1. Chapter 2: Forces
2.1: Forces change motion

2. Vocabulary
A push or a pull is called a
____________.
force

3. Vocabulary
A ___________________ is when one object pushes or pulls another object by touching it. The first object is applying this force to the second object.
contact force
The ground produces a contact force on the skier as she pushes against the ground.

4. Vocabulary Gravity masses
_______________ is the force of attraction between two masses.
The strength of the gravitational force between two objects depends on their _______________.
Gravity
Gravity pulls the balls toward the ground.
masses

5. Vocabulary
_______________ is a force that resists motion between two surfaces that are pressed together.
Friction
There is friction between the wheels and the ground.

6. Types of Forces Gravity: Friction: Contact Force:
Pulls the skater toward the ground.
Contact Force:
The skater applies a contact force as he pushes against the ground. The ground applies a contact force that pushes the skater forward.
Friction:
There is friction between the ground and the blades.

7. Friction
Which surface would produce more friction? How would that affect the motion?

8. YES – it has both size and direction!
Thinking Question
Is force is a vector?
YES – it has both size and direction!

9. How to read a force diagram…
Red arrows show size and direction of force.
Blue boxes show mass

10. Vocabulary
The overall force acting on an object when all the forces are combined is called the
_____________________.
net force

11. Balanced Forces zero no
If the net force on an object is _______________, the forces acting on it are balanced.
2. Balanced forces have the same effect as ________ forces at all. no

12. Unbalanced Forces
Only an unbalanced force can change the ____________ of an object.
motion

13. No – if the forces are balanced it will not move.
Thinking Question
Does an object always move when a force acts on it?
No – if the forces are balanced it will not move.

14. Quick Review of Balanced and Unbalanced Forces
__________________ forces cannot change an object’s speed or direction.
An __________________ force is needed to change an object’s motion.
Balanced
Was I a balanced or unbalanced force?
unbalanced
Unbalanced – I caused motion!

15. Forces on Moving Objects…
To increase the speed of your bike, you may exert ___________ forward force by pedaling harder or changing gears. The net force moves the bike faster.
To turn your bike, you apply an ____________ force by leaning to one side and turning the handlebars.
To stop the bike, you use the extra force of ____________ that your brake pedals provide.
more
unbalanced
friction

16. Vocabulary
An object at rest stays at rest, and an object in motion stays in motion unless acted upon by another force is known as
_______________________________.
Newton’s first law

17. An object at rest remains at rest unless acted upon by an unbalanced force.
Unbalanced force (from the foot)
Object at rest (ball)

18. Object in motion (ball)
An object in motion remains in motion with the same velocity, unless acted upon by an unbalanced force.
Unbalanced force (from the hand)
Object in motion (ball)

19. What unbalanced forces change the motion of a volleyball that is hit hard over the net? How will its velocity change?
The contact force of a hand changes the ball’s direction, and may increase its velocity.

20. Vocabulary
The resistance of an object to a change in the speed or the direction of its motion is known as
_____________.
inertia
When you measure the _________ of an object you are also measuring its inertia!
mass

21. How would inertia affect a person in the back seat of this car?
The passenger will continue to move forward and will hit the back of the front seat.

22. Watch the video on Newton’s First Law