# High School Physics : Understanding Newton's Third Law

## Example Questions

### Example Question #8 : Newton's Laws

If a tennis ball hits a racket with  of force, what is the force of the racket on the ball?

Explanation:

Newton's third law states that every force has an equal and opposite force.

That means that . Since the force of the ball equals , the force of the racket must be equal and opposite.

.

Remember, this force will be negative as it is equal and OPPOSITE. That means it is moving in the opposite direction.

### Example Question #9 : Newton's Laws

A bat hits a baseball with  of force. How much force does the baseball exert on the bat?

Explanation:

Newton's third law states that when object A exerts a force on object B, object B exerts a force equal in magnitude but opposite in direction on object A.

That means that if the force of the bat on the ball is , then the ball on the bat must be .

### Example Question #1 : Understanding Newton's Third Law

A hammer strikes a nail with  newtons of force. How much force does the nail exert on the hammer?

Explanation:

Newton's third law states that for every force, there is an equal and opposite force. The force of the hammer on the nail will be equal in magnitude, but opposite in direction, to the force of the nail on the hammer.

Mathematically, that would mean .

The nail would exert  force on the hammer.

### Example Question #2 : Understanding Newton's Third Law

Two skaters push off of each other in the middle of an ice rink. If one skater has a mass of  and an acceleration of , what is the mass of the other skater if her acceleration is ?

Explanation:

For this problem, we'll use Newton's third law, which states that for every force there will be another force equal in magnitude, but opposite in direction.

This means that the force of the first skater on the second will be equal in magnitude, but opposite in direction:

Use Newton's second law to expand this equation.

We are given the acceleration of each skater and the mass of the first. Using these values, we can solve for the mass of the second.

Notice that the acceleration of the second skater is negative. Since she is moving in the opposite direction of the first skater, one acceleration will be positive while the other will be negative as acceleration is a vector. From here, we need to isolate the mass of the second skater.

### Example Question #3 : Understanding Newton's Third Law

Two skaters push off of each other in the middle of an ice rink. If one skater has a mass of  and an acceleration of , what is the acceleration of the other skater if her mass is ?

Explanation:

For this problem, we'll use Newton's third law, which states that for every force there will be another force equal in magnitude, but opposite in direction.

This means that the force of the first skater on the second will be equal in magnitude, but opposite in direction:

Use Newton's second law to expand this equation.

We are given the mass of each skater and the acceleration of the first. Using these values, we can solve for the acceleration of the second.

From here, we need to isolate the acceleration of the second skater.

Notice that the acceleration of the second skater is negative. Since she is moving in the opposite direction of the first skater, one acceleration will be positive while the other will be negative as acceleration is a vector.

### Example Question #4 : Understanding Newton's Third Law

A rock strikes a window with  of force. How much force does that window exert on the rock?

We need to know the mass of the rock to solve

We need to know how long the two were in contact to solve

Explanation:

Newton's third law states that when one body exerts a force on another body, the second body exerts a force equal in magnitude, but opposite in direction, on the first body.

Mathematically, this process can be written as:

Since the rock exerts  of force on the window, then the window must exert of force on the rock.

### Example Question #1 : Understanding Newton's Third Law

A boy falls out of a tree and hits the ground with  of force. How much force does the ground exert on the boy?

We must know the mass of the boy to solve

Explanation:

Newton's third law states that when one object exerts a force on another object, that second object exerts a force of equal magnitude, but opposite in direction on the first.

That means that:

Using the value from the question, we can find the force of the ground on the boy.

### Example Question #6 : Understanding Newton's Third Law

A hunter fires a rifle. The rifle kicks back with  of force. How much force does the hunter's shoulder exert on the rifle during kick back?

We don't have enough information to solve.

Explanation:

If the rifle exerts a force of  on the hunter's shoulder, then the hunter's shoulder will exert  of force on the rifle.

This is because of Newton's third law, which states that when one object exerts a force upon another object, the second object exerts a force equal in magnitude but opposite in direction to the first force. Mathematically that means . Since our first force was , the second force will be .

### Example Question #7 : Understanding Newton's Third Law

Which of Newton's laws explains why it is easy for you to lift a 1L jug of milk from the fridge, but impossible to lift a 1000L jug?

Newton's second law

Newton's first law

More than one of these

Newton's third law

None of these