PHYSICS UNIT 3: FORCESNewton’s 1st Law of Motion

Newton’s 2nd Law of Motion

Newton’s 3rd Law of Motion

Friction

Who was Isaac Newton?

He was probably the most influential scientist who has ever lived. He was the first person to determine the laws of motion that govern almost everything in the universe. He also invented calculus!!!!

This unit is about the three laws of motion he first described in 1686.

What are Newton’s Laws of Motion:

1st Law: Objects at rest will stay at rest and objects in motion will stay in motion until acted upon by an external force.

Ex: An asteroid flying through space will keep going until it hits a planet.

2nd Law: F = ma (Force equals mass times acceleration.

Ex: The harder you push on a shopping cart, the faster it will speed up.

3rd Law: All interacting objects push on each other with a force equal in magnitude but opposite in direction.

Ex: Both cars in a collision are damaged, not just one.

Topic 1: Newton’s First Law of MotionLearning Goal: You will be able to use Newton’s First Law of Motion to describe the behavior of moving objects.

Success Criteria 1: I can differentiate between mass, weight, force, inertia, and momentum.Success Criteria 2: I can determine the net force acting upon an object.

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Success Criteria 1: I can differentiate between mass, weight, force, inertia, and momentum.

Image from Bing Images

First, some vocabulary:

Task 1 (4 pts): Complete the table of definitions below as we go.

Word Definition

Force

Weight

Inertia

Momentum

A force is a type of interaction when an object pushes or pulls on another object. Gravity and electromagnetism are examples of field forces. There is no contact required between objects for these forces to be felt. When two objects push or pull on each other directly, such as when you kick a soccer ball or lean on a wall, it is a contact force.

Weight is the force that an object’s mass exerts downward due to gravity. On the moon, you would have a lower weight because the moon’s gravity doesn’t exert as great a force on you as does the Earth’s gravity. Your mass, however, is the same everywhere.

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Why does it take a large force to move a massive object, such as a car vs a skateboard?

Anything with mass possesses something called inertia. Inertia is the tendency of an object to remain either at rest or to remain moving until it is acted upon by some external force. It can be thought of as the resistance of an object to changing its state of motion.

A more massive object has more inertia, thus it requires a greater force to alter its state of motion. It takes more pushing to start a car moving than it does a skateboard, and it takes more pushing to stop a car from moving than it does a skateboard. This is because a car has more mass, thus more inertia, than a skateboard.

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When there is no force acting upon an object, it just keeps going because of its inertia. This is why the moon continues to orbit the earth without slowing down. While gravity is attracting the moon to the Earth, there is no force pushing against the moon as it travels through space.

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What is the difference between inertia and momentum?

All objects have inertia, which as you just learned, is the resistance to changing its state of motion.

Moving objects also have momentum. Momentum, as we’ll learn more about in a later unit, is the mass of an object times its velocity. A fast-moving, massive object, such as a car driving down the freeway, has more momentum than a slow object or light object.

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Task 2 (4 pts): Answer the following questions.a) What is the difference between mass and

weight?

b) What is the difference between inertia and momentum?

c) _________ and _________ depend on where an object is or how it’s moving.

__________ and _________ are intrinsic to the object and have nothing to do with where it is or how it’s moving.

d) Why is it harder to move a massive object than a light one?

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Success Criteria 2: I can determine the net force acting upon an object.

Force is usually measured in newtons, abbreviated N.

One pound is about 4.45 newtons. So if you weigh 150 pounds, you are pushing down on the ground right now with a force of 150 x 4.45 = 668 newtons.

A 1kg mass on earth pushes downward with a force of 9.81N (Remember this number from somewhere?)

So, using scales on earth:

1.00kg = 2.20lbs = 9.81NImage from Bing Images

Task 3: What values would the following objects show on scales measuring kilograms, pounds, and newtons? (Hint: Use unit cancelling)

1.00kg = 2.20lbs = 9.81N

Kilograms Pounds Newtons

A A 16 pound cat.

B A 43kg teenager

C A 12N rock

D A 920kg car

E A 84922N bus

F A .033lb pinecone

When we study Newton’s second law, we’ll see how the force on an object determines how it will move. First, let’s look at how to determine the forces and the net force acting upon an object.

Net Force: 7N to the left + 10N to the right = 3N to the right

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Right now, as you sit in your seat, you are not moving. More specifically, you are not being accelerated in any direction. That means the net force, the sum of all forces acting upon you, is zero. We can represent this with a free body diagram.

Your chair pushes up on you with the same force that gravity pulls you down, thus you don’t accelerate up or

down. Image from Bing Images

When the forces, represented as vectors, are added together, the net force can be determined. If the net force is zero, the object will not undergo acceleration. If the net force is not zero, the object will accelerate. Note that the force vectors in a free body diagram are drawn pointing outward from the center of the object.

Usually, the force of the surface pushing up on an object preventing it from falling, such as the road pushing upwards on a car or a table pushing upwards on a book, is called the normal force.

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Task 4: Draw free body diagrams for the following objects, calculate the net force including the direction, and say if it would accelerate or not and in which direction.

Object and Forces Free Body Diagram Net Force?(Yes or No)

Acceleration? (Yes or

No)

A

You push sideways with a force of 50N on a desk

weighing 200N, and friction pushes back with a force of

50N.

B

You push sideways with a force of 80N on a desk

weighing 200N, and friction pushes back with a force of

60N.

C

A skydiver reaches terminal velocity where gravity pulls

down with 400N and air resistance pushes up with

400N.

Object and Forces Free Body Diagram Net Force?(Yes or No)

Acceleration? (Yes or

No)

D

You throw a 35N rock through the air and it accelerates toward the ground. Never

mind air resistance.

E

An 11,000N car drives down the road at a constant speed. The engine pushes it forward with a force of 2500N, which is exactly countered by air

resistance and friction.

F

You pull on a 600N weight sitting on the ground with a 240N force at 30° above the

horizontal and it doesn’t move.

As we’ve seen, since force is a vector quantity, the magnitude and direction of a force can be added to another force to find the net force. Also, since they are vectors, the x and y-components of forces can be calculated.

Task 5: Find the wanted force component or resultant. In each of these cases, the net force is zero. We’ll deal with cases where the net force is not zero in the section on Newton’s 2nd Law.a) An 80N mass pushes down on a scale. What would the scale read if you pulled up on the

object with a 30N force?

b) You drag a box at a constant speed. If the force of friction (the horizontal “counter force”) were 45N, what force would you be pulling the box with if you were pulling it at a 37° angle above the ground?

c) You tie a rope to a piano and you pull on it with a 135N force. If you pull it at 21° above the ground, what is the force of friction if the piano doesn’t move?

d) Wind blowing at 52° S of E exerts an 89N force on a flag. What is the magnitude of the force on the flag to the south?

e) You put a 15kg chair on a scale and you sit on it. You have a mass of 53kg. If someone pulls straight up on you with a 73N force, what would the scale read (the scale is in newtons).

f) An asteroid hits you at a 71° angle above the ground. If it exerts a 4000N force on you while colliding with you, and you weigh 142 pounds, what force is exerted on the ground?

Task 6: Write a two-paragraph summary (four sentences per paragraph) on what you learned in this topic.

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