Lesson 10.4.2: Newton's 2nd and 3rd Law of Motion

Lesson 10.4.2: Newton's 2nd & 3rd Law of Motion

Multiple Choice

Review: Which of the following equations is the equation for Acceleration?

• $$a=\frac{V_{final}-V_{initial}}{Time}$$  

• $$a=V\cdot time$$  

• $$a=\frac{dis\tan ce}{time}$$  

• $$a=\frac{Speed\cdot Velocity}{Time}$$  

Fill in the Blanks

A car is driving down the highway going 50 kilometers/hour. The road clears so the driver changes their speed to 75 km/hr. The time this took was 7 seconds. What is the acceleration? (Round to two Decimal Places)

Multiple Choice

The greater the mass of an object,

• the easier the object starts moving.

• the greater its inertia.

• the more balanced it is.

• the more space it takes up.

Multiple Choice

All objects have inertia

• True

• False

Multiple Choice

The force of gravity on a person or object on the surface of a planet is called

• mass.

• weight

• Terminal Velocity

• free fall.

Multiple Choice

The law of universal gravitation states that any two objects in the universe that have mass, without exception,

• attract each other

• repel each other.

• combine to provide a balanced force.

• create friction.

Lesson Objectives

• Describe Newton's 2nd Law of Motion

  • Calculate Acceleration when given a Force and a mass

• Describe Newton's 3rd Law of Motion

Recall: Acceleration

• Acceleration is the change in velocity over time

  • Involves changes in speed as well as changes in direction​

• Acceleration occurs in both horizontal movement as well as vertical movement

Mass and Acceleration are Related

• More massive Objects are more difficult to accelerate

  • Experiments show that doubling the mass of an object halves the acceleration, tripling thirds it, etc.

  • Acceleration and mass are inversely related​

Newton's Second Law of Motion

• ​When an Unbalanced Force is applied to a mass, it causes it to accelerate

  • The acceleration of an object is directly proportional to the net force on the object and is inversely proportional to the mass of the object

    • ​a = f/m

    • f=m*a​

Fill in the Blanks

Review: While biking you are traveling at 35 km/hr to 40 km/hr over the course of 1 second. What is your acceleration?

Fill in the Blanks

Review: If you apply a net force of 5 N on a cart with a mass of 5kg, what is the acceleration of the cart?

A Force is Part of an Interaction

• Force is not a trait of an object, it is something an object does

• Force is more than just a single push or a pull and is an interaction between two objects

  • Both the object exerting the force and the object receiving the force is part of the interaction

• Example: If a truck collides with a car, the truck exerts a force on the car and the car exerts a force on the truck at the same time

  • These forces are equal in strength and occur in opposite directions

Newton's 3rd Law: The Law of Action & Reaction

• Whenever one object exerts a force on an second object, the second object exerts an equal and opposite force on the first

  • To every action, there is an equal and opposite reaction

  • One force is the action force, the other force is the reaction force

Identifying Action and Reaction

• First identify the interaction

  • Example: Object A interacts with Object B

    • ​Action: Object A exerts a force on Object B

    • Reaction: Object B exerts a force on Object A

  • If the action is A on B the reaction is B on A

Multiple Choice

Think about it: When a heavy football player and a light one run into one another, does the light player exert as much force on the heavy player as the heavy player exerts on the light player?

• Yes

• No

• Depends on the situation

Multiple Choice

Think about it: Would the damage to the heavy player be the same as the damage to the lighter player

• Yes

• No

Action & Reaction on Objects of Different Masses

• Recall Newton's Second Law: a= F/M

• Newton's Third law​ states that every action has an equal and opposite reaction, so why do objects respond differently?

  • Because they have Different Masses

  • A given Force Exerted on a small mass produces a large acceleration, while the same force exerted on a large mass produces a small acceleration

Action & Reaction on Objects of Different Masses

• ​If Forces are equal and Opposite, why don't they cancel out to zero?

  • Because they act on different bodies

    • When you kick a ball, an action force acts on the ball, the reaction force is on the foot​

Defining a System

• A System is any object or collection of objects that you are studying​

  • Can be adjusted to focus on different parts​

Summary of Newton's Laws

• Newton's First law: an object in motion stays in motion, an object at rest stays at rest

• Newton's Second Law: a=F/m

• Newton's Third Law: every action has an equal and opposite reaction​

Multiple Choice

When you push a marble with a 0.5 N force, what does the marble do?

• accelerate at 10 m/s²

• Resists being pushed

• Stay at rest

• Pushes on you with a 0.5 N Force

Multiple Choice

A karate chop delivers a force of 3000 N to a board that breaks. The force that the board exerts on the hand during this event is

• less than 3000 N

• 3000 N

• Greater than 3000 N

• Cannot tell from the given information

Multiple Choice

When you throw a basketball, your force on the ball accelerates it. The ball pushes back on you with an equal and opposite force. Why don't you accelerate as much as the ball?

• The reaction force is only on your hand, not the rest of you

• Friction stops you from acceleration

• Your acceleration is much smaller than the ball's because of your larger mass

Multiple Select

Review: Which of the following equations is the equation for Acceleration?

• $$a=\frac{V_{final}-V_{initial}}{Time}$$  

• $$a=V\cdot time$$  

• $$a=\frac{Force}{Mass}$$  

• $$a=\frac{Speed\cdot Velocity}{Time}$$