Unit 2 Lesson 4: Newton's Laws of Motion

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Newton's Laws of Motion

Multiple Choice

Which measurement refers to how fast an object is moving.

• Speed

• Velocity

• Acceleration

• Motion

Multiple Choice

Which is an example of VELOCITY?

• A dog running at 3 m/s

• A deer sprinting at 4 m/s

• A migrating bird headed south for the winter at 3 m/s

• A person not moving at all

Fill in the Blanks

Multiple Choice

Which two factors does speed depend on?

• distance and direction

• direction and time

• distance and time

• distance, time and direction

Lesson Objectives

• Define Newton's First Law of Motion and connect it to inertia

• Define Newton's Second Law of Motion

  • Relate acceleration, force and mass to each other

  • Describe how Gravity and free fall are related

• Define Newton's Third law of Motion

  • Identify Action & Reaction Forces

Newton's First Law of Motion

• Galileo's studies on inertia helped lay the foundation for Isaac Newton

• Newton built off of Galileo's concept of inertia to develop his laws of motion

  • Published in his book Philosphiae Naturalis Principia Mathematica

• Newtons First law of motion states: Every object continues in its state of rest, or a uniform speed in a straight line, unless acted on by a nonzero force

  • An Object will continue to do what it is already doing unless a force acts on it

  • Also called the law of Inertia

Newton's Second Law of Motion

• Newton used Galileo's observations of acceleration, force, and mass to explain a new relationship

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

Visualizing Newton's Second Law of Motion

Newton's 2nd Law of Motion Sample Problem 1

• A five kg bag of sand has a weight of 50 N. What is its acceleration in m/s² when dropped?

  • Step 1: Identify your Variables

    • F=

    • m=

    • a=

  • Step 2: Substitute your variables in and solve

Newton's 2nd Law of Motion Sample Problem 1

• A five kg bag of sand has a weight of 50 N. What is its acceleration in m/s² when dropped?

  • Step 1: Identify your Variables

    • F= 50

    • m=5

    • a=?

  • Step 2: Substitute your variables in and solve

Newton's 2nd Law of Motion Sample Problem 2

• What is the acceleration of the 15 kg box that has 500 N of force applied to it?

  • Step 1: Identify your Variables

    • F=

    • m=

    • a=

  • Step 2: Substitute your variables in and solve

Newton's 2nd Law of Motion Sample Problem 2

• What is the acceleration of the 15 kg box that has 500 N of force applied to it?

  • Step 1: Identify your Variables

    • F=500

    • m=15

    • a=?

  • Step 2: Substitute your variables in and solve

Multiple Choice

You have three grocery carts, one with 1 kg of clothes, one with 2 kg of food, and 3 kg of tools. You push on all of them with a force of 1 N. Which one will have the largest acceleration?

• The clothes

• The Food

• The Tools

• They will all have the same acceleration

Multiple Choice

You have three grocery carts, one with 1 kg of clothes, one with 2 kg of food, and 3 kg of tools. You push on all of them with a force of 1 N. Which one will have the smallest acceleration?

• The clothes

• The Food

• The Tools

• They will all have the same acceleration

Multiple Choice

You have three grocery carts, one with 1 kg of clothes, one with 2 kg of food, and 3 kg of tools. You push on all of them with a force of 1 N. How much bigger will the acceleration on the clothes be compared to the food?

• Twice as much

• Three times as much

• 1.5 Times as much

• Four times as much

Free Fall

• Newton's second law explains why falling objects all fall at the same rate

• Free Fall: When the force of gravity is the only force acting on an object and the object accelerates at a rate of ~10m/s²

  • When air resistance is negligible

  • is shown by the constant variable g

• Recall: A=F/M

  • A larger mass has a larger gravity force pulling on it than a smaller one, so the increase in mass does accelerate any slower

Newton's Second Law of Motion & Non-Free Fall

• Most of the time, air resistance is non-negligible, so the acceleration of an object is less than g.

• Recall: Air Resistance is the force of friction acting between an object and the surrounding air

  • Air Resistance is related to the speed of an object and its surface area

• When air resistance is present: Net force weight-air resistance

Terminal Velocity

• What happens when the force of air resistance is equal to the weight of an object?

  • Example: weight= 10 and air resistance=10

  • Acceleration=10-10=0

    • The Object continues to move, but at an unchanging speed

• Terminal Velocity: The point at which acceleration terminates for a falling object

  • Causes objects to fall and hit the ground at almost the exact same time

    • Lighter objects hit terminal speed faster than heavier objects

Gliding

• Gliding: A mode of locomotion where animals move through the air in a controlled fall

  • Uses Air Resistance to increase their ability to control the rate of falling

    • Maximizes Surface Area

  • Decreases the amount of energy needed to get from one location to the next

Multiple Choice

A coin and feather are dropped inside a vacuum (an area of space with no air resistance) and fall at the same rate. Are the forces of gravity on these objects equal?

• Yes

• No

Multiple Choice

You Drop a pillow off the tallest building in town. While the pillow falls, its speed_________

• And Acceleration Increases

• Increases and Acceleration Decreases

• And Acceleration Decrease

• Decreases and acceleration increases

Multiple Choice

What is the acceleration when an object is in free fall?

• 0 m/s²

• 10 m/s²

• 32 m/s²

• 100 m/s²

Forces & Interactions

• A force is an interaction between two objects

  • If you push on one object, it has to be pushing back on you

  • Forces Come in Pairs

  • Both objects are involved in the interaction

Newton's Third Law of Motion

• When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first at the same time

• Every Action has an Equal and opposite reaction

  • One force is called the Action Force

  • One force is called the Reaction Force

Identifying Action & Reaction Forces

• Identify the two objects

• Make one object the "Actor" And the other the Reactor"

  • Example: the hammer is the actor and the nail is the reactor

    • When the hammer pushed on the nail, the nail pushes back on the hammer

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​

• ​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​

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

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

• Gravity cancels out the force of the ball

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

• Equal to 3000 N

• Greater than 3000 N

• More information is needed to answer this question