What is Newton’s 3rd Law?

The Questions

Newton’s 3rd 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 3rd 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.AndThe door exerted a force on _______.

Newton’s 3rd 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 3rd Law

http://science.discovery.com/interactives/literacy/newton/newton.html

Newton’s 3rd Law

Solving ProblemsA woman with a weight of 500N is sitting on a chair.

Describe one action-reaction pair of forces in this situation.

Give the magnitude of each force and what object the force acts on.

Solving ProblemsLooking for:

…pair of action-reaction forces

Given…girl’s weight which is the force of…FG = -500 N (down)

Relationships:Action-reaction forces are equal and opposite and act on different objects.

SolutionDraw a free body diagramThe 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.

Fw = -500 N

Fc = 500 N

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: …ForceG = 12,000N (up);

…ForceE = 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: …ForceG = 12,000N (up);

…ForceE = 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 accelerationGiven

…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/s2

What is Newton’s 3rd 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.

The Answers

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.

FE = 200 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.

The forward engine force balances the friction force so the friction force is −2,000 N.

FF = -200 N

FW

= -

12,0

00N

FN

= 1

2,0

00N

Newton’s 3rd 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 NSig. fig. = 200 N

Solving Problems

1. Looking for: … the velocity of the astronaut (backward)

2. Given …velocity1 = 10 m/s; mass1= 2 kg;

...mass2 = 100 kg;

3. Relationships: m1v1 = m2v2

4. Solution Draw a free body diagram.

What is Newton’s 3rd Law?

For every action there is an equal and opposite reaction.

Simple English:

Forces always occur in pairs and act on different objects.

The Answers