1. What are the four main types of friction?
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?

2. The wind pushes against the man and his umbrella
The wind pushes against the man and his umbrella. The push from the wind is a _____________
force

3. What Is a Force?
A force is a _________ or a _________ that acts on an object.
push
pull
A force can cause…
a resting object to ______________
a moving object to __________________
move
accelerate

4. What Is a Force?
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.
spring
scale
weight

5. What Is a Force?
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 = N
1 kg
· m/s2

6. What Is a Force?
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?
length
downward
picture B

7. What Is a Force?
Representing Force
What upward force is keeping the apples from plummeting to the ground?
The spring

8. Combining Forces
When the forces on an object are balanced, the net force is _____________, and there is no change in the object’s motion.
zero
When an unbalanced force acts on an object, the object _________________.
accelerates

9. Combining 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.
overall force
add
subtract
cancel

10. Forces can add together or subtract from one another.
Combining Forces
Forces can add together or subtract from one another.
Adding forces
Subtracting forces
Equal and opposite forces

11. Combining Forces yes opposite zero (canceled out)
Are the forces shown equal?
Are they moving towards each other or in opposite directions?
What is the net force?
yes
opposite
zero (canceled out)

12. Friction
Friction is a force that ________________ the motion of objects that touch as they move past each other.
opposes
There are four main types of friction:
______________ friction
Static
Sliding
Rolling
Fluid

13. 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.
not moving
push
Static friction

14. Sliding friction is ___________ than static friction. slides
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.
slides
less
push
movement
Sliding friction

15. Rolling Friction rolling less
Rolling 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.
rolling
less
______________________ in these wheels greatly reduce friction by replacing sliding friction with rolling friction.
Ball bearings

16. 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

17. Gravity is a force that acts between any _______ ________________. two
Gravity causes objects to accelerate ________________, whereas air resistance acts in the direction __________________ to the motion and reduces acceleration.
downward
opposite
Gravity is a force that acts between any _______ ________________.
Gravity is an attractive force.
Gravity can act over _______________ distances.
two
masses
large

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

19. No…forces are not equal
Gravity
Falling Objects
Terminal velocity is the ______________ velocity of a falling object when the force of air resistance ____________ the force of gravity.
constant
equals
Has the flying squirrel reached
terminal velocity? Check the size of
the arrows!
No…forces are not equal

20. Projectile Motion
A thrown ball follows a ____________ path.
Projectile motion is the motion of a falling object (projectile) after it is given an initial _______________ velocity.
curved
forward

21. Assessment Questions
If an object is at rest, which of the following statements must be true?
There are no forces acting on the object.
There is no friction acting on the object.
The forces acting on the object are unbalanced.
The net force acting on the object is zero.

22. Which of the following is not a type of friction?
Assessment Questions
Which of the following is not a type of friction?
static friction
sliding friction
fluid friction
pull friction

23. In which direction does Earth’s gravitational force act?
Assessment Questions
In which direction does Earth’s gravitational force act?
opposite the direction of motion
downward toward the center of Earth
upward away from the center of Earth
in the direction of motion

24. Assessment Questions
A ball thrown into the air follows a projectile course due to the initial velocity and the
force of gravity.
effect of air resistance.
motion of Earth beneath it.
mass of the ball.