1. Common Forces

2. Force of Gravity Recall g = 9.8 m/s2 [downward]
and so Fg = m g = m (9.8 m/s2)
where m = mass (kg)
This holds near earth’s surface
as the distance from earth changes the value of “g” changes so this formula doesn’t hold true.

3. Universal Gravitation
Every object in the universe exerts a gravitational force of attraction on every other object in the universe.

4. Universal Gravitation
Every object in the universe exerts a gravitational force of attraction on every other object in the universe. M m Fg

5. Universal Gravitation
Every object in the universe exerts a gravitational force of attraction on every other object in the universe.
The amount of force depends on: m M Fg Fg

6. Universal Gravitation
Every object in the universe exerts a gravitational force of attraction on every other object in the universe.
The amount of force depends on:
the distance between the objects and m M Fg Fg

7. Universal Gravitation
Every object in the universe exerts a gravitational force of attraction on every other object in the universe.
The amount of force depends on:
the distance between the objects and
the masses of the objects. m M Fg Fg

8. Universal Gravitation
Equation Fg = GmM / R2
m = mass of one object(kg)
M = mass of other object(kg)
G = 6.67 x 10-11Nm2/kg2(universal constant)
R = distance between objects (m)
R is measured center to center! M m Fg

9. Universal Gravitation
Equation Fg = GmM / R2
m = mass of one object(kg)
M = mass of other object(kg)
G = 6.67 x 10-11Nm2/kg2(universal constant)
R = distance between objects (m)
R is measured center to center! M m Fg

10. Friction Friction is a force that always opposes motion.
The force of friction occurs whenever
- there are 2 or more objects
- in contact and
- one object moves compared to the other
- or there is a tendency for motion.

11. Friction
The frictional force always acts in the direction opposite to the motion
or opposite to the tendency for motion.

12. 2 Types of Friction
1) Static Friction
2) Kinetic Friction

13. 1) Static Friction (from the word stationary)
Occurs when there is no motion between the objects
but the there is a tendency for motion.
i.e. there would be motion except friction is preventing motion from occurring.

14. 1) Static Friction - examples
a person standing still on a sloped roof
trying to slide a large appliance across a floor but it just won’t move
trying to push a stopped car with the brakes on.

15. 2) Kinetic Friction (or moving friction)
Occurs when there is motion between the objects.
i.e. one object slides across another object

16. 2) Kinetic Friction - examples
sliding a cheeseburger across the counter at Wendy’s
brake pads sliding on the metal wheel of a bicycle
a hockey puck sliding across ice
the tires of a car sliding across the pavement with the brakes locked and the wheels stopped.

17. The Amount of Friction depends primarily on two factors:
the smoothness/condition of the surfaces in contact and
the amount of force squeezing the surfaces together.

18. Formula for Friction Ff = N Ff = amount of friction in Newtons
 = the coefficient of friction
 is a number that represents the smoothness of the surfaces
N = the “Normal” force in Newtons
N is the amount of force squeezing the surfaces together

19. FBD showing Frictional Force
Object

20. FBD showing Frictional Force
Object
Table top

21. FBD showing Frictional Force
Object
Table top

22. FBD showing Frictional Force
Object
Table top
Force of
Gravity

23. FBD showing Frictional Force
Object
Table top
Force of
Gravity

24. FBD showing Frictional Force
Normal
Force
Object
Table top
Force of
Gravity

25. FBD showing Frictional Force
Normal
Force
Object
Table top
Force of
Gravity

26. FBD showing Frictional Force
N = Fg
Normal
Force
Object
Table top
Force of
Gravity

27. FBD showing Frictional Force
N = Fg
Normal
Force
Object
Table top
Force of
Gravity

28. FBD showing Frictional Force
N = Fg
Normal
Force
Applied
Force
Object
Table top
Force of
Gravity

29. FBD showing Frictional Force
N = Fg
Normal
Force
Motion
Applied
Force
Object
Table top
Force of
Gravity

30. FBD showing Frictional Force
N = Fg
Normal
Force
Motion
Applied
Force
Object
Table top
Force of
Gravity

31. FBD showing Frictional Force
N = Fg
Normal
Force
Motion
Force of
Friction
Applied
Force
Object
Table top
Force of
Gravity

32. FBD showing Frictional Force
N = Fg
Normal
Force
Ff =  N
Motion
Force of
Friction
Applied
Force
Object
Table top
Force of
Gravity

33. Coefficients of Friction
no units for the coefficient of friction
there are actually 2 coefficients of friction for every possible pair of surfaces
one coefficient for static friction
and one coefficient for kinetic friction
the symbols s and  k are used

34. Coefficients of Friction
For example
when rubber is in contact with asphalt
the  values are:
s = and  k = 0.90
the static coefficient is always greater than or equal to the kinetic coefficient!
It is harder to get an object to start moving than it is to keep it in motion.

35. Applications of Friction
Friction is a very important force that must be considered.
Friction can be helpful to us in many situations
and in other situations friction can be a real problem that must be dealt with.
When friction occurs usually heat is produced. Energy is being changed into heat energy!

36. Friction - Examples (helpful)
walking or running
brake pads
tires on the road
standing on a roof
turning a doorknob

37. Friction - Examples (hindrance)
metal parts inside a motor rubbing together
air resistance
pushing heavy objects across a floor
Many times we use lubricants or other means to reduce the amount of friction in situations where the friction is a burden to be overcome.