# Worksheet: Force Pairs and Newton's Third Law of Motion

In this worksheet, we will practice applying Newton's third law of motion to calculate the forces acting on pairs of mutually attracting or repelling objects.

Q1:

An artillery shell of 1,100.0 kg is fired from a battleship, and the shell is accelerated at m/s2.

What is the magnitude of the net external force exerted on the shell?

What is the magnitude of the force exerted on the ship by the shell’s acceleration?

Q2:

When starting a race, a sprinter of mass 55.0 kg exerts an average force of 635 N backward on the ground for 0.560 s.

What is the sprinter’s speed after the acceleration?

How far does the sprinter move during the acceleration?

Q3:

A truck collides with a car and, during the collision, the net force on each vehicle is equal to the force exerted by the other vehicle. The mass of the car is kg and the mass of the truck is kg. The magnitude of the truck’s acceleration is 8.63 m/s2. Find the magnitude of the car’s acceleration.

Q4:

Which of the following is true?

• AThe Moon orbits Earth because Earth exerts a force on the Moon and the Moon exerts a force equal in magnitude and opposite in direction on Earth.
• BA rocket taking off exerts a force on Earth equal in magnitude to the force Earth exerts on it.
• CEarth exerts a force due to gravity on your body and your body exerts a smaller force on Earth because your mass is smaller than the mass of Earth.
• DAn airplane cruising at a constant speed is not affected by gravity.

Q5:

Which force causes a boat to move forward when someone rows it?

• AThe frictional force between the oars and the water.
• BThe force of gravity acting on the oars.
• CThe frictional force between the boat and the water.
• DThe force of the oars pushing against the water.

Q6:

What is Newton’s third law of motion also known as?

• Athe law of force
• Bthe law of action-reaction
• Cthe law of gravitational attraction
• Dthe law of conservation of energy
• Ethe law of inertia

Q7:

A sports car of mass 550 kg collides with a truck of mass 2,200 kg, and, during the collision, the net force on each vehicle is the force exerted by the other. The magnitude of the truck’s acceleration is 10 m/s2. What is the magnitude of the sports car’s acceleration?

Q8:

A roller coaster car travels along a circular track, where the bottom of the track is on the ground and the top of the track is above the ground, as shown in the diagram. The wheels of the car stay in contact with the track even when the car is upside down. At the top of the loop, the car’s momentum makes its wheels push upward on the track with a force of 6,000 N. The car’s weight is 4,500 N. What force does the track apply to the car’s wheels when the car is at the top of the loop? 