8.3 Newton’s laws of motion

Loose change experiment, p.269

First law of motion: a body remains at rest or in motion with a constant velocity unless acted upon by an external force.

First law of motion examples The net force is the total force. It could be the sum of two forces or more than two forces.

Inertia

Inertia is the resistance of any physical object to a change in its state of motion or rest,

Second law of motion When several forces act on an object, they may cancel one another out to leave no net force. Only a net (or unbalanced) force can accelerate an object.

Second law of motion

Third law of motion For every action there is an equal and opposite re- action.

Principle of a Launch Vehicle Thrust to propel a rocket is based on Isaac Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. The principle of a rocket motor may be understood by considering the example of a closed container filled with a compressed gas. Within this container the gas exerts equal pressure on every point of its walls. If a hole is punched in the bottom of the container, however, the gas at the bottom escapes and the pressure against the top of the container is no longer equalized. The internal gas pressure then pushes the container upwards in reaction to the jet of air escaping downwards. The amount of thrust developed by a rocket motor depends mainly on two factors, the velocity with which the burning gases leave the combustion chamber, and the mass of the burning gases.

ACTIVITIES

Action: the tires on a car push on the road… Reaction: the road pushes on the tires. Action: while swimming, you push the water backwards... Reaction: the water pushes you forward. Action: a rocket pushes out exhaust… Reaction: the exhaust pushes the rocket forward. One of the original arguments that flight in the vacuum of space was impossible was that there would be nothing to push against. This action- reaction explains how a rocket can fly in space where there is no air to push against. Action: the earth pulls down on a ball… Reaction: ball pulls up on the earth.

If I push on a lawn mower, it pushes back on me with an equal, but opposite force. Explain why we don’t both just stay still. The answer is that these forces are acting on different bodies (and there are other forces to consider). It doesn’t matter to the lawn mower that there is a force on me… all that matters to the lawn mower is that there is a force on it, so it starts to move! Another action-reaction pair you need to consider is that I am pushing backwards on the ground, and it pushes forwards on me.