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More about forces

More about forces

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What is the name of the force that the floor exerts on a table, so that the table won't fall through the floor?

More about forces

​​Force is a word we use in our everyday language. There it means roughly the same thing as energy, effort, or power. But in physics, the term force has a very specific meaning, different from words like energy and power. In physics, a force is something that pushes or pulls. You can't see a force, but sometimes you can feel it.

It's something going on between things, the interaction between objects. Forces have a tendency to change the motion or shape of objects. That doesn't mean the force always succeeds in doing so. But it does have a tendency, an intention, a direction to change the motion or shape of objects. A force, that's what pushes or pulls. ​ ​Here's a box, standing on the floor.

Gravity pulls the box straight down with a force of 500 Newtons. The downward force arrow illustrates the force of gravity, but if there is a downward force then why doesn't the box go down through the floor? Well, the floor pushes back on the box. If you lift the box up a bit and place your hand between the box and the floor, then you'll feel this force. The box pushes your hand from above, but the floor pushes your hand just as much from below.

That's the normal force you feel from the floor. Normal, in this case means that it is perpendicular to the floor. It's the normal force that keeps the box from going through the floor. As long as the box stands on the floor, the gravitational force and the normal force cancel each other out. But if you're to lift the box from the floor, you have no help from the normal force anymore.

Then you have to use your strength and pull with a force at more than 500 Newtons straight up. This upward force arrow shows you how to pull on the box. But your upward force is too weak so nothing happens. A friend comes to help you and there are two upward forces. Two force arrows pointing in the same direction can be added like this.

The combined upward force is now greater than the downward force of gravity. Take the length of the upward arrow minus the length of the gravity arrow and you get the value of the combined force, the net force that affects the box at this moment. The upward force is greater than the gravitational force, so the net force is directed upwards. You are lifting the box. If the force arrows are pointing in the same direction you can add them together.

The net force is a longer arrow with the same direction. If the force arrows point in opposite directions the net force is the difference of their lengths pointing in the direction of the strongest force. ​ ​But what happens with force arrows that point in different directions? Here is a large ship being towed by two tugboats. The Tugboats pull equally hard but in different directions. Their forces can be shown as arrows like this.

Then what is the net force from these two tugboats? Well, it looks like this. You can add force arrows by placing them one after another like this. It doesn't matter in which order you place them, the result is the same. The large ship is pulled with a force directed straight forward even though none of the individual tugboats pull in that direction.

When we lift up a box to tow a ship or push a glass across the surface of a table we have contact with the object that we expose to the force. Contact forces are easy to think of since we use them all the time when moving things or ourselves. But there are also forces acting between objects that are not in contact with each other. Gravity is a force that pulls everything that comes near without having to touch it. Magnets and electric charges also give rise to forces that influence objects without touching them.

Non-contact forces.