1. Unit 3: Forces
Lesson 1: Gravity

2. The Force of Gravity “The attraction between masses”
“Gravitational field strength”
On Earth: F=mg
g = 9.8 N/kg
m = mass (kg)
Note:
the acceleration due to gravity = the gravitational field strength
mass = amount of matter; weight = force of gravity
10 min
E.g. Find the weight of a 70 kg man.
F = mg
= (70)(9.8)
= 686 N

3. Try This! A 100 kg astronaut weighs 620 N on planet X. Find:
a) the gravitational field strength.
b) the weight of the 6000 kg space ship on planet X.
c) the weight of a “moon-rover” that weighs 5000 N on Earth.
15 min

4. Brain Break!

5. Called the “inverse square law”
What Affects Gravity? M1 M2 F d
Called the “inverse square law”
Any two objects attract each other!
The force of attraction (gravity) depends on
the mass of both objects
the distance between the centers of the objects
5 min
F ∝ m (directly proportional)
F∝ (inversely proportional to the square of the distance)

6. Using the Inverse Square Law
A spaceship 100,000 km from planet X experiences a gravitational pull of 50 N. Find the force of gravity when the ship is
a) 50,000 km away from the planet’s center.
b) 500,000 km away from the planet’s center.
10 min

7. 6400 km above Earth’s surface. 100 km above the ground.
2) A 500 kg meteor approaches Earth (which has a radius of 6400 km). Find the force of gravity when the meteor is
6400 km above Earth’s surface.
100 km above the ground.
100km
6400km
6400km
10 min

8. Homework
Gravity Problem Set #1 and 6

9. Lesson 2: Newton’s Law of Gravity
Unit 3: Forces
Lesson 2: Newton’s Law of Gravity

10. Newton’s Law of GravityM1 M2 F d
10 min
E.g. #1: Find the gravitational force of attraction between a 50 kg girl and 60 kg boy 1.5 m apart.

11. E.g. #2: Find the mass of planet Earth, given that R = 6380 km and g = 9.8 N/kg.
m=1kg
10 min

12. Brain Break!

13. E.g. #3: (Try It!) A rock is dropped on planet X and it accelerates at 19.3 m/s2. Planet X has a radius of 9500 km.
Find: a) The weight of a 1300 kg space pod on planet X.
b) The mass of a 135 kg astronaut on planet X.
c) The mass of planet X.
F=mg
15 min c)

14. Homework
Work on the Gravity Problem Set (you can now do #2-5, and #7 if you’re up for a challenge).
20 min

15. Lesson 3: Gravity Practice
Unit 3: Forces
Lesson 3: Gravity Practice

16. Work Block
Here are your priorities: 1. Gravity Problem Set 2. Velocity & Time Lab 3. Work from other classes (such as Chemistry)
60 min

17. Lesson 4: Elastic Force Lab
Unit 3: Forces
Lesson 4: Elastic Force Lab

18. Unit 3: Forces
Lesson 5: Hooke’s Law

19. Hooke’s Law Hooke’s Law: F=kx k = spring constant (N/m)
F = force needed to stretch/compress an elastic object
x = length of extension from relaxed state
F (N)
x (m)
Equation: y=mx
5 min
This is a constant for a given spring, indicating its stiffness.
Hooke’s Law: F=kx k = spring constant (N/m)

20. Try This: A force of 27 N extends a spring 1.5 m.
Find: a) the spring constant.
b) the force needed to extend it 2.0 m.
c) the extension length from a force of 62 N.
d) the extension length if a mass of grams is hung vertically from the spring.
F = kx
k = F/x = 27/1.5
k = 18 N/m
b) F = kx
= 18 x 2.0
= 36 N
c) F = kx
x = F/k
= 62/18
= 3.4 m
d) F = mg = (0.5) (9.8)
= 4.9 N
F = kx
x = F/k
= 4.9/18
= 0.27 m
= 27 cm
20 min

21. Brain Break!

22. You may use the rest of class to work on your Elastic Force Lab report.
30 min

23. Unit 3: Forces
Lesson 6: Friction

24. Back Front Ashley Amira Jacob Z McKenna Travis Carli Finley Anna Nate
Aidan
Kally
Clare
Taylor
Annie
Jacob L
Dani
Chase
Balance
Back

25. Friction Force
“resistance caused by the rubbing of 2 surfaces together”
Coefficient of friction ()
is a measurement of the “roughness” of a surface
f = friction force
N = normal force
The normal force is the force that acts perpendicular to the two surfaces. On a horizontal surface, the normal force is equal to the force of gravity.
10 min N mg

26. Important note: When an object is at rest or moving with uniform motion, the forces acting on it are balanced.
E.g. A box of mass 10 kg is pulled at a constant speed by a force of 45 N. Find the coefficient of friction. N
10 min f
45 N mg

27. Brain Break!

28. Static and Kinetic Friction
Static friction acts on an object at rest.
Kinetic friction acts on a sliding object.
10 min

29. E.g. A 600 kg fridge sits on a tile floor. s = 0.7 and k = 0.5. P
(push) mg
E.g. A 600 kg fridge sits on a tile floor. s = 0.7 and k = 0.5.
Find the force needed: a) to just get it moving.
b) to keep it moving at 1.2m/s.
10 min

30. Practice
Pg. 88 #1-3
Pg. 92 #3-6
10 min

31. Unit 3: Forces
Lesson 7: Friction Lab

32. Unit 3: Forces
Lesson 8: Catch-Up

33. Work Block
Options: 1. Elastic Force Lab 2. Friction Lab 3. Friction practice: pg. 88 #1-3 and pg. 92 # Forces review: pg. 98 #1-9
60 min

34. Unit 3: Forces
Lesson 9: Review