1. Forces Vocabulary Force Newton (unit) Contact Force Field Force
Free Body Diagram (FBD)
Weight
Normal Force
Tension
Net Force
Balanced Forces
Unbalanced Forces
“Boating on the Seine” (1879)
Pierre-Auguste Renoir
This topic can be found in your textbook on pp

2. A force is a push or a pull.
What is a force?
A force is a push or a pull.

3. The SI unit for force is a Newton (N).
What is a force?
The SI unit for force is a Newton (N).

4. Types of forces Contact Forces
Force due to physical contact between objects.

5. Types of forces Field Forces
Forces that act at a distance with no contact (e.g., gravity and electromagnetism.)

6. Forces are Vectors
Forces have both magnitude and direction.

7. Forces are Vectors
Pushing on the left…

8. Forces are Vectors
has a different effect than pushing on the right…

9. Forces are Vectors
which has a different effect than pulling it upward…

10. Forces are Vectors
or pushing it downward.

11. Forces are Vectors
Arrows are used to indicate the direction of forces.
The length of the arrow indicates the relative magnitude of the force.
small force to the left
large force to the right

12. Free Body Diagrams
We use what are called free body diagrams (FBD) to show all of the forces that are acting on an object.
Object are represented as rectangles and each force acting on the object is represented by an arrow

13. Free Body Diagrams Example 1:
Draw a FBD for a moving car. What forces need to be included?
force of engine
force of air resistance
force of friction
force of gravity
(called “weight”)
force of the ground (this
is the “normal force”) Fn Fa Fe Ff Fg

14. Free Body Diagrams Example 2:
Draw a FBD for Batman hanging on the end of a rope. What forces need to be included?
Force of gravity (weight)
Force of rope
(called “tension”) FT Fg

15. Free Body Diagrams
Example 2 (continued): If there are forces acting on Batman, why isn’t he moving? The forces are balanced! FT Fg

16. Balanced vs. Unbalanced Forces
Balanced forces are forces acting on an object that are equal and opposite. This means that the net force (the forces added together) is zero.
Balanced forces always cause an object to keep doing what it’s doing (stay at rest or continue moving at constant velocity, but more about this later.)

17. Balanced Forces Fnet = ΣF = Fwings + Fgravity = 0 N
Example: Hovering bird
Fwings
Wings push the bird up with the exact same force that gravity pulls the bird down. Fnet = 0 N
Fgravity
Fnet = ΣF = Fwings + Fgravity = 0 N

18. Balanced vs. Unbalanced Forces
Unbalanced forces occur when the net force on an object is not zero.
Unbalanced forces always cause an object to accelerate (but more about this later.)

19. Unbalanced Forces Fnet = ΣF = Fair res + Fgravity < 0 N
Example: Bungee jumper just after jumping
Fair resistance
Fnet = ΣF
= Fair res + Fgravity
< 0 N
Fgravity

20. Finding Net Forces
To find net forces, the forces need to be broken into x-direction forces and y-direction forces. Then the net x-direction force and the net y-direction force can be found separately.

21. Finding Net Forces
Example: Draw a free body diagram of Joy Rider’s 1200 N sled being pulled on level ground with a force of 240 N and a friction force of 15 N.
1200 N
15 N
240 N
1200 N

22. Finding Net Forces Fnet,x= ΣF = Fpull + Ffric = 240N + (-15N) = 225 N
Example: What are Fnet,x and Fnet,y for Joy’s sled?
Fnet,x= ΣF
= Fpull + Ffric
= 240N + (-15N)
= 225 N
1200 N
15 N
240 N
1200 N

23. Finding Net Forces Fnet,y= ΣF = Fgrav + Fnormal = 1200N + (-1200N)
Example: What are Fnet,x and Fnet,y for Joy’s sled?
Fnet,y= ΣF
= Fgrav + Fnormal
= 1200N + (-1200N)
= 0 N
Which way will the sled accelerate?
1200 N
15 N
240 N
1200 N

24. Force Summary Equation Summary Fnet = ∑F Fnet,x = ∑Fx Fnet,y = ∑Fy
Measurement
Symbol
SI Unit
force F N
Equation Summary
Fbuoyant
Fnet = ∑F
Fnet,x = ∑Fx
Fnet,y = ∑Fy
Fpaddle
Fdrag
Fgravity