1. Gravity, Weight
9/14/11

2. Part 1: Gravity: A Basic Force
we have been learning about forces and motion that mainly occur front to back or side to side. Today we are learning about motion that occurs up and down as a result of the force of gravity.
gravity = the attractive force between any two objects.
this force depends on:
the mass of the two objects, and
the distance between the two objects
the law of universal gravitation shows this relationship.
you will not have to calculate with this equation, but you need to know it.
G = gravitational constant = × Nm2/kg2
gravitational = G  mass1  mass2
force distance2

3. G = gravitational constant = 6.673 × 10-11 Nm2/kg2
the force of gravity between most everyday objects is barely noticeable because the mass difference between the two objects is not large enough to calculate to a noticeable force.
for instance, let’s say we have two elephants, each weighing ,000 kg, standing 1 m apart If we plugged in their masses and distance into the equation above, we would get only about 0.01 N of gravitational force between them.
Why? Because “G” is really small × =
in order to really feel the force of gravity, you have to have HUGE differences between the masses of your two objects AND a short distance between them.

4. in order to really feel the force of gravity, you have to have HUGE differences between the masses of your two objects AND a short distance between them.
let’s say we have two objects that are VERY different in mass: The Earth and an apple.
Earth = × 1024 kg, apple = .25 kg
distance between them = radius of Earth
plugging these values into the equation above would give a total force of about 1.0 N.
that’s about 100 times the amount of gravitational force that was experienced by the 2 elephants!
planets orbit the sun due to gravitational force. The moon orbits the Earth for the same reason. ALL objects are exerting gravitational forces on all other objects.

5. Part 2: Acceleration Due to Gravity
that’s about 100 times the amount of gravitational force that was experienced by the 2 elephants!
planets orbit the sun due to gravitational force. The moon orbits the Earth for the same reason. ALL objects are exerting gravitational forces on all other objects.
Part 2: Acceleration Due to Gravity
according to Newton’s 2nd Law of Motion, F = ma
when an object is dropped, it falls to Earth due to the acceleration of gravity.
so another way of writing F = ma is F = mg
the acceleration of gravity (g) is a constant (it’s always the same)
for example, the gravitational force on a skydiver who’s mass is 60 kg would be:
F = ma
Fgravity = mg
g = 9.8 m/s2

6. for example, the gravitational force on a skydiver who’s mass is 60 kg would be:
F = mg = (60 kg)(9.8 m/s2) = 588 N
the acceleration of gravity is the same for all objects, no matter what their mass is.
the two pictures to the right show objects in free fall. Time lapse photography was used to take the pictures.
notice how the feather falls at the same rate as the apple. In a vacuum, there is no air resistance to prevent the feather from falling more slowly.
Fgravity = mg
g = 9.8 m/s2

7. notice how the feather falls at the same rate as the apple
notice how the feather falls at the same rate as the apple. In a vacuum, there is no air resistance to prevent the feather from falling more slowly.
also, notice how the objects are “speeding up” as they fall. What do we call this? Acceleration!!!
Part 3: Weight vs. Mass
even if you are not falling, the force of Earth’s gravity is still pulling you downwards. The force of the floor you are standing on pushes up with the same amount of force, so the net force on you is zero.

8. Part 3: Weight vs. Mass
even if you are not falling, the force of Earth’s gravity is still pulling you downwards. The force of the floor you are standing on pushes up with the same amount of force, so the net force on you is zero.
weight = the gravitational force exerted on an object.
mass = a measure of the amount of matter a substance contains
mass and weight are NOT the same!!!
the weight of an object varies from one planet to another. For instance, the gravitational force on the moon (gmoon) is 1.6 m/s2.

9. mass and weight are NOT the same!!!
the weight of an object varies from one planet to another. For instance, the gravitational force on the moon (gmoon) is 1.6 m/s2.
if you weigh 700 N (about 160 lbs) on Earth, you’d only weigh 119 N on the moon (about 26 lbs)!
would your mass change on the moon?

10. “weightlessness” is a term to describe what an object experiences when it seems to be “floating” relative to its surroundings.
objects on the space shuttle seem to be floating, but really, they are just falling towards the Earth with the same acceleration as the shuttle itself.