1. Working with Forces
Gravity, Apparent Weight & Hooke’s Law

2. Force of Gravity

3. Force of Gravity: Any two objects will feel a force of attraction that depends on their masses and distance apart. Mass (kg): The amount of matter in an object. Weight (N): The amount of gravitational force felt by an object. Mass is constant throughout the universe whereas weight changes depending on where you are.

4. Fg = mg The formula for force of gravity is: Where: m = mass (kg)
g = acceleration due to gravity (m/s2)
= gravitational field strength

5. g varies depending on the size of the planet and distance to the object
For Example:
On Earth at sea level g = 9.80 m/s2
On the moon’s surface g = 1.60 m/s2
On Jupiter g = 24.5 m/s2
The Sun g = 274 m/s2
Determine your weight (in Newtons) on Earth, the moon and Jupiter.

6. The Elevator Problem

7. When a person is accelerating upwards or downwards they can sometimes feel heavier or lighter than they actually are. Although their actual weight (force of gravity) is the same, their apparent weight differs. Apparent weight (how heavy we feel) is equal to the normal force supporting us.

8. Consider a student jumping off of a desk
a) What is their apparent weight while standing on the desk? Fg = FN so they feel their normal weight

9. Consider a student jumping off of a desk
b) What is their apparent weight while in the air? FN = ZERO, so they feel WEIGHTLESS!!!

10. Consider a student jumping off of a desk
c) What is their apparent weight when they hit the ground? Their acceleration is upwards and so is their net force, therefore… …FN > Fg and they feel HEAVIER than normal.

11. The same thing can happen elevators
The same thing can happen elevators. Ex 1: A 65 kg person in an elevator is traveling upwards at 5.0 m/s. What is their apparent weight? Since a = 0, then Fnet = 0 and so Fg = FN

12. Ex 2: The same 65 kg person is in an elevator that accelerates upwards at 4.9 m/s2. What is their apparent weight? When the net force is upwards, FN > Fg

13. Ex 3: The elevator reaches the top floor and decelerates at 4. 9 m/s2
Ex 3: The elevator reaches the top floor and decelerates at 4.9 m/s2. What is their apparent weight? When the net force is downwards, FN < Fg

14. In reality they are constantly falling towards the Earth.
When objects are in “free fall” there is no normal force acting on them. During this time they will feel totally weightless.
This is why astronauts orbiting the earth seem to float as though gravity does not affect them.
In reality they are constantly falling towards the Earth.
In order to prepare themselves for this experience astronauts train on a plane affectionately named “The Vomit Comet”

15. Part A: Elevator Is At Rest.
You have just boarded the elevator, so it (with you inside) is at rest... Question 1: What does the scale read?
Question 2: If you let go of the apple, what does it do?

16. Part B: The Elevator Accelerates Upward.
The elevator, (with you inside) begins to accelerate upward from rest at 2 m/s2. Complete the FBD!
Question 3: What will the scale read now?
Question 4: If you let go of the apple now, what does it do?

17. Part C: Elevator Moves Up With Constant Velocity
It now moves with this constant upward velocity of 10 m/s.
Question 5: What does the scale read now?
Question 6: If you let go of the apple, what does it do?

18. Part D: The Elevator Slows Down (While Going Up)
Now it begins to slow down as it reaches the top. Its acceleration (or deceleration) is 2 m/s2 downward.
Question 7: What does the scale read now?
Question #8: If you let go of the apple now, what does it do?

19. Part E: The Elevator Speeds Up (While Going Down)
The elevator stops for a while, and then begins to accelerate downward. Its acceleration is 2 m/s2 downward. Question 9: What does the scale read now?
Question #10: If you let go of the apple now, what does it do?

20. Part G: Oh, No. The elevator cable snaps, and the elevator falls
Part G: Oh, No! The elevator cable snaps, and the elevator falls! You have time for one last Physics observation! Question 11: What does the scale read as the elevator falls? Question 12: If you let go of the apple now, what does it do?

21. Hooke’s Law

22. FElastic: The force that works to return a distorted object to its equilibrium (rest) position.

23. Hooke’s Law: The size of an elastic force is proportional to the distortion of the object.
FE = kΔx

24. Where: k (N/m) = the spring constant The stiffer the spring the higher it is Is constant for given material. Δx (m) = distortion from equilibrium

25. Ex: A student stretches an elastic band with a spring constant of 50
Ex: A student stretches an elastic band with a spring constant of 50.0 N/m by 15 cm. How much force are they applying?

26. Ex: Al McInnis uses a wooden stick with a spring constant of 850 N/m
Ex: Al McInnis uses a wooden stick with a spring constant of 850 N/m. What is the distortion on the stick if he exerts 525 N while taking a slapshot?

27. Ex: A 65 kg girl sits in a redneck sling shot that has a spring constant of 10.5 N/m. If the sling is stretched by 45 m, what is her initial acceleration when released?