Newton’s Laws & Forces

Newton’s Laws & Forces

Key Topics:
●

History of Sir Isaac Newton & gravity

●

Newton’s 3 Laws

●

Unbalanced/balanced forces,

●

Inertia

●

Types of forces

Open Ended

So far, we have been talking about objects moving
●But we haven’t talked about why they move yet
●Something must make an object move

What do you think causes something to move? Give an
example.

Think About it Video

What is a force- asking around town:
https://www.youtube.com/watch?v=GmlMV7bA0TM

Sir Isaac Newton
● Most have heard of Isaac Newton and his laws, but you might not

know what they say

● You may have heard about him being hit by an apple and

discovering gravity, but he did a lot more than that!

● Fun Fact: He invented Calculus!

It all starts with an apple…

● One beautiful spring day in 1655, a man named Isaac

Newton was sitting under an apple tree in his garden,
enjoying a glass of tea.

● Suddenly, one of the apples fell and crashed on his

head.

Disclaimer: Most believe that the apple did not actually hit

Newton on the head, but rather fell nearby and caught
his attention.

A STORY ABOUT AN APPLE

● That got Newton thinking (once the bump had

gone down of course).

● “Why did the apple fall towards the Earth?”
● “Why did it not “shoot upwards'' when it came

away from the branch?”

● And, “why did I plant that apple tree anyway?!!''

A STORY ABOUT AN APPLE

■ Newton knew that unbalanced forces are

necessary to move or change the motion of
objects.

■So, he came up with the idea that the Earth must

attract the apple towards it with some “unseen
force''.

■He named this force gravity.

WHAT IS A FORCE?

●A force is a push or a pull, that one object exerts on

another

●Forces make things move

●Force is a VECTOR, which means the direction is

important!

●Units🡪 Newtons

Forces

● A force is defined as any pushor pullthat is exerted on an object

○ Forces cause objects to speed up, slow down, or change direction

○ This means: forces cause objects to accelerate

● The direction of a force tells you what direction an object accelerates in

Push a box right,

it moves right

Push a box left,

it moves left

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Force is measured in what units:

BALANCED & UNBALANCED
FORCES

●What did Newton mean that unbalanced forces are

necessary to move or change the motion of objects?

●In the last example, the rope was pulling up an object.

The force of the rope would need to be larger than the
force of gravity to make the object move, and the
forces would be unbalanced

●Balanced Forces = NO acceleration

●Unbalanced Forces= Acceleration

●This leads to Newton’s 1st Law…

BALANCED FORCES

●Do not change motion
●Cancel each other
●Combine to produce a net

force of zero

UNBALANCED FORCES

●Do not have the same

strength (size)

●Cause acceleration

Balanced and Unbalanced

Forces

● What would happen if two players hit each other and they both push with a

force of 200 N?

200N
200N

Open Ended

What would happen??

Balanced and Unbalanced

Forces

● What would happen if the left team pulled with 100 N and the right team

pulled with 150N?

● What would be the total force? What direction?

100N

150N

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What would happen??

Balanced and Unbalanced

Forces

● When forces don’t cancel out, they are called unbalanced forces

● The net force is whatever is left over after they cancel out

HOW MANY TYPES OF FORCES DO YOU THINK
THERE ARE?
●There are 5 main ones that we will

focus on!
○Applied Force
○Tension Force
○Normal Force
○Force of Friction
○Force of Gravity

Common Forces

● Applied Force (Fa): any force you are applying to an object
● Force of Gravity (Fg): force that pulls an object towards

the center of Earth

● Force of Friction (Ff): A force that tries to slow objects down

○ It opposes motion

● Normal Force (FN): A force that acts perpendicular to a

surface
○ It cancels out gravity

● Tension Force (FT): A force that is exerted on a rope or

string

NET FORCE

●When two or more forces act on an object at the same

time, the forces combine to form the net force.

» The combination of all forces on an object

●Example: If an object is tied to a rope and is being

pulled up, the net forces would be the combination of
the force of gravity trying to push it down and the force
of the rope pulling the object up.

Open Ended

Net Force is the force:

Think About it Video

Newton’s 1st Law- Football: https://www.youtube.com/watch?v=08BFCZJDn9w

NEWTON’S 1ST LAW

● Newton’s First Law:An object at rest will stay at

rest, and an object in motion will stay in motion
(with constant velocity), until some unbalanced
force acts on it.
- This means the VELOCITY of an object will

not change unless a force acts on it. So

the

direction or the speed won’t change.

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Fill in the blank: Newton’s 1st Law states an object will stay at rest/motion unless:

Newton’s 1st Law

● When you hit the ball,

it takes off.

● According to this law,

it will continue to move
forever until another
force acts on it

● But this isn’t what happens. Why does the ball eventually come to a stop?

○ Friction is an unbalanced force that slows it down!

Inertia

● Newton’s 1st Law is sometimes called the Law of Inertia

● Inertia is an object’s tendency to not change its velocity (speed or direction)

● Have you ever been in a car that speeds up/slows down suddenly and you

are thrown back/forward?
○ Inertia!

Newton’s 1st Law is also known as the Law of:

Open Ended

Newton’s 1st Law is also known as the Law of:

Think About it Video

Newton’s 2nd Law- Football:

NEWTON’S 2ND LAW

●The relationship between force, mass, and

acceleration:

○ Force= mass x acceleration

●Which is an equation you may have seen before

:

F=ma

●Force and acceleration are BOTH vectors. They

will always be in the same direction as each
other

Newton’s 2nd Law

● Newton’s 2nd Law: An object will accelerate in the direction of an

unbalanced force acting on it

● F= Force

● m= mass

● a= acceleration

The SI unit for force
is the Newton (N)

F=ma

F=MA UNITS

●Force🡪 Newtons (N)

●Mass🡪 kilograms (kg)

●Acceleration🡪 meters/second/second

(m/s2)

●Do not use this equation until all units are

accurate, convert if needed! *If mass if
given in grams, convert to kg*

Open Ended

F=ma

An object with a mass of 4 kg is pushed
and accelerates at 3 m/s2right. Find the
force required in order to have pushed it.

Open Ended

F=ma

If I push on a 2kg book with a
force of 10N, how much will it accelerate?

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F=ma

If a force of 15 N pushes an
object to accelerate at 3 m/s2.
Find the mass of the object.

Think About it Video

Newton’s 3rd Law- Football:

NEWTON’S 3RD LAW

●For every action there is an opposite and

equal reaction force

●The size of the forces on the first

object equals the size of the force on the second
object.

●The direction of the force on the first object

is opposite to the direction of the force on the
second object.

●Forces always come in pairs - equal and

opposite action-reaction force pairs.

Rocket Science is just Newton’s 3rd

Law!

● When a rocket is taking off, a bunch of

reactions create hot gas inside the engine

● The gas escapes at the bottom and

pushes down. The force down also pushes
up!

● This makes the rocket move upwards!

N3L EXAMPLE:
PROPULSION OF A FISH
●A fish uses its fins to push water backwards. But

a push on the water will only serve to accelerate
the water, the water must also be pushing the fish
forwards, propelling the fish through the water.

● The size of the force on the water = the size of

the force on the fish

●The direction of the force on the water

(backwards) is opposite the direction of the force
on the fish (forwards)

N3L EXAMPLE: BIRD FLYING

● A bird flies by use of its wings. The wings of a

bird push air downwards. Since forces result from
mutual interactions, the air must also be pushing
the bird upwards.

●The size of the force on the air = the size of the

force on the bird

●The direction of the force on the air (downwards)

is opposite the direction of the force on the bird
(upwards)

FREE BODY DIAGRAMS

● Free body diagrams use vectors to show all the forces acting on an

object.

● Rules of Drawing Free Body Diagrams:

1)Identify the object you will draw a diagram for. (If there are multiple objects of
interest, you will need to draw multiple diagrams.)

2)Identify all the forces acting directly on the object.

3)Draw a dot to represent the object of interest.

4)Draw a vector to represent each force. Draw it in the direction the force is being
exerted, and label it

The vectors should be drawn somewhat to scale. If the force applied on an object is
300 Newtons, and the force of friction is 10 Newtons… Then the 300 Newtons vector
should be much longer than the 10 Newtons vector

Force Game

-Working with your table partner, if they are done with their quiz!!

-Create a team name

-Use only 1 laptop to answer the questions

-Write your team name when you answer each question, so you get points
if you get it right!

-We will keep the score up on the board

Open Ended

What direction is the force of gravity?

Open Ended

What direction is the normal force?

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What is a free body diagram?

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What force always exists in all situations on earth?

Open Ended

List the 5 types of forces:

Open Ended

F= ma

If I push on a 2kg book with a force
of 10N, how much will it accelerate?

Balanced and Unbalanced

Forces

● What would happen if the left team pulled with 100 N and the right team

pulled with 150N?

● What would be the total force? What direction?

100N

150N

Open Ended

What direction would the total force be in?

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Fill in the blank: Newton’s 1st Law states an object will stay at rest/motion unless:

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At what rate does gravity acceleration? “g”

Open Ended

Balanced or Unbalanced: A box sitting on the
ground.

Open Ended

Net Force is the force:

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If a 2 kg object is not moving and just sitting on a table, what is the normal force?

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When is the net force 0 Newtons?

Open Ended

If a rope is pulling up with a force of 20 Newtons
and gravity is pushing down 4 Newtons, what is the net force?

Open Ended

If an object is pushed with a force of 40 Newtons
right across a rough floor with a friction force of 8 Newtons. What is the net force?

Open Ended

If an object is pushed with a force of 25 Newtons
right across a rough floor with a friction force of 6 Newtons. What is the net force?

Open Ended

What is the difference between force of gravity and weight?

Open Ended

Force is measured in what units:

Open Ended

Balanced or unbalanced: a ball being kicked to the right

Open Ended

F= ma