1. Forces and Free Body Diagrams
Physical Science

2. Types of Force Contact Forces Friction Normal force/ support force
Tension force
Applied force

3. Friction-Ff Force is opposite direction of motion Surface and weight
Always parallel to surface
Air drag

4. Normal Force/ Support Force
Fn or Fs
force exerted upon an object which is in contact with another stable object
Perpendicular force

5. Tension Force
Ft: force transmitted through a string, rope or wire

6. Applied Force
Fa : force which is applied to an object by a person or another object
Ex: pushing a desk

7. Action at Distance 1. magnetism 2. electrostatic 3. Gravity
Fg : force at which the earth, moon or other objects attract another object towards itself

8. Free Fall: the constant acceleration of an object moving only under the influence of gravity is g
The acceleration caused by gravity is 10 m/s2
If there was no air, all objects would fall at the same speed
Doesn’t depend on mass
After 1 second falling at 10 m/s
After 2 seconds 20
m/watch?v=KDp1tiUsZ w8

10. Force of gravity is weight
All objects experience a force of gravity downward towards the center of the earth
Force of gravity is weight
What’s the difference between mass and weight?

11. 800 N Gravity On the earth, a person’s weight is 800 N.
I love physics!
800 N

12. Gravity
On the moon, his weight is around 130 N.
Why?
130 N

13. Calculating weight (in Newtons not Pounds!!)
To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are.
Weight (N) = mass (kg) x gravitational field strength (N/kg) Or
W= m x g
Fg = m x g
Common gravity values:
On Earth:
g = 9.8 m/s/s
or about 10 m/s/s
On the moon:
g = 1.67 m/s/s

14. Examples w = m x g 1. The force exerted by gravity on 1kg=
3. The force exerted by gravity on ____kg= 98 N
4. The force exerted by gravity on 70kg= ________N
1. The force exerted by gravity on 1kg=
2. The force exerted by gravity on 5 kg = _____ N

15. Draw force diagrams for the following:
If an object’s net force is zero, then it is either A or B.
If A. The object is not moving or at rest on a surface.
(So if an object is being supported on a surface the weight(Fg) and the normal force (Fn) are always the same!)
If B. It is moving at constant speed.
If on a surface:
(So if an object is at constant speed on a surface then the force applied (Fa) and the force of friction are always the same!)
If not on a surface:

16. Practice Problem 1 w= mg A 55kg person is sitting on a chair.
a. What is their weight (in Newtons!)
b. What is the normal force (support force)?
c. Draw a force diagram showing the forces.
d. What is the net force on the block?

17. Practice Problem 2
A block with a mass of 6 kg is being dragged across a table at a constant speed. It requires 3N of force to drag it because of friction. Draw a force diagram showing the forces involved. What is the net force on the block?

18. Free Body Diagrams
Show magnitude and direction of all forces acting on an object