Download 4.1 Newton Laws and Gravity

SPH 3U
Force of Gravity
What force is responsible for the moon orbiting the earth? What force is
responsible for the earth orbiting the sun? Why don’t objects “fall” upwards?
Gravity
- it is a fundamental force
-
any object that has mass will exert a __________
on another object that has mass
-
mass is the _______________ an object is
composed
the greater the amount of ____, greater the
ability for an object to resist changes in motion
ie  it is easier to move a gymnast versus a
football player
-
-
regardless of where you are standing on earth, the
gravitational force always points towards the
center of the earth and is __________
-
the force of gravity that is exerted is proportional to the mass of the
objects
near the Earth’s surface, the force of gravity due to the earth’s mass on
any object is given by the formula
-
Fg = mg
m is the mass of the object, g is the acceleration due to the force of gravity
near the earth’s surface
g = -9.8 m/s2[U] = 9.8 m/s2[D]
(these values are identical, we use Up value in problems almost always since
other information is given in terms of Up value)
- the moon also has mass, hence also exerts a force of gravity on objects.
However, since the moon’s mass is much less than the earth, it exerts a
smaller force of gravity on the object. The acceleration of an object near
the surface of the moon due to the moon’s gravitational force of attraction is
about 1/6 that of earth.
gmoon = 1/6 x (-9.8 m/s2)[U]
Example 1: If two identical objects each having a mass of 1 kg were dropped 1 m
above the moon’s and earth’s surface, which one would hit the ground first
(assume there is no air resistance on the earth)?
Example 2: Two objects of mass 1 kg and 1000 kg are dropped from a height of 1
m above the ground (on the moon). Which object will hit the moon first?
So why does it take a 1kg box of feathers a lot longer to fall on the earth then a
1.0 kg piece of lead?
Practice Problems:
1) Calculate the force of gravity acting on a 3 kg object?
(Ans: 29.4 N)
2) Calculate the force of gravity acting on a 50 gram object?
(Ans: 0.49 N)
3) The force of gravity acting on an object is 7500 N. Calculate its mass.
(Ans: 765.31 kg)
4) A 7.5 kg box is resting stationary on a countertop. Calculate the acceleration
of the object, the force due to gravity and the
normal force.
(Ans: 0m/s2, 73.5 N, 73.5 N)
Air Resistance
The force due to air resistance depends upon several factors. In general, air
resistance increases as
a) the speed of the object increases
b) the size of the
object increases
ie.
sleek sports car versus a dump truck
Example 1:
Person falling on earth (mass 100 kg),
no air resist
a=?
Example 2:
Person falling on earth (m= 100 kg),
air resist. is 600 N

a=?
Why is there no normal force for both examples?
Terminal Velocity – occurs when a falling object reaches a maximum velocity and
no longer accelerates
How does this occur?
Weight and Normal Force
A bathroom scale measurement of weight is proportional to the normal force
acting on a person, which in turn is proportional to the mass of the person.
Regardless of where the person is, his/her mass does not change.
However, a person’s weight changes depending on where they are. In an
accelerating elevator, the person’s weight depends on the acceleration of the
elevator.
If a person is on the moon, his/her weight registered on a bathroom scale would
be less than on the earth.