1. Forces
Ms. MacGillivray

2. Force A push or a pull on an object.
Objects exert forces on each other.

3. Force Units: Newton (N)
example – 5 Newtons of force pushing a box to the right would be written like this:
5 N

4. Force Example Example: You sit on a chair;
Your weight is pushing down on the chair.
The chair is pushing up.
Because the forces are balanced, you don’t move.
Force of Chair
Force of Gravity

5. Net Force The combination of all forces acting on an object.
If the forces are equal and
opposite, then the net force
is ZERO and there’s no
motion.
Force = 10 N
Force = 10 N

6. Balanced Forces Net Force = zero!
Balanced forces produce no change in the motion of an object. (i.e., the object doesn’t move)

7. Unbalanced Forces Net Force does not equal zero!
Unbalanced forces produce a change in the motion of an object.
A motionless object starts to move OR
A moving object changes direction or speed.

8. Types of Forces
Force of Gravity
Centripetal force
Force of Friction

9. 1. Gravity
The force of attraction between objects that have mass.

10. Mass
The amount of “stuff” in an object.

11. Gravity
Since all objects have mass, gravity acts between all objects.

12. Examples of Gravity
Example 1: the force of attraction between you and the Earth pulls you toward the Earth (toward the ground).
Example 2: The force of attraction between the Earth and the Moon pulls them toward each other, causing the Moon to orbit around the Earth.

13. Strength of Gravity Depends on: The mass of the objects.
More mass = more attraction (more gravitational force)
The distance between the objects.
Objects that are closer together = more attraction (more gravitational force)

14. Mass vs. Weight Mass – the amount of “stuff” in an object.
Weight – A measure of the force of gravity on an object. (i.e., how much an object is pulled toward another object because of gravity)

15. Mass vs. Weight Examples
Your mass is how much matter you are made out of.
Your weight is how much Earth’s gravity is pulling you down (toward Earth).

16. 2. Centripetal force
The force that causes objects to move in a circular path.
Example #1: water in a bucket demonstration
Example #2: the moon revolves around the Earth.

17. Centripetal Force TWO forces are involved:
ONE force pushes the object in a forward motion.
ANOTHER force pulls the object toward another object due to gravity.
When you add the two forces, you get a circular motion.

18. 3. Force of Friction
The force that opposes motion between two surfaces that are in contact with each other.

19. Two Kinds of Friction
Static Friction – friction that prevents the movement of the two objects rubbing against each other.
Sliding Friction – friction that occurs between two surfaces rubbing against each other.

20. Practice Problem #1
Ms. Mac tries to push a table across the classroom, so she applies a force to the table. The table pushes an equal and opposite force against Ms. Mac.
Does the table move?
Are the forces “balanced forces” or “unbalanced forces”?
Is this an example of the “force of gravity”, “centripetal force”, or the “force of friction”?

21. Practice Problem #2
Ms. Mac swings a bucket full of water around in a circle.
What forces are acting on the bucket of water?
Are the forces “balanced forces” or “unbalanced forces”?
Is this an example of the “force of gravity”, “centripetal force”, or the “force of friction”?

22. Practice Problem #3
Ms. Mac jumps out of a plane (with a parachute of course) and is falling through the air.
What forces are acting on Ms. Mac?
Are the forces “balanced forces” or “unbalanced forces”?
Is this an example of the “force of gravity”, “centripetal force”, or the “force of friction”?