1. Forces and Free Body Diagram Notes

2. What is a force and how is it illustrated?
A force is a push or a pull.
The metric unit: Newtons.
A force is usually shown as a vector, which includes both magnitude (size) and a direction.
head
tail

3. These measurements have a size and direction:
Vector vs Scalar
These measurements have a size and direction:
Vector Quantities
These have only a size:
Scalar Quantities
Postion (displacement)
10 meters South
Velocity
10 m/sec South
Force!
10 Newtons South
Also: Acceleration
Distance
10 meters
Speed
10 m/sec
Also: Temperature, Time, etc.

4. Spooky At a Distance Forces
Types of Forces
Spooky At a Distance Forces
Contact Forces
Friction
Tension
Normal Force
Air Resistance
Applied Force
Gravity
Electrical
Magnetic

5. Force of Gravity AKA: weight
Caused by the gravitational pull of a planet (LARGE object).
To calculate weight:
multiply mass times acceleration of gravity Fg = m x g
Basically, that’s just F=ma!
On Earth, g = 9.8 m/s2

6. Weight and Mass are NOT the Same
Amount of matter an object has
The same everywhere
Measured in kilograms (kg)
Weight:
Force of gravity pulling on the object
Different on different planets
Measured in Newtons (N)

7. Mass and Weight: Example
10 kg
m = 10 kg
Fg = 98 N weight
m = 10 kg
Fg = 16 N weight
g = 1.6 m/s2
g = 9.8 m/s2
G =

8. What is a free-body diagram?
A free-body diagram illustrates the relative magnitude and direction of all forces acting upon an object.
Dog sitting on chair FN
dog Fg

9. Normal Force
Normal Force (FN): the force the chair exerts on the dog. Any time the object is on a surface there will be a normal force! FN Fg

10. Net Force Net Force: sum of all forces acting on an object.
Net force is zero if all forces cancel.
We also say “No net force” or EQUILIBRIUM!
If all forces don’t cancel,
We say there is a
“Net force”
But, if there is a
“Net force”
the object is:
So, if there is
“No net force”
the object is:
Not moving or
Moving with
constant velocity
Accelerating

11. Example:
Net Force? FN
FNet = 0
No Net Force!
The dog is not moving Fg

12. Example: tennis ball falling into commons
Net Force? Fg
FNet ≠ 0
NOT ZERO!
Accelerating

13. Tension
Tension: force exerted by a string or rope supporting an object. FT Fg

14. Air Resistance
Air Resistance: Force opposing the motion of an object falling in air.
Fair Fg

15. Air Resistance FNet ≠ 0 If the frog is accelerating as it falls. FairFg

16. Air Resistance FNet = 0 If the frog is falling at a constant velocity.
Fair
FNet = 0 Fg

17. Friction
Friction: Force opposing the motion of an object sliding against a surface. Fa Ff
Applied Force (FA): Someone applying a force on the object.

18. Friction
The force of friction between the desk and the floor is 100 N. To keep the desk moving at a constant velocity you need to push with a force of : Ff Fa