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Newton’s Third Law of Motion: Action Reaction Pairs

Newtons Third Law of Motion: Action Reaction Pairs

  • The third law states that an applied force creates an equal and opposite force.
  • When you push a wall the wall pushes you with equal force but in the opposite direction.
Newton's Third Law: Action Reaction Pairs

Learning Targets

  • I know the main difference in Newton’s second and third law
  • I understand what an action reaction pair is
  • I can solve for the force, mass, or acceleration of two object that result from a force applied from one object to the other

Review of Newton’s First Two Laws

Q1: What was Newton’s first law?

Law of Inertia: An object in motion stays in motion and an object at rest stays at rest unless acted upon by an outside force

Q2: What was newton’s second law?

(Fnet = ma)

“Net force causes acceleration”

Q3: Using Newton's second law, what would the acceleration of this cart be?

Fnet = ma

Givens:

F = 20 N

m = 10 kg

a = ?

Equation and Work:

F = ma

a = F/m

a = 20/10

Answer:

a = 2 m/s²

Q4: Does the car or butterfly exert more force when the butterfly hits and squishes against your windshield?

Both the same force: Newton’s 3rd Law

Forces: Newtons Third Law Question and Answer

Newton’s 3rd Law of Motion  (Action-Reaction Pairs)

For every action there is an equal and opposite reaction

This means that Forces always occur in pairs and are:

  • equal in strength
  • opposite in direction

Equation:   F1 = -F2

  • F1 = Force of the first object on second
  • F2 = Force of the second object on first
Newton's Third Bug and Car

Q5:  What is the reaction force when a bat applies a 20 N force to a baseball?

The ball applies a 20 N force to the bat in the reverse direction

Q6: What is the reaction force when the earth pulls a person down by his weight of 800 N?

The person pulls the earth up by 800 N of force

 

Same force but different mass results in different accelerations

  • large mass means less acceleration
  • small mass means more acceleration
Acceleration Inversely Related to Mass

Major differences between Newton’s 2nd and 3rd Law

Newton’s 2nd Law of Motion (Fnet = ma)

Involves one object and the net force resulting from all the forces creating that one object to accelerate

  • Fnet = Sum of forces only on the box
Force Causes Acceleration

Newton's 3rd Law of Motion (Action-Reaction Pairs)

Involves two objects each with an equal and opposite force

    • F1 = Force of the first object on second
    • F2 = Force of the second object on first
Newton's Third Law

Q7: A 60 kg person pushes a 10 kg box with a force of 30 N to the right.  What is the force on the person?

Action Reaction Problem

30 N to the left

Q8: A 60 kg person pushes a 10 kg box with a force of 30 N to the right.  What is the acceleration of the box?

Answer

Q9: A 60 kg person pushes a 10 kg box with a force of 30 N to the right.  What is the acceleration of the person?

Answer 2

Newton's Third Law in equation form states

  • F1 = -F2

Newton's Second Law states net force causes acceleration on an object

Object 1

  • F1 = m1a1

Object 2

  • F2 = m2a2

With Substitution of F1 and F2 in Newton's 3rd Law Equation

  • m1a1 = -m2a2
  • The negative represents an opposite force causing an opposite acceleration
Newton's Third Law Equations

Q10: A 60 kg person pushes a 10 kg box that accelerates 25 m/s² to the right.  What is the acceleration of the person while force is applied?

Answer 10

Q11: 95 kg Joe jumps off a 0.8 kg skateboard and he accelerates at 1.5 m/s² right what is the acceleration of the skateboard?

Answer 11

Problem Set

1A. What is the weight of a 50 kg child, the force the Earth pulls a child down by? (use g = 10 m/s2)

FW = 500 N

1B. What would the 50 kg child pull the earth up by?

500 N, equal and opposite

(Use for #2A-C) 82 kg Tommy applies a force of 200 N on a 1,000 kg car while pushing it forward on a frictionless surface

2A. How much does the car accelerate?

a = 0.2 m/s2

2B. How much force does the Tommy feel from the car?

FTommy = 200 N Backward

2C. How much does Tommy accelerate?

a = 2.44 m/s2

3. A 76-kilogram person jumps off a 100 kg canoe accelerating the canoe at 5.0 m/s2 right. What is the acceleration of the person?

a1 = -6.58 m/s2 or 6.58 m/s2left

(Use for #4A-C) 67 kilogram Jody swats at 0.012 kg bug that landed on her applying 15N of force to squish it.

 4A.  As a result Jody feels how much force? (Choose one)

A) Less than 15 N of force

B) More than 15 N of force

C) Exactly 15 N of Force

C) Exactly 15 N of Force

Equal and opposite force

4B. What is the acceleration of the bug while the force is applied?

a = 1250 m/s2

4C. Would the magnitude of Jody’s acceleration be

A) More than the bug

B) Less than the bug

C) Exactly the same as the bug

B) Less than the bug

While force is equal and opposite, Jody has much more mass and would accelerate less

5A. When you push on a wall with 50 N of force the wall pushes you back with 50 N of force?

Newton’s 3rd Law (Action Reaction Pairs)

 

5B. When you hit cars breaks, your body leans forward.

Newton’s 1st Law (Inertia)

 

5C. Joe and Jill both push a 159 kg refrigerator with a force of 200 Newtons and the refrigerator accelerates at 2.52 m/s2 forward.

Newton’s 2nd Law (Fnet = ma)

 

Newton's Laws and The Third Law of Motion Quiz

1 / 8

Which of Newton's Laws is most related to a larger object resisting a change of motion more than a less massive one?

Newtons First Law: Law of Inertia.  An object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force. (More mass more inertia or resistance to change)

Newtons Second Law: Force causes acceleration (F=ma)

Newtons Third Law: All forces are paired, equal and opposite.  When you push on an object with 15 Newtons of force it pushes back on you with 15 Newtons of force.

2 / 8

Which of Newton's Laws is most related to all the forces on an object causing the object to accelerate?

Newtons First Law: Law of Inertia.  An object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force. (More mass more inertia or resistance to change)

Newtons Second Law: Force causes acceleration (F=ma)

Newtons Third Law: All forces are paired, equal and opposite.  When you push on an object with 15 Newtons of force it pushes back on you with 15 Newtons of force.

3 / 8

Which of Newtons Laws are best represented by this scenario.

If you push on a wall with 50 N of force, the wall pushed on you with 50 N of force

Newtons First Law: Law of Inertia.  An object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force. (More mass more inertia or resistance to change)

Newtons Second Law: Force causes acceleration (F=ma)

Newtons Third Law: All forces are paired, equal and opposite.  When you push on an object with 15 Newtons of force it pushes back on you with 15 Newtons of force.

4 / 8

When you are on earth, which of the following is true?

Newtons First Law: Law of Inertia.  An object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force. (More mass more inertia or resistance to change)

Newtons Second Law: Force causes acceleration (F=ma)

Newtons Third Law: All forces are paired, equal and opposite.  When you push on an object with 15 Newtons of force it pushes back on you with 15 Newtons of force.

5 / 8

When hitting a baseball you apply 150 N of force with the bat on the ball.  How does the magnitude of force the ball applies to the bat compare?

Newtons Third Law: All forces are paired, equal and opposite.  When hit a baseball with 15 Newtons of force forward it hits you with 15 Newtons of force backwards.  Magnitude means ignore the direction and just look at the number and unit.

6 / 8

95kg Joe jumps off a 0.8kg skateboard and accelerates at 1.5 m/s/s right what is the acceleration of the skateboard?

ma = -ma

F = ma

a = F/m

Forces on both are equal but if mass is less (its inversely related to acceleration) the acceleration caused by an equal force is more.

7 / 8

When a more massive skater jumps of a less massive skateboard. Which is true of the scenario?

8 / 8

When a bug hits your windshield.  Which of the following are true?

Newtons Third Law: All forces are paired, equal and opposite.  When you push on an object with 15 Newtons of force it pushes back on you with 15 Newtons of force.  The forces are equal.

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Unit 1: One Dimensional Motion
Unit 2: 2D Motion
Unit 3: Newton’s Laws and Force
Unit 4: Universal Gravitation and Circular Motion
Unit 5: Work, Power, Mechanical Advantage, and Simple Machines
Unit 6: Momentum, Impulse, and Conservation of Momentum
Unit 7: Electrostatics
Unit 8: Current and Circuits
Unit 9: Magnetism and Electromagnetism
Unit 10: Intro to Waves
Unit 11: Electromagnetic Waves
Unit 12: Nuclear Physics

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