12.1 Forces

12.1 Key Concepts• How do forces affect the motion of an object?

• What are the four main types of friction?

• How do gravity and air resistance

affect a falling object?

• Why does a projectile follow a

curved path?

12.1 Forces

The wind pushes against the man and his umbrella. The push from the wind is a _____________force

12.1 Forces

A force is a _________ or a _________ that acts on an object.

A force can cause…• a resting object to ______________• a moving object to __________________

What Is a Force?

push pull

move

accelerate

12.1 Forces

Measuring Force

You may have measured forces using a _________ ___________

The stretch of the spring in the scale depends on the amount of ____________ (a type of force) acting on it.

What Is a Force?

springscale

weight

12.1 Forces

Units of Force

Force is measured in newtons, abbreviated as ___

One newton (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2).

1 N =

What Is a Force?

N

1 kg · m/s2

12.1 Forces

Representing Force

The ___________ of the arrows show relative amounts of force.

In which direction does the weight (force) of the apples point?

In which picture is the weight (force) greater?

What Is a Force?

length

downward

picture B

12.1 Forces

Representing Force

What upward force is keeping the apples from plummeting to the ground?

What Is a Force?

The spring

12.1 Forces

When the forces on an object are balanced, the net force is _____________, and there is no change in the object’s motion.

Combining Forces

When an unbalanced force acts on an object, the object _________________.

zero

accelerates

12.1 Forces

The net force is the ___________________ acting on an object after all the forces are combined.

• Forces in the same direction ___________ together.• Forces in opposite directions _________________ from

one another or ________________ each other out.

Combining Forces

overall force

addsubtract

cancel

12.1 Forces

Forces can add together or subtract from one another.

Combining Forces

Adding forces Subtracting forces

Equal and opposite forces

12.1 Forces

Are the forces shown equal?

Are they moving towards each other or in opposite directions?

What is the net force?

Combining Forces

yes

oppositezero (canceled out)

12.1 Forces

Friction is a force that ________________ the motion of objects that touch as they move past each other.

Friction

There are four main types of friction: • ______________ friction• ______________ friction• ______________ friction• ______________ friction

opposes

Static

Sliding

Rolling

Fluid

12.1 Forces

Static Friction

Static friction is the friction force that acts on objects that are ________________.

Static friction always acts in the direction opposite to that of the applied force.

push

Static friction

not moving

12.1 Forces

Sliding Friction

Sliding friction is a force that opposes the direction of motion of an object as it _______________ over a surface.

Sliding friction is ___________ than static friction.

push

Sliding friction

movement

slides

less

12.1 Forces

Rolling FrictionRolling friction is the friction force that acts on ________________ objects.

The force of rolling friction is about 100 to 1000 times ______________ than the force of static or sliding friction.

______________________ in these wheels greatly reduce friction by replacing sliding friction with rolling friction.

rolling

less

Ball bearings

12.1 Forces

Fluid Friction• Fluid friction opposes the motion of an object

through a _______________.

– Fluids are anything that _____________• Gases, liquids

– Fluid friction acting on an object moving through the air is known as _________ ___________________.

fluid

flows

air resistance

12.1 Forces

Gravity

Gravity causes objects to accelerate ________________, whereas air resistance acts in the direction __________________ to the motion and reduces acceleration.

Gravity is a force that acts between any _______ ________________.

• Gravity is an attractive force. • Gravity can act over _______________ distances.

downwardopposite

twomasses

large

12.1 Forces

Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force on the boulder. The forces are balanced.

Gravity

12.1 Forces

Falling Objects• Terminal velocity is the ______________

velocity of a falling object when the force of air resistance ____________ the force of gravity.

Gravity

Has the flying squirrel reached terminal velocity? Check the size ofthe arrows!

constant

equals

No…forces are not equal

12.1 Forces

A thrown ball follows a ____________ path.

Projectile motion is the motion of a falling object (projectile) after it is given an initial _______________ velocity.

Projectile Motion

curved

forward

12.1 Forces

Assessment Questions

1. If an object is at rest, which of the following statements must be true? a. There are no forces acting on the object.

b. There is no friction acting on the object.

c. The forces acting on the object are unbalanced.

d. The net force acting on the object is zero.

12.1 Forces

Assessment Questions

2. Which of the following is not a type of friction?a. static frictionb. sliding frictionc. fluid frictiond. pull friction

12.1 Forces

Assessment Questions

3. In which direction does Earth’s gravitational force act? a. opposite the direction of motion

b. downward toward the center of Earth

c. upward away from the center of Earth

d. in the direction of motion

12.1 Forces

Assessment Questions

4. A ball thrown into the air follows a projectile course due to the initial velocity and thea. force of gravity.

b. effect of air resistance.

c. motion of Earth beneath it.

d. mass of the ball.