Newton's Third Law

- I can describe Newton's 3rd Law of motion.

- I can name the force pairs acting on an object.

- I understand how inertia is important in collisions.

1st Law - an object at rest will stay at rest, an object in motion will stay in motion unless there's an outside force.

2nd Law - an object's acceleration is directly proportional to the force, and inversely proportional to the mass.

This is because all forces come in pairs.

Newton's 3rd Law of Motion - for every action force, there is a force that is ¹equal in strength and ²opposite in direction.

This image shows the force pairs between the force on the cannon and the force on the ball. Newton's 3rd Law says that they must be equal, which is why the canon is pushed backwards.

Include this diagram in your notes.

This image shows the force pairs between the force on the chair and the force on the fire extinguisher. The resulting action is for them to travel in a direction opposite of the extinguisher force.

This image shows the force pairs between the force on the ground from your foot, and the force on you from the ground.

The forces that combine to propel you forward are equal and opposite.

Newton's 3rd Law says that any time 2 objects hit each other, they exert equal and opposite forces on each other. However, the effect of the force is not always the same.

The two people in this example may have exerted the same force on one another, but the result was not the same. Why?

• The person with more mass has more inertia, so more force is needed to change his motion.

• So even though the forces are equal and opposite, there are two very different results based on Newton's 1st Law of Motion, inertia.

The forces that act on each truck may be the exact same, but due to the yellow trucks greater mass and therefore inertia, the results are very different.