1. Universal law of Gravitational rotation
Physics
Mr. Villa

2. Weight and Mass
Weight is the force an object applies as a result of gravity pulling it downward. It is directed downward and it varies with gravity.
Mass is a universal constant which is a measure of the matter that makes up an object. It is always constant regardless of location
Because weight is a force created by the downward acceleration of gravity, we can make this substitution.

3. Weight and Mass: Examples
What is the weight of a 10-kg block?
The weight of an object is the force it causes as a result of gravity.
9.8 m/s2 W m
10 kg
W = mg = (10 kg)(9.8 m/s2)
W = 98 N
The weight of an object is also referred to as the force of gravity

4. Always Remember!!
In Physics, the use of Newton’s second law and many other applications makes it absolutely necessary to distinguish between mass and weight. Use the correct units!
Metric SI units: Mass is in kg; weight is in N.
Pounds should never be used!!!
Always give preference to the SI units.

5. Practice Assignment Kg N W=mg W= (3kg)(9.8 m/s²) W= 29.4 N W=mg
Use g = 9.8 m/s2 for earth’s gravity and on the moon use1.7 m /s2 for the Moon’s gravity.
What is the SI unit for mass?
2. What is the SI unit for weight?
3. What is the weight of a 3-kg object when on the earth?
4. When on the moon? Kg N
W=mg
W= (3kg)(9.8 m/s²)
W= 29.4 N
W=mg
W= (3kg)(1.7 m/s²)
W= 5.1 N

6. Universal Law of Gravitation

7. Gravity!
The strength of attraction between any two objects can be found based on a relationship between the two objects masses and their distance from each other.
Simply put, the bigger the masses are and the closer together they are, the larger the gravitational pull on each other.

8. This is Newton’s Universal Law of Gravitation
So because of Newton’s 3rd law every body in the universe exerts a force of attraction on every other body.
This is Newton’s Universal Law of Gravitation
The force between two objects, due to their masses, is called the gravitational force (Fg)- in this case it’s not Earth specific and is not 9.81 m/s2).

9. gravity

10. Universal Gravitational Constant- 6.67 x10-11
Masses of the 2 objects
Distance between
the objects 10

11. Example 1:What is the gravitational force between the Earth and the Moon?
mEarth = M = 6.0 x 1024 kg
mMoon = m = 7.4 x 1022 kg
r = 3.8 x 108 m
G = 6.67 x 10-11 11

12. Example 1:What is the gravitational force between the Earth and the Moon?
GIVEN:
mEarth = M = 6.0 x 1024 kg
mMoon = m = 7.4 x 1022 kg
r = 3.8 x 108 m
G = 6.67 x 10-11
F= G(m1*m2) r2
F = (6.67 x 10-11) (6.0 X 1024)(7.4 X 1022 )
((3.8 x 108)2)
F = 2.05 x 1020 N 12

13. Example 2: What is the gravitational force between the Earth and Venus?
mEarth = M = 6.0 x 1024 kg
mVenus = m = 5.0 x 1024 kg
r = 3.8 x 1010 m
G = 6.67 x 10-11
Answer:
F = 1.386x1018 N 13

14. To be continued………

15. There is also a way to determine the gravitational field around one object:
This is now
the gravitational
Field Strength(GFS)

16. Quick check Mass of Object 1 (kg) Mass of Object 2 Separation Distance
Force of Gravity
(N) a.
Football Player100 kg
Earth
5.98 x1024 kg
6.38 x 106 m
(on surface)
Fgrav = 980 N b.
Ballerina40 kg
Earth
5.98 x1024 kg
6.38 x 106 m
(on surface)
Fgrav = 392 N c.
Physics Student
70 kg
Earth
5.98 x1024 kg
6.60 x 106 m
(low-height orbit)
Fgrav = 641 N

17. Example F = 2.26 X 1017 N F= G(m1*m2) r2
Calculate the magnitude of the interplanetary force of attraction between Uranus and Neptune. (The mass of Uranus is 8.73 X 1025 kg and the mass of Neptune is 1.03 X 1026 kg)
F= G(m1*m2) r2
F = (6.67 x 10-11) (8.73 X 1025)(1.03 X 1026 )
((1.63 x 1012)2)
F = 2.26 X 1017 N