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Newton’s 2 nd and 3 rd Laws. Objectives Describe the acceleration of an object in terms of its mass and the net external force acting on it. Predict the direction and magnitude of the acceleration caused by a known net external force . Identify action-reaction pairs.

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newton s 2 nd and 3 rd laws
Newton’s 2nd and 3rd Laws


  • Describe the acceleration of an object in terms of its mass and the net external force acting on it.
  • Predict the direction and magnitude of the acceleration caused by a known net external force.
  • Identify action-reaction pairs.
  • Explain why action-reaction pairs do not result in equilibrium.
net force
Net Force

If there is NONET FORCE on an object, then it is at EQUILIBRIUM and either:


So a “net” or “unbalanced” force will


Changing velocity means ACCELERATION

force acceleration
Force  Acceleration

How much acceleration?

Depends on:



Acceleration is directly related to force



Acceleration is inversely related to mass

newton s second law
Newton’s Second Law

“The acceleration of an object is directly proportional to

the net external force acting on the object and inversely

proportional to the mass of the object.”

Unit of force is theNEWTON (N)







If mass is held constant,

    • doubling of the net force results in …
      • a doubling of the acceleration,
    • halving of the net force results in …
      • a halving of the acceleration.
  • If force is held constant,
    • doubling of the mass results in …
      • a halving of the acceleration
    • halving of the mass results in …
      • a doubling of the acceleration.

A 2 kilogram box is pushed with a net, unbalanced force of 10 newtons.

What is the acceleration experienced by the box?

a = Fnet / m

a = (10 N) / (2 kg)

a = 5 m/s2

the big misconception
The Big Misconception
  • The most common misconception is one that dates back for ages; it is the idea that sustaining motion requires a continued force.
  • Newton's laws declare loudly that a net force (an unbalanced force) causes an acceleration;
are you infected with the misconception
Are You Infected with the Misconception?
  • Two students discussing an object that is being acted upon by two individual forces as shown. During the discussion, Anna Litical suggests to Noah Formula that the object under discussion could be moving.
  • Noah Formula objects, arguing that the object could not have any horizontal motion if there are only vertical forces acting upon it.
  • Who do you agree with?
class work
Class work
  • Read page 137 Sample Problem 4B
  • Page 138 – practice 4B
  • Page 138 – conceptual challenge
newton s third law
Newton's Third Law
  • For every action, there is an equal and opposite reaction.
  • Forces always come in pairs - equal and opposite action-reaction force pairs.
  • Examples:
    • The propulsion of a fish through the water.
    • The flying motion of birds.
    • The motion of a car on the way to school.
third law examples
Third Law Examples

A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose?

A man getting out of a rowboat jumps north onto the dock. What happens to the boat?

There will be a force on the hose to the WEST

The boat will move to the SOUTH

identifying action and reaction force pairs
Identifying Action and Reaction Force Pairs
  • Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements describing who is pushing on whom and in what direction.

Action: air pushing the balloon out.

Reaction: balloon pushing air in.

Action: bat hit the ball

Reaction: ball hit the bat

Action: ball hit the mitt

Reaction: mitt hit the ball

action reaction forces vs equilibrium forces

Force on the car


Force on the ground


Action/reaction forces vs. equilibrium forces
  • Action and reaction forces don’t cancel out because they act on different bodies.
  • Equilibrium forces act on samebody.
check your understanding
Check Your Understanding

1. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

2. For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ...
  • ... space is void of air so the rockets have nothing to push off of.
  • ... gravity is absent in space.

c. ... space is void of air and so there is no air resistance in space.

d. ... nonsense! Rockets do accelerate in space and have been able to do so for a long time.

3. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...
  • greater than the acceleration of the bullet.
  • smaller than the acceleration of the bullet.

c. the same size as the acceleration of the bullet.

class work1
Class work
  • Page 140 – section review
  • 4-3 section review