What is a Force

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Forces

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What are forces?

by JRodriguez

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What Are Forces?

Forces are ________ or

______ in a given direction.

Forces affect how objects move.

a push

pull

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Forces Affect Motion

At your table, make a list of ways forces can affect objects.
There are many ways. Write a few of these ways below.

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Forces

Acceleration

cause

The Big Idea...

Since forces can cause changes in the speed or direction of an
object, we can say that forces cause changes in velocity, so
forces cause acceleration!

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Units of Force

Forces are measured in newtons (N).

You probably measure yourself on a scale in pounds.
One pound is equal to 4.448 newtons.

Just like velocity, force has direction. When forces are
demonstrated both magnitude and direction should be shown.

10 N

What is the magnitude of the force shown above? How about the
direction? the magnitude is 10 N - the direction is to the right

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Balanced Forces

If you and a friend both push with the same strength on each side of
a table, will it move?

Two or more opposite forces acting on an object are considered
balanced if their effects cancel each other out. Balanced forcesdo
not cause a change in an object's motion.

8 N

8 N

The box to the right is
at rest and will remain
at rest since the
opposite forces acting
on it are balanced.

not

Net force = the total sum of all the forces acting on an object

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Unbalanced Forces

If the effects of the forces don't cancel each other out (one force is
stronger than others), the forces areunbalanced forces. Unbalanced
forces do cause a change in motion; speed and/or direction.

Think about Tug-Of-War.
How does one side win?

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Unbalanced Forces

15 N

8 N

Two ways to interpret this diagram are to say:

there is a 15N force to the right and an 8N force to the left

OR

we can say there is a +15N force and a -8N force.

What does the negative on the -8N tell us?

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Unbalanced Forces

15 N

8 N

The box was initially at rest. Since the forces acting on the box
are unbalanced, the box will start moving.

Does the box accelerate?

Which way do you think the box will move and why?

the box will accelerate and move to the right since that is the direction that
the stronger force is pushing (15 N is stronger than 8 N)

Multiple Select

Forces are all around us. Which of the following

do you think are examples of forces? (choose all

that apply)

• Gravity

• Friction

• Muscles

• Wind

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18 Forces are all around us. Which of the following
do you think are examples of forces? (choose all
that apply)

A

Gravity

B

Friction

C

Muscles

D

Wind

muscles can be used to cause a force but it is not

a force in itself, and wind is a result of forces such
as gravity but again, it's not a force alone

Multiple Choice

What is the SI unit for force?

• 
  Pounds

• Kilograms

• Newtons

Multiple Choice

Calculate the Net Force. 

• 40N, Left
• 40N, Right
• 0N, Balanced
• 75N, Right

Multiple Choice

What is the net force?

• 400 N Up
• 400 N Down
• - 400 N Down
• 800 N Down

Multiple Choice

How will this object move? 

• It won't move. 
• It will move to the right. 
• It will move to the left. 
• It will move up. 

Multiple Choice

A +10 N force acts on a car and at the same time,
a -20 N force acts on the car. What is the net
force acting on the car and is it balanced?

• -30N

  Unbalanced

• -10 N

  Unbalanced

• -10 N

  Balanced

• +30 N

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20 A +10 N force acts on a car and at the same time,
a -20 N force acts on the car. What is the net
force acting on the car and is it balanced?

A -30 N unbalanced

B -10 N unbalanced

C -10 N balanced

D + 30 balanced

Hint: Net force is the total
(or sum) of all the forces
acting on an object.

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Friction

Forces are present all around us, but can not usually be seen.
Friction is a force that resists motion and we experience daily.

When you run, walk, sit on the couch,
brush your hair, and write you
experience friction.

You are experiencing friction right
now!

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Friction

The force of friction is caused by microscopic particles touching each
other. These microscopic pieces on both surfaces cause friction.

Friction is affected by how hard the surfaces push together and the
types of surfaces involved.

Click here to see the force of friction clip

There are many types of
friction.

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Static Friction

Static friction acts on objects that are not moving. Have you ever
wondered why it is so hard to start moving a heavy object like a
dresser or couch, but then once it starts moving it is easier?

That is because you have to overcome the force of static friction!
Static friction holds the couch in place and keeps it from moving.

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Sliding Friction

Sliding friction occurs when objects slide over each other. Sliding
friction is easier to overcome than static friction. That is why the
couch is easier to move once it starts sliding!

Click here to see sliding friction in action.

Sliding friction is also known as kinetic friction.

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Fluid Friction

Fluid friction occurs when objects move through a fluid.

Remember, air is a fluid, so you continuously experience fluid friction!

Click here to see the effects of fluid

friction.

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Rolling Friction

Rolling friction exists
when objects roll
across surfaces.

With a partner, make a
list of other examples
of rolling friction. Hint:
There are many
examples in sports.

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Friction

The force of friction is measured in Newtons like all other forces.
When you are trying to determine where to put the friction force,
just remember thatfriction acts opposite to motion!

stationary box

force applied by
person pushing box

static friction
force

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21 Friction acts more on objects in motion than it
does on objects at rest.

True

False
it acts on both equally

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22 Friction always opposes the motion of an object.

True

False

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Gravity

Forces are present all around us and always act in pairs, so we usually
experience more than one force. Gravity is a force that pulls objects
towards each other.

How does gravity act on objects
here on Earth? Use the picture to
the right to help you with your
answer.

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Law of Gravitation

The Law of Universal Gravitation tells us that gravity acts between
all objects in the universe. This means that without exception, any
two objects in the universe attract each other!

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Factors Affecting Gravity

Gravity is affected by both mass and distance. The greater the distance
between two objects, the less the gravitational force between them. The
greater the mass of the object, the greater the object's gravitational
force.

You have learned about mass in the past. Can you define mass?

Which of these
examples has
more gravity?
Why?

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23 The force of gravitation between an object
and a planet is increased as they move
away from each other.

True

False

it's the opposite

closer = stronger gravity
further = weaker gravity

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24 The force of gravitation between an object
and a planet is decreased if the object
increases mass.

True

False

it's the opposite

greater mass = greater gravity
lower mass = lower gravity

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Mass and Weight

What is weight?

You have also learned the difference between mass and weight in the

past. What can you recall about mass and weight?

Think about

astronauts on the

International

Space Station.
What do you

know about their

masses and

weights?

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Wait, so mass affects

gravity?

Weight is a measure of the gravitational force exerted on an object.
Weight varies depending on gravitational force, but mass does not.

Weight = mass x gravity

Our moon has a gravity that is

one sixth of that on Earth.

What would that do to your

weight on the moon?

Would your mass change on

the moon?

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What is "g"?

"g" is the acceleration due to gravity.

On Earth, our "g" is approximately 9.8 m/s2.

On other planets, acceleration due to gravity will vary depending
on the mass of the planet. In general, the more massive the
object, the larger the value of g!

Jupiter
g = 26.1 m/s2
Earth's Moon
g = 1.67 m/s2

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25 A 50 kg kid is on planet Earth, where g = 9.8 m/s2.
What is the boy's weight?

A

5.1 m/s

B

490 N

C

490 kg

D

5.1 N

I know we normally measure weight in pounds here in the U.S., but since

we are talking about weight due to the FORCE of gravity use Newtons

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26 A 50 kg kid is on planet Earth, where g = 9.8 m/s2.
What is the boy's mass?

A

50 kg

B

490 N

C

490 kg

D

5.1 N
this is a trick question...anytime you see kg it's

automatically the mass

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27 A 50 kg kid is on the moon, where g = 1.67 m/s2.
What is the boy's weight?

A
83.5 kg

B
29.9 kg

C
83.5 N

D
29.9 N

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28 A 50 kg kid is on the moon, where g = 1.67 m/s2.
What is the boy's mass?

A
83.5 kg

B
50 kg

C
83.5 N

D
50 N
again, it's a trick question...no need to solve for
mass when it's provided in the question

kg = mass units

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29 The larger the planet, the larger the __________.

A mass

B weight

C gravity

D all could be correct

*larger planets have larger gravity

which automatically means we will weigh

more there since weight depends on gravity

*mass on the other hand will remain the

same no matter where we go

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Gravity and Motion

Galileo showed that falling objects accelerate equally no matter
what their mass is. This is strictly true when gravity is the only
force acting on a falling object and is known as free fall.

When an object is in free fall, it accelerates at 9.8 m/s2

10kg

If these two stones are dropped
from the same height at the same
time, which hits the ground first?

1kg

Click here to see a bowling

ball and golf ball being

dropped

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Gravity and Motion

The following video shows a feather and a ball bearing being
dropped from a small height.

Which simulation showed the objects in free fall?

Critical thinking: What was happening in the simulation that did not
illustrate free fall?

Click here to see a feather and a ball bearing

being dropped.

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Air Resistance

Try this: Crumple up two pieces of paper individually and drop
them both from the same height at the same time. Which hits
the ground first? Now drop a crumpled piece of a paper and a
non-crumpled piece of paper in the same way. What's the
difference?

Objects are not always truly in free fall because they
experience air resistance. Air resistance is a fluid friction
experienced by falling objects. When objects experience air
resistance, they don't fall at a rate of 9.8 m/s2.

Click here to see a feather

and a hammer being dropped
on the moon. What happens

with no air resistance?

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Air Resistance

The flat piece of paper fell at a slower rate because it had more
surface area. This greater surface area resulted in the paper
experiencing air resistance. For the crumpled paper, air
resistance was probably very tiny and thus could be ignored.
The crumpled paper was essentially in free fall.

People use parachutes when they
jump out of planes. Why?

The greater the surface area the more air resistance

will occur.

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Net Force

As you know, many forces are acting on us and other objects.
To determine the total force acting on an object, the forces are
added and subtracted as appropriate to find the net force.

When several forces are acting on the same object, the net
force might be zero...

5 N

-5 N
Net Force = 0

If the net force on an object is zero, then it is in equilibrium.
When an object is at rest, the net force is zero.

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If the net force is not equal to zero, then there is a change in the motion
of the object. The object is not in equilibrium. What is the net force in the

example below?

5 N

-12 N
Net Force =

Unbalanced Forces

What direction is this box going to

move?

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5 N

-12 N
Net Force = -7 N

In this case, the object will accelerate towards the left because

the NET FORCE is toward the left.

Unbalanced Forces

On Earth, gravity and friction are two of the unbalanced forces

that

frequently change an object's motion.

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30 What is the net force acting on the object below?
Is the object in equilibrium?

7 N

-8 N

-10 N

it's not in equilibrium

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31 What is the net force acting on the object below?
Is the object in equilibrium?

25 N

-15N

-10 N

yes, this is in equilibrium and at rest

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32 Acceleration due to gravity on Jupiter is 26.1 m/s2.
How much would a 60 kg person weigh on Jupiter?

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33 Acceleration due to gravity on Earth's Moon
is 1.67 m/s2. How much would a 60 kg person weigh
on Earth's Moon?

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Sticky Sneakers Lab

How does the amount of friction between a shoe and a surface
compare for different shoes?

Which shoes would be best for playing basketball?

Which would be best for bowling?

These questions will be answered in this lab.

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Newton's Laws of Motion

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