Forces and Interactions
Force and Newton's three laws
Add forces to find the net force, use F = ma and F = mg, say what Newton's three laws claim, and treat two connected objects as one system.
A force has a size and a direction, and it is measured in newtons — a unit named after Newton, so its symbol is a capital N even though the word itself is lowercase. Hardly anything in the world has just one force on it. A pushed box also has friction on it; a floating ball has gravity pulling down and buoyancy pushing up. What decides the motion is the net force, which is what is left once the forces have been added up.
Three laws in one paragraph
First law: with no net force, a resting object stays at rest and a moving one keeps a constant velocity. Second law: with a net force, the object accelerates along it, more for a bigger force and less for a bigger mass — F = ma. Third law: whenever one object pushes a second, the second pushes back just as hard the other way. That third pair acts on two different objects, which is why the two do not cancel.
Have a play
Two everyday situations, and the forces acting in each.
Tap any part of the picture.
Worked example
A block on a table has two horizontal forces on it, 10 N and 6 N. Work out the net force when the two point the same way, and when they point opposite ways.
Same way, say both to the right: add them. 10 N + 6 N = 16 N to the right.
Forces pointing the same way reinforce, so the strength of the pair is the total of the two.
Try it together
Now push a block that friction is holding back.
A 25 kg block sits on a floor whose friction opposes motion with 50 N. A student pushes with 55 N. Answer with a bare number each time.
1.First the net force. What is 55 N of push against 50 N of friction, in N?
Have a go
Have a go on your own. What is the gravitational force the Earth exerts on a 20 kg suitcase, in N? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.