1. Forces
An Introduction

2. Force Force (F) A push or pull
copy
Force (F)
A push or pull
Causes objects to change their motion and/or shape
Is a vector
Is measured in newtons (N)
1 N = 1 kg∙m/s2

3. Try it out…. Mini Investigation: page 113 Two spring scales
One 100 g object
One 200 g object

4. Four Types of Forces 1. Gravitational Force
copy
1. Gravitational Force
- known as “force of gravity” or “weight”
- attraction ONLY
- acts between all objects in the universe
- weak force but long range
Electromagnetic Force
- caused by electric charges
- most common force (i.e. light, electricity, magnetic attraction…)
- strong force and long range

5. 3. Strong Nuclear Force
- holds protons and neutrons together in the nucleus of an atom
- strongest force but very short range
4. Weak Nuclear Force
- responsible for radioactive decay - strong force but very short range
8/8/2018

6. Mini Investigation Pg 116 Materials: Spring scale
Set of objects of known mass

7. Everyday Forces
copy
Force of Gravity (Fg)  force of attraction between Earth and object
Normal Force (FN)  force perpendicular to surface upon which object rests
Friction Force (Ff)  force that opposes (attempted) motion of object
Applied Force (Fa)  force exerted on object
Tension Force (FT)  force exerted by string or rope on object
Note:
The force of gravity is a non-contact force (i.e. it does not require contact in order to be experienced)
Fg = mg
Free-Body Diagrams (FBD) are simple drawings representing the object and all external forces acting upon it 7

8. Free-Body Diagrams (FBD)
copy
A simplified picture showing all of the forces acting on the object
Examples:
Free-falling object (no air resistance)
A book on a desk
Pulling a wagon with a rope

9. External & Internal Forces
In this course, we will usually be concerned with external forces
External forces are caused by one object pushing or pulling on another
Internal forces occur when an object exerts a force on itself
For example, when skate 1 pushes on skater 2, the force on skater 2 is external
If skater 1pulls forward on her own arms, then it is an internal force

10. Practice Draw a Free-Body Diagram for the following:
A) a cup sitting a rest on a table
B) a large trunk is pulled by a rope to the right side, the trunk does not move
C) a baseball player is sliding to the left across the ground
D) a desk is pushed to the left across the floor

11. Textbook:
Read over pages 114 – 121
Answer #1-5, 7

12. Net Forces Balanced Forces:-
When two forces acting on an object cancel each other out because they are equal in magnitude, but opposite in direction. Net Force = 0 N
When all of the forces acting on an object do not cancel each other out there is an Unbalanced Force or Net Force
 Net Forces
Fnet = F1 + F2 + F3 + …

13. Adding Forces
A Free Body Diagram can be used to construct a vector diagram which can then be used to calculate the net force acting on an object.
Example 1:
An object has two forces being applied to it:
F1 = 120 N [right]
F2 = 50 N [left]
What is the net force acting on the object?

14. Example 2:
An object has two forces being applied to it:
F1 = 9 N [right]
F2 = 3 N [down]
What is the net force acting on the object?

15. Worksheet
Pg #15