1. Let’s begin with some skydiving movies.
Types of Forces
Let’s begin with some skydiving movies.

2. Gravity Downward force one of two forces where no contact is required
Magnitude depends on the mass of objects and distance between them

3. Friction The force that acts to resist the motion of objects that are in contact

4. Friction
Friction is the FORCE that RESISTS the MOTION between objects.

5. Friction
Your teacher (or a volunteer) is going to push a book across the table.
Predict what is going to happen to the motion of the book when the person stops pushing it.
Why do you think the book moved the way it did after the person stopped pushing it?

6. Friction
The force of friction when objects are not yet sliding past one another is called STATIC FRICTION.
The force of friction when objects are sliding past one another is called KINETIC FRICTION.

7. Friction Does FRICTION depend on the TYPE OF SURFACE?
Does FRICTION depend on WEIGHT?

8. Air Resistance ① Definition
Air resistance is the FORCE that OPPOSES the motion of another object through the air.

9. Predict Observe Explain ② ③ ④
Your teacher (or a volunteer) is going to drop
two pieces of paper: one piece of paper is flat
and the other is crumpled up.
Which piece of paper is going to hit the ground first? ②
Predict ③
Observe ④
Explain

10. ⑤
Draw arrows and label the force of gravity (Fg) and the force of air resistance (Far)
Far Fg

11. ⑥
Terminal Velocity
Terminal velocity is the CONSTANT velocity (speed) that a falling object reaches when GRAVITY and AIR RESISTANCE are balanced.

12. ⑧
Draw arrows and label the force of gravity (Fg) and the force of air resistance (Far).
Far
ANIMATION Fg

13. How would skydiving be different on the Moon?Fg

14. ⑨ The Moon has no ATMOSPHERE (no air).
Therefore there is NO air resistance on the Moon.

15. ⑩
Empty space (a place with no air)
is called a VACUUM.

16. ⑪
What will happen if a flat piece of paper and crumpled piece of paper
are dropped at the same time on the Moon? Will one piece land first?

17. ⑫
FREE FALL is the name given to objects falling that experience the force of gravity but no air resistance.
Simulation of Galileo’s free fall experiment...ignoring air resistance.

18. NORMAL FORCE ⑬ The symbol for NORMAL FORCE is FN
Normal force is a force (usually a push) that
is PERPENDICULAR to the surface of contact.
The symbol for NORMAL FORCE is FN

19. Perpendicular means ⑭ AT RIGHT ANGLES or AT 90 DEGREES.
DEFINITION:

20. Push against a wall. Draw an arrow and label the normal force⑮
Push against a wall. Draw an arrow and label the normal force FN

21. The force a table exerts on a book.FN

22. ⑯ Comparing normal force to friction:
Friction force is PARALLEL to the contact surface.
Normal force is PERPENDICULAR to the
contact surface.

23. ⑰
Tension Force
The pull of a ROPE or STRING is called a TENSION force.
The symbol for the force of tension is FT

24. ⑱ Spring (Elastic) Force The pull or push of a SPRING, RUBBER BAND, or
BOUNCY BALL is called SPRING FORCE.
The symbol for the spring force is Fs

25. Now let’s try Identifying Interactions in Realistic Situations