Third Law of Motion

Newton's Third Law

Objectives:

- 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.

The first 2 laws of motion apply to individual objects.

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.

The third law of motion deals with pairs of objects.

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.

¹Equal in Strength

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.

¹Equal in Strength

Include this diagram in your notes.

²Opposite in direction

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.

²Opposite in direction

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.

Collisions

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.

Collisions

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

Open Ended

Why do you think the result of the force pairs in the previous example was different between the boy and the girl?

THE ANSWER IS INERTIA!

• 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.

Which truck would you rather be riding in?

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.

Real life example

A good way to explain Newton's Third Law is baseball. When a batter hits a ball, the ball exerts the same amount of force on the bat, but in the opposite direction. The only reason that the ball goes farther is that the ball is a lot less massive.

(remember f=ma)

Complete now

1. Outline the definition of newtons third law.

• For every force applied to an object, there is another force applied by that object, equal in magnitude and opposite in direction

• For every force there is a force of the same size acting in the opposite direction.

2. Create a table with 4 columns.

3. Research how 3 of newtons laws exist in your favourite sport (do not copy and paste I will know)

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.