia is the tendency to stay at its state of rest or motion.

2. The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force.

3. For every action (or force) there is a reaction (or opposing force) of equal but opposite direction.

They may sound complex, but it is really not that difficult to understand them once we start applying them to our experiment.

When we start, neither you nor the weight you are holding is moving on the scale. Newton's first law of motion tells that it will require some force to make a change in the objects movement. It takes force to start it moving. It takes force to stop it from moving. You are providing the force when you move your arm up or down.

Newton's second law tells us that the more mass the object has, the more force it takes to make it move or change its motion once it is moving. The heavier your object is, the more the reading on the scale will change. Notice I said mass, not weight. Here on Earth, weight and mass mean pretty much the same thing, but while weight changes with gravity, an object's mass remains the same.

Newton's third law tells us that when you push upwards on the object, you are pushed downwards with the same amount of force. That downwards force on your body is what changes the reading on the scale. When you pull downwards on the object, you are pulled upwards, again with the same amount of force. Again, this changes the reading on the scale.

These three simple laws describe the motion of everything from a baseball to the space shuttle. Too bad they won't negate the impact of all that ice cream.