1. 12/09 (p.66) Forces- Net force
IQ: Copy the graph that shows the correct relationship between an object’s mass and distance from the earth’s center?

2. What is force?
Force is a push away from the source (towards the object) or pull towards the source (away from the object). It has a tendency to cause a change in an object’s motion or position. Ex. A person pushing a cart, an engine pulling a train, gravity, friction.
Source: stars.eng.usf.edu/.../Force.../Balanced%20and%20Unbalanced%20Forces...‎

3. Force (study jams)
Forces have a magnitude (size or strength) and a direction. Ex.
Magnitude: 5N
Direction: upward
5N, up
or 5N

4. Forces are measured in Newtons
SI unit of force
Symbol: N
Measured by using a spring scale.
Weight is a type of force– measure of force of gravity on an object.

5. Drag the weight on your table. Measure the force in Newtons
Drag the weight on your table. Measure the force in Newtons. Drag the weight on the carpet. Be sure to hold the carpet down, measure the force in Newtons.

6. Free-body or Force diagrams
Free-body or force diagrams are used to show the relative magnitude and direction of all forces acting on an object. A larger arrow indicates a larger force.

7. This diagram shows four equal forces acting upon an object
This diagram shows four equal forces acting upon an object. Although, all four are usually present for most moving objects, there aren’t always four forces, For example, there could be one, two, or three forces.
Fnorm= Normal force (against gravity), Ffrict= Friction, Fapp= Force applied, Fgrav= Gravitational pull

8. Problem 1
A book is at rest on a table top. Diagram the forces acting on the book.
In this diagram, there are normal and gravitational forces on the book.

9. Problem 2
An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

10. Gravity is the only force acting on the egg as it falls.

11. Problem 3
A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance (Fair). A free body diagram for this situation looks like…

12. Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

13. Problem 4
A rightward pull force is applied to a book in order to move it across a desk. Consider frictional forces. Construct a free-body diagram.

14. Note the larger applied force arrow pointing to the right since the book is accelerating to the right. Friction force opposes the direction of motion. The force due to gravity and normal forces are balanced.

15. Problem 5
A skydiver is descending with a constant velocity. Consider air resistance. Draw a free-body diagram.

16. Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

17. Problem 7
A football is moving upwards toward its peak after having been booted by the punter. Draw a free-body diagram. (Neglect air friction)

18. The force of gravity is the only force described. (no air resistance).

19. Resultant/Net force = overall force
If two or more forces are acting in the same direction (helping) at a 180 degree angle, they get added to get the net force.
Copy diagrams:
Net force= 2N + 4N = 6N

20. Which of the following shows the correct equation and net force for the above diagram? A. 200N left – 100N right = 100N left B. 200N right – 100N right = 100N right C. 200N right + 100N right = 300N right D. 200N left + 100N right = 300N left

21. If forces are acting in opposite directions, subtract the smaller force from the bigger force to get the net force which will be in the direction of the bigger force.
Net force = 4N – 2N = 2N right

22. Draw 2 push forces with net force = 0

23. When there are more than 2 forces
To get net force
Add forces in same dir.
Subtract forces in opp. directions
Ex.
7N left, 5N right, 2N left
7N left + 2N left = 9N left
9N left – 5 N right = 4N left
Phet simulation

24. When forces are at 90 degrees angle, subtract the ones at 180 degrees from each other and get net force.
50N – 50N = 0N
20N – 0N = 20N
Net force = 20N

25. Try to solve for net force:
Source:

26. OQ: For the following situation, on the left hand side page, draw a force diagram, write equation and solve for the net force on the ball.
150 N
175 N
Source:

27. Balanced and Unbalanced forces

28. 12/10 (p. 68) Balanced and Unbalanced Forces
IQ: Draw a force diagram and find the net force, writing equations: Three dogs are fighting over a bone. One is pulling to the left with a force of 20N, another to the right with a force of 20N and the third upward with a force of 15N. Who will win?

29. Balanced and Unbalanced Forces
Opposite forces on an object can be either balanced or unbalanced.
Balanced Forces = No acceleration
Unbalanced Forces = Acceleration

30. Balanced Forces
Balanced forces do not cause change in motion or position- no acceleration.
They are equal in size and opp. in dir.
Net force = 0, no motion or change in motion, 0 acceleration.
Ex. 6N, left – 6 N right = 0N
10 N, left – 10N, right = 0N

31. Objects at rest
Even objects at rest have forces being applied on them. The forces are completely balanced, no net force, so no acceleration.

32. Constant speed= balanced forces
Objects moving at constant speed have balanced forces, (no net force, no acceleration) applied on them.
Source:

33. Unbalanced Forces
An unbalanced force always causes a change in motion or position- acceleration. They can make an object
Start moving,
Speed up,
Change direction,
Slow down, and/or
Come to a stop

34. Unbalanced Forces
6N left– 3 N, right = 3N, left

35. Unbalanced Forces
4 N, left – 10 N, right =
6N, right

36. Unbalanced forces resulting in positive acceleration:

37. Calculating Net force:
The block has 2 opposing forces being applied to it:
7N to the right and 4N to the left. They are Unbalanced Forces.
To Calculate the Resultant or Net Force (Fr),
subtract one from the other: 7N, right – 4N, left = 3N to the right
This means that the box will accelerate (either move from rest or get faster) to the right. 3N

38. Forces on moving objects
When will the car go at a constant speed?
When would it be slowing down?

39. Effects of forces Forces on bike (12:30)

40. Forces on a flying object
Which forces will be balanced and/or unbalanced when the plane
Speeds up? Lift & Weight bal., Thrust greater than Drag Unbal.
Slows down?
Flies at a constant speed?
Flies at a constant height?
Rises and speeds up?
Lands and slows down to come to a stop?

41. Forces on a Skydiver

42. 10N A B C
Balanced forces: _______ Unbalanced: _______ Net force: _________________= ______ Object will not move/accelerate in _______ direction.

43. Problem:
A man of mass 75kg has a weight of 750N. This 750N will act downward on the table and the table will exert a 750N force upwards on the man.
The forces on the man are balanced/unbalanced, and there will/will not be any acceleration.
What would happen if the upward force were less than 750N?
Force down 750N
Force up 750N

44. Another way to look at balances and unbalanced forces

45. OQ Draw a force diagram for the following situation and find the net force: A car is going forward with a force of 5000N. The force of friction from the road is 500N and the air resistance is 1000N. Forces are balanced/unbalanced. Car will be accelerating/decelerating/at constant speed.