The Questions
What is Newton’s 3 rd Law?

Newton’s 3 rd Law
 Newton’s 3 rd Law says that for every action force there must be an equal and opposite reaction force.
 The reaction force must be equal in strength and opposite in direction.
 There can never be a single force, alone!!
 Forces always occur in pairs.

Newton’s 3 rd Law
 Newton’s Third Law (action-reaction) applies when a force is pushed (or pulled) on any object, like a door.
 _______ exerted a force on the door.
And
 The door exerted a force on _______.

Newton’s 3 rd Law
 When figuring out action and reaction forces it is helpful to draw a picture.
The action force (thing that is doing something) is the
________________.
The reaction force (force that happens because of the thing that is doing something) is the
_______________.

Newton’s 3 rd Law
 http://science.discovery.com/inte ractives/literacy/newton/newton.
html

Solving Problems
 A woman with a weight of 500N is sitting on a chair.
 Describe one actionreaction pair of forces in this situation.
 Give the magnitude of each force and what object the force acts on.

Solving Problems
Looking for:
…pair of action-reaction forces
Given
…girl’s weight which is the force of…
Fc = 500 N
F
G
= -500 N (down)
Relationships:
Fw = -500 N
Action-reaction forces are equal and opposite and act on different objects.
Solution
Draw a free body diagram
The downward force of 500N exerted by the woman on the chair is the action force.
Therefore, the chair acting on the woman provides an upward force of 500 N and is the reaction.

Solving Problems
A car drives along the highway at constant
velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000N between the tires and the road and the normal force on the car is 12,000 N.
 Describe one action-reaction pair of forces in this situation.
 Give the magnitude of each force and what object the force acts on.

Solving Problems
Looking for:
Reaction force Pairs.
Given:
…Force
G
…Force
E
= 12,000N (up);
= 2,000N (forward)
Relationships:
Newton’s 3rd Law: For every action there is an equal and opposite reaction.

Solving Problems
Looking for:
Reaction force Pairs.
Given:
…Force
G
…Force
E
= 12,000N (up);
= 2,000N (forward)
Relationships:
Newton’s 3rd Law:
Car pushes down on road with 12000N Force.
Road pushes up on car with 12000N Force
Tires push on road with 2000N Force.
Road pushes on tires with a 2000N Force.

Solving Problems
A car has a mass of 1,000 kilograms. If a net force of 2,000 N is exerted on the car, what is its acceleration?
Looking for:
…car’s acceleration
Given
…mass = 1,000 kg; net force = 2,000 N
Relationships:
Newton’s Seconde Law: a = F / m
Solution:
2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s 2

The Answers
What is Newton’s 3 rd Law?
1. For every action force there is an equal and opposite reaction force that occurs on the other object.
2. Forces occur in pairs.

Solving Problems
A car drives along the highway at constant
velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000N between the tires and the road and the normal force on the car is
12,000 N.

Solving Problems
4.
Solution
 Draw a free body diagram.
 There is no net force upward, so the weight of the car is an equal downward force of
−12,000 N.
F
F
= -200 N
 The forward engine force balances the friction force so the friction force
is −2,000 N.
F
E
= 200 N

Newton’s 3 rd Law

Solving Problems
 Calculate the weight of a 60-kilogram person (in newtons) on Earth and on
Mars.
1.
Looking for:
 …weight of person in newtons on both planets
 Given:
 …mass = 60 kg; g = 3.7 N/kg on Mars;
 …implied g = 9.8 N/kg on Earth
2.
Relationships:
 W = m x g
3.
Solution:
 60 kg x 9.8 N/kg = 588 N
 60 kg x 3.7 N/kg = 222 N
Sig. fig. = 600 N
Sig. fig. = 200 N

Solving Problems
1.
Looking for:
 … the velocity of the astronaut (backward)
2.
Given
 …velocity
1
 ...mass
2
= 10 m/s; mass
= 100 kg;
1
= 2 kg;
3.
Relationships:
 m
1 v
1
= m
2 v
2
4.
Solution
 Draw a free body diagram.

The Answers
What is Newton’s 3 rd Law?
For every action there is an equal and opposite reaction.
Simple English:
Forces always occur in pairs and act on different objects.