1. DAY 19 LETS GO! THE END OF THE QUARTER Topic 3 Forces continues
Exam next class

2. END OF QUARTER EXAM
Purpose: A summary of the class since the last exam
Major ideas from the ungraded work, HW, labs, demos
Occurring: next class
Topics: All on website: Videos as well

3. I LOVE THIS QUESTION (because…)
All that matters is the time in air. Which direction controls time in air? Y!
So which shot has less Y velocity?
The one to B, because it did not go as high.

4. Remember
Remind yourself of each of Newton's Laws AND what they mean.

5. Remember
Remind yourself of what WEIGHT, NORMAL FORCE, INERTIA, and BOTH KINDS OF FRICTION. And the units for them.

6. And it depends if you go DOWN or UP….
ELEVATORS
In an elevator, you feel lighter/heavier in your knees at the start/end of the elevator ride.
And it depends if you go DOWN or UP….

7. Elevator going UP at a constant velocity:
LETS TRY
Elevator going UP at a constant velocity:

8. Elevator going UP and speeding up
LETS TRY
Elevator going UP and speeding up

9. Elevator going UP and slowing down
LETS TRY
Elevator going UP and slowing down

10. Elevator going DOWN at constant velocity
LETS TRY
Elevator going DOWN at constant velocity

11. Elevator going DOWN and speeding up
LETS TRY
Elevator going DOWN and speeding up

12. Elevator going DOWN and slowing down
LETS TRY
Elevator going DOWN and slowing down

13. 6) An elevator lifts a panda up at an upwards acceleration of 2m/s2.
a) Which way is the imbalance? How can you tell?
b) Which is larger then, the up force of the elevator cable or the weight?

14. 6) An elevator lifts a panda up at an upwards acceleration of 2m/s2.
c) Draw a FBD of the panda, call the elevator force the normal force, the panda's mass M.

15. 6) An elevator lifts a panda up at an upwards acceleration of 2m/s2.
d) Create a net force equation for the panda's situation.
e) If the elevator had 600N of force up, how much mass was the panda?

16. The Example I do
A force of 20N is needed to overcome the frictional force of 5N and accelerate a 3kg mass across the floor. What is its acceleration?

17. YOU TRY…
A 10kg mass is moving to the right at 6m/s, and you are pulling it right with a force of 20N. Friction is present.
A) If you want the mass to come to a stop in 12 seconds, what acceleration would it have?

18. YOU TRY…
A 10kg mass is moving to the right at 6m/s, and you are pulling it right with a force of 20N. Friction is present.
B) How much friction is required to stop it after 12 seconds even though you are pulling it.

19. MY Example
A block of 2kg is moving at a velocity of 3m/s. The coefficient of kinetic friction is .2. If it is moving left, and you push right with a force of 6N, what is the acceleration of the object?

20. Example
A 7kg block is on a rough surface, and is being pulled at an angle.
1) Draw a FBD with X and Y split up

21. Example
A 7kg block is on a rough surface, and is being pulled at an angle.
2) Is the normal force more, less or equal to the weight?

22. Example
A 7kg block is on a rough surface, and is being pulled at an angle.
3) If it was moving right, could the net force be left?

23. FORCES AT ANGLES AGAIN
A force on a surface will mean that the angled force will have an effect on the normal force.
You use Sin and Cos to find the X and Y parts of the force
If there are other forces in the Y direction, then the normal force does NOT equal the weight

24. LETS DO ONE
You pull a cat again at an angle. There is friction opposing your pull.
1) Draw a FBD and then another with the X and Y components

25. Ff Weight Fn Pull in Y Pull Pull in X
Pull Y Force of person on the cat vertically
Pull X: Force of person on the cat horizontally
Pull: Force of person on the cat
Weight: Force of Earth on the cat
Fn: Force of ground on cat Fn
Ff: Force of ground on cat
Pull in Y
Pull Ff
Cat
Pull in X
Weight

26. QUESTIONS 1) Which is larger, the normal force or the weight?
Pull Y Force of person on the cat vertically
Pull X: Force of person on the cat horizontally
Weight: Force of Earth on the cat
1) Which is larger, the normal force or the weight?
The weight, since the cat is in equilibrium in Y. And the weight is balanced by the normal force AND the Y pull.
Fn: Force of ground on cat Fn
Ff: Force of ground on cat
Pull in Y Ff
Cat
Pull in X
Weight

27. QUESTIONS
Pull Y Force of person on the cat vertically
Pull X: Force of person on the cat horizontally
Weight: Force of Earth on the cat
2) If you pull at a larger angle, will the weight or the normal force change?
The weight will NOT, since it’s the Earths pull on the cat. The normal force WILL, since you are still in equilibrium in Y, your larger pull in Y will decrease the normal force needed from the table.
In other words…
Fn: Force of ground on cat Fn
Ff: Force of ground on cat
Pull in Y Ff
Cat
Pull in X
Weight

28. QUESTIONS
Pull Y Force of person on the cat vertically
Pull X: Force of person on the cat horizontally
Weight: Force of Earth on the cat
3) If the cat was moving at a constant velocity, would increasing the angle change the friction?
YES! Since friction depends on normal force (Ff = µFnormal) then more of a Y pull will decrease the normal force, resulting in less friction
In other words…
Fn: Force of ground on cat Fn
Ff: Force of ground on cat
Pull in Y Ff
Cat
Pull in X
Weight

29. QUESTIONS
Pull Y Force of person on the cat vertically
Pull X: Force of person on the cat horizontally
Weight: Force of Earth on the cat
4) Say that the coefficient of friction is µk and pulled at angle θ. Lets create 2nd Law equations for X and Y
X: Lets say the acceleration will be with the pull (my guess) so +. There is an imbalance then.
X: F1x – Ff = imbalance
Y: No imbalance, call down –
Y: -Weight + FNormal = equilibrium
Fn: Force of ground on cat Fn
Ff: Force of ground on cat
Pull in Y Ff
Cat
Pull in X
Weight

30. Y: -Weight + FNormal = equilibrium Y: -mg + FNormal = 0
QUESTIONS
X: F1x – Ff = imbalance
X : F1Cosθ – µFnormal = ma
Y: -Weight + FNormal = equilibrium
Y: -mg + FNormal = 0

31. SUMMARY
When changing the angle of a force on a surface, the normal force changes.
Which changes the friction as well.
A downward push results in MORE normal force, an upward push results in LESS normal force.
Friction is the coefficient of friction times the normal force. Use static for non-moving, kinetic for moving

35. LETS TRY A FEW AP PROBLEMS
Alone, with a formula sheet. Just the first, please.

38. What this Reinforced Static friction used to get things to move.
Friction cannot make things speed up, so if friction is a larger force AND will cause a speed up, we have issues.
We needed to solve for the normal force using Y info to put in X

39. Lets use our knowledge to define a term….

42. WHAT THIS REINFORCED
25: Increasing the angle in this case is taking the burden of the weight off the normal force, so it decreases.
26: Increasing the angle will result in less forward force, which will lead to a smaller acceleration.

43. Draw the FBD from 40.

45. Now its different….What words have changed everything?

48. The 3 Y forces are added together
The 3 Y forces are added together. 2 are negative because they are opposite the imbalance direction.