Science · Force and motion

Balanced and unbalanced forces

In a tug of war, why does the rope stay still while both teams pull as hard as they can? Learn to calculate the resultant force, and find out that balanced forces don't change motion while unbalanced forces change speed or direction.

18 min
2 simulations
Mini quiz

Updated: 22 October 2026

Preview

The question

In a tug of war, both teams are pulling as hard as they can, but for a while the rope doesn't move at all. Then suddenly one team gets the upper hand and the rope starts to slide their way. Forces were acting on the rope the whole time, so why didn't it move at first? And is there really no force at all acting on a book lying on a table?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Number line

Resultant force in a tug of war

Move the marker in steps of 10 to the correct value and submit your answer.

Predict first

If the Blue team pulls harder than the Yellow team, what do you think the rope will do?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Projectile motion

Land the ball on the target

Choose the launch speed with the slider and press the Launch button. Watch the path the ball follows and where it lands. When you find the speed that lands the ball on the target, submit your answer.

Predict first

Why does a launched ball follow a curved path through the air instead of a straight line?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Frequently asked

4 questions

If no force acts on an object, does it stop?

No. If the net force is zero, an object at rest stays at rest and a moving object keeps going at the same speed in the same direction. A ball you roll stops not because the force "runs out" but because friction and air resistance slow it down.

Is there really no force acting on a book lying on a table?

There is. Earth pulls the book down, and the table pushes it up with a force of the same size. Because the net force is zero, the book stays where it is. "No forces" and "balanced forces" are not the same thing.

I am pushing a heavy wardrobe but it won't move; is my force wasted?

If the wardrobe does not move, your push is balanced by the friction between the floor and the wardrobe, and the net force is zero. When you push harder, or a friend helps you push in the same direction, the forces become unbalanced and the wardrobe starts to move.

Do unbalanced forces always make an object speed up?

No. Unbalanced forces change an object's motion: if the net force points the same way as the motion, the object speeds up; if it points the opposite way, it slows down; if it points sideways, the object changes direction. A car that is braking is also under unbalanced forces.