Gravity
lesson 1
chapter 4
What is Gravity?
Objective
You will be able to describe gravitational forces
qualitatively and quantitatively.
What is Gravity?
Newton proposed that there is a force of gravitational
attraction between any two bodies and that it is:
proportional to the product of the two masses
inversely proportional to the square of the separation between
the centers of the two masses.
Newton's Formula
If a constant is included, the formula becomes
F g=
How was G determined?
Henry Cavendish (1798) did an experiment
using a torsion balance. This is
demonstrated here.
Example 1
Example 2
Determine the force of gravitational attraction between a 50.0
kg girl and a 100 kg boy whose centers are separated by 2.00
m.
A 100 kg person is standing on the surface of the Earth, what
is the Fg?
Example 3
Example 4
The 100 kg person is now at an altitude of 4000 km. Fg?
A 30 000 kg satellite is placed midway between the Earth
and Moon. If the orbital radius of the Moon is 3.85 x 108m
and the mass of the moon is 7.36 x 1022 kg, find the net
gravitational force of attraction on the satellite.
Homework
The Moon orbits the Earth at a distance of 3.85 x 108m and
it has a mass of 7.36 x 1022kg. The Earth orbits the Sun at
an average distance of 1.496 x 1011m. The Sun’s mass is
2.00 x 1030kg. Compare the gravitational force of the Sun
on the Moon to the Earth’s gravitational force on the Moon.
Comment on any conclusions you draw or questions that
arise from this comparison.
Read text P. 203-207
Do practice problems p. 207 #1,2 and
next day: SNAP p. 190 #1-13
Answers to even numbers 2=1.94 x 1020N, 4= 2.34 x 103N,
6=0.85m, 8=3.44 x 102N, 10=3.06 x 102N, 12=1.33 x 10-8N
p. 209 #1,2