PS CW 03.1 Forces

Forces

Lesson 1

How does friction affect motion?

Focus Question

weight

field

gravity

friction

net force

force

New Vocabulary

mass: the amount of matter in an object

Review Vocabulary

A force is a push or a pull.
Sometimes it is obvious that a force has been applied, but other times it’s not as
noticeable. For example, the
floor exerts a force on your
feet when you stand. The
atmosphere pushes against
your body. Gravity pulls on
your body.

A force can cause the motion
of an object to change.

What is force?

ThePhoto by PhotoAuthor is licensed under CCYYSA.

• Weight: force of gravity pulling on an object

• Applied force: a push or pull exerted on an object by a

person or another object

• Friction: force that opposes motion between two surfaces

that are touching

• Normal force: force from a floor or other horizontal

surface that supports an object

• Elastic (or spring) force: force that causes a material to

spring back to its original shape

Types of Force

The net force on an object is the sum of all the
forces acting on an object.

Forces are measured in SI units of newtons (N).

What is force?

ThePhoto by PhotoAuthor is licensed under
CCYYSA.

ThePhoto by PhotoAuthor is licensed under
CCYYSA.

Open Ended

What are the base units of force (measured in Newtons) and what are the base units for momentum?

• The amount of force that will cause a 1 kg object to

accelerate at 1 m/s2 is called a Newton (abbreviated N)

What is force?

Forces have direction so they follow the same
addition rules as displacement

Forces can be balanced or unbalanced.

The image below shows students pushing the box
in the same direction. The forces are combined or added together.

What is force?

Open Ended

What is the net force?

The students in this image are pushing in opposite directions. The direction of the net force is the same as the direction of the greater force. Therefore, the student who pushes harder causes the box to move in the direction of that push.

What is force?

Open Ended

What is the net force?

If the students push with the same size force but in opposite directions, as shown below, the net force on the box is zero because the two forces cancel each other. These are called balanced forces.

What is force?

Rules for Adding Forces:

•

Add forces that act in the same direction

•

Subtract forces that act in opposite directions

•

Forces that are neither in the same direction nor in
opposite directions cannot be directly added together.

What is force?

Balanced Forces

What is force?

Unbalanced Forces

What is force?

• There are many types of forces. We will focus on only a

few.

• Friction: force that resists motion between two

surfaces that are touching

• Spring: the force that causes a material to return to its

original shape when it is stretched or compressed

Friction

ThePhoto by PhotoAuthor is licensed under CCYYSA.

Suppose you give a skateboard a push with your hand. After you let go, the skateboard slows down and eventually stops. Because the skateboard’s motion is changing as it slows down, there must be a force acting on it. The force is called friction.

Friction is the force that opposes the motion between two surfaces that touch.

Friction always acts opposite the direction of the object’s motion

Friction

• The surface of any object is rough. Even an object that

feels smooth is covered with tiny bumps.

• When the bumps are in contact with each other, they

stick. The more the bumps press together, the more
friction

Friction

• The amount of friction depends on:

• Roughness of the surfaces
• The amount of force pushing the surfaces together

What Causes Friction?

• Static friction occurs when force applied to an object

does not cause the object to move.

• Static friction is the strongest form of friction.

Types of Friction

ThePhoto by PhotoAuthor is licensed under CCYYSA.

• Sliding Friction: occurs when an object slides across a

surface

• Sliding friction is also called kinetic friction

Types of Friction

ThePhoto by PhotoAuthor is licensed under CCYYSA.

Rolling Friction: occurs
when an object rolls
across a surface

Rolling friction is less than sliding friction.

Types of Friction

Why do objects fall to earth?

What keeps the planets in motion in the sky?

The answer to both questions is the force of gravity.

Gravity

• Gravity is one of the four forces called the

fundamental forces, because all other forces are included in them.

• The four forces are gravity, the electromagnetic force,

and the strong and weak nuclear forces.

Gravity

Gravity

• ​Isaac Newton proposed

that gravity is a force
that exists between any
two objects that have
mass.​

• The more mass the

objects have, the
stronger the force of
gravity.​

• And, the closer together

the objects are, the
stronger the force of
gravity.​

Open Ended

Gravity is an attractive force between two objects that depends on their masses and distance from each other

Effect of mass:
Compare the gravitational force between the two objects at the top and the two at the bottom.

Open Ended

Gravity is an attractive force between two objects that depends on their masses and distance from each other

Effect of distance:
Compare the gravitational force between the two objects at the top and the two at the bottom.

Gravity is called a field force.
A gravitational field is a
region of space surrounding
an object in which the force
of gravity exists.

We can measure the
strength of Earth’s
gravitational field at various
distances from Earth’s
surface.

Gravity

We have a special name
for the force of gravity
on an object

Weight is the force of
gravity on an object.

As usual for forces, the
SI unit for weight is a
newton or N.

Weight is calculated as
mass (in kg) X 9.8 m/s2
= weight in Newtons

Gravity

•

WEIGHT = the gravitational force exerted on an object

MASS = the amount of matter in an object

Weight = mass(kg) x 9.8(m/s2)

Weight is measured in Newtons
Mass is measured in kilograms

Gravity

1. An elephant has a mass of 5,000 kg. What is the

elephant's weight?

Practice Problems

2. A squirrel has a mass of 0.5 kg. What is its weight?

Practice Problems

3. A boy weighs 400 N. What is his mass?

Practice Problems

4. An astronaut has a mass of 100 kg and has a weight of 370 N on Mars. What is the gravitational strength of
Mars?

Practice Problems

Multiple Choice

What is the gravitational force exerted on an object called? ​

• centripetal force​

• weight​

• momentum​

• friction speed​

Multiple Choice

Which explains why astronauts seem weightless in orbit?​

• Earth’s gravity is much less at that distance from Earth. ​

• The spacecraft is in freefall. ​

• The gravity of Earth and the Sun cancel. ​

• The centripetal force on the shuttle balances Earth’s gravity. ​

Multiple Choice

Which term best describes the forces on an object with a net force of zero? ​

• unbalanced forces​

• balanced forces​

• inertia​

• acceleration​

Multiple Choice

Which body exerts the strongest gravitational force on you? ​

• moon

• earth

• sun

• your chair

Multiple Choice

Which is not a force?

• weight

• friction

• air resistance

• momentum

Summary

A. A force is a push or a pull of an object.

B. The net force on an object is the combination of all the forces acting on the object.

C. Unbalanced forces cause the motion of objects to change.

D. Friction is the force that opposes the sliding motion of two surfaces that are in contact.

E. Gravity is an attractive force between all objects that have mass.

Open Ended

EXPLAIN: Can there be forces acting on an object if the object is at rest?

Open Ended

EXPLAIN: Must there be an unbalanced force acting on a moving object?

Open Ended

PREDICT: Suppose Earth's mass increased but Earth's diameter did not change. Describe how the gravitational force between Earth and an object on its surface would change.

Math Response

On Earth, what is the weight of a large screen TV that has a mass of 75 kg?

Type answer here

Correct Answer: 735

Deg^°

Rad

Open Ended

Two students push on a box in one direction and a third student pushes in the opposite direction. What is the net force on the box if each student pushes with a force of 50 N