Newton's Third Law of Motion: Action Reaction Pairs

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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?

(F_net = ma)

“Net force causes acceleration”

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

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 3^rd 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

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

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

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

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

F_net= Sum of forces only on the box

Newton's 3^rd 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

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?

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?

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?

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 F₁and F₂in Newton's 3rd Law Equation

m₁a₁= -m₂a₂
The negative represents an opposite force causing an opposite acceleration

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?

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?

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)

F_W = 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?

F_Tommy = 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 1^st 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 2^nd Law (F_net = ma)