Questions on Newton's Third Law

Newton's Third Law explains how things move.

Question

A balloon is taped to a straw. The straw is free to move along a length of tight thread. The balloon is held at the neck so that air does not escape from the balloon.

A balloon full of air, attached to a straw which has a thread through it. A hand is holding the untied end of the balloon.

The balloon is then released and it accelerates along the thread. What makes the balloon move?

A balloon attached to a straw, moving along a thread. Arrows show the balloon pushing on air and the air pushing on the balloon, resulting in its motion.

The balloon pushes on the air inside it. The air is pushed out of the neck of the balloon. We know from Newton's Third Law that as air is being pushed out by the balloon and the air is pushing back on the balloon. There is now an unbalanced force acting on the balloon and so it accelerates along the thread.

Rockets

The movement of the balloon along the thread is similar to the motion of a rocket at lift off. At lift-off, hot burning gas is pushed downwards by the rocket motors. The hot gas pushes back on the rocket in an upwards direction. When this upwards push, or thrust, exceeds the weight of the rocket, the forces acting on the rocket are unbalanced and the rocket accelerates upwards.

Rocket at launch, with 'hot burning exhaust gases' labelled.

The Space Shuttle uses rocket propulsion.

Use your knowledge of Newton's Third Law to solve the following problem.

Question

You are standing in the middle of a frozen pond. Imagine that the ice on the pond is very slippy and completely frictionless. Because there is no friction, running, walking and crawling are impossible. The ice is also very hard so you cannot dig into it to get a grip. How can you get off the pond?

You could do it by blowing! When you blow air from your mouth you apply a force to the air. By Newton's Third Law, the air that is pushed from your mouth pushes back on you. The air pushing on your mouth would cause you to start sliding backwards and you would eventually reach the edge of the pond.

Summary of Newton's three laws

Newton's three Laws of Motion