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Chapter 6
Forces in Motion
Gravity and Motion
Q: If you were to drop a marble and a
baseball at the same time from the
same height which would hit the
ground first?
A: They will hit the ground at the SAME
time!
http://www.youtube.com/watch?v=z_sJ15feNGw
ALL objects fall with the SAME
acceleration.

Mass DOES NOT affect the rate at which
an object falls.
Acceleration at a Constant
Rate
ALL objects accelerate at 9.8 meters per
second per second or 9.8 m/s/s
Examples:

What is a penny’s velocity after if has
fallen for 2 seconds?

The penny hits the ground in 4
seconds. What is its final velocity?
Terminal Velocity

A falling object will continue to fall faster and
faster….until the upward push of air
resistance becomes equal to the downward
pull of gravity. The object then falls at a
constant velocity known as terminal velocity.
Free Fall
Free fall occurs when there is NO air
resistance.
An object is in free fall only if the gravity
is pulling it down and NO other forces
are acting on it.
Free fall can only occur where there is
NO air!
On Earth…


Air resistance slows down acceleration.
Air resistance: is the friction caused by air
that occurs between the surface of the
falling object and the air that surrounds it.
Projectile Motion
Objects orbiting the Earth are in free fall.
Two motions combine to cause orbiting:
1. Forwards/Horizontal
2. Down/Vertical
Projectile Motion
Learn360: Projectile Motion
Quiz 6.1
A feather and a rock dropped at the same time from the
same height would land at the same time when dropped by:
1.
a.
b.
c.
d.
Galileo in Italy
Newton in England
An astronaut on the moon
An astronaut on Earth
Falling objects stop accelerating at _________
2.
a.
b.
c.
d.
Free fall
Terminal velocity
Momentum
Inertia
Quiz #6.1 Continued
3.
An object in motion tends to stay in motion because it has
_________.
a.
Mass
b.
Weight
c.
Inertia
d.
None of the above
Extra Credit:
A boy standing on a high cliff dives into the ocean below and
strikes the water 3 seconds later. What is the boy’s
velocity when he hits the water? Don’t forget your units?
Newton’s 3 Law’s of Motion
Inertia
2. Acceleration
3. Action/Reaction
1.
Newton’s First Law

Inertia: An object at rest will stay at
rest and an object in motion will stay in
motion unless acted on by an outside
force.
Newton’s Second Law

Acceleration of an object depends on
the mass of the object and the amount
of force applied.
Example 1:
Example 2
A 50kg skater pushes off from a wall with
a force of 200N. What is the skater’s
acceleration?
F=M x A
Answer
A 50kg skater pushes off from a wall with a force of 200N. What is
the skater’s acceleration?
F=MxA
Or
200N = 50Kg x A
200N/50Kg = A
A = 4 m/s/s
Newton’s Third Law
Action/Reaction: Whenever one object
exerts a force on a second object, the
second object exerts and equal and
opposite force on the first.
Examples
Momentum
Momentum: is a property of a moving
object that depends on the object’s
mass and velocity.
Momentum = Mass x Velocity
Examples
1. If a semi-truck and a slug-bug are both
traveling down the highway at 50MPH
North, then the ___________ has more
momentum because it has a greater
_______.
2. If the semi-truck and slug-bug are both
parked on a hill and the slug-bug begins to
roll down the hill, then the __________ has
more momentum because
______________________________.
Examples cont.
Which has more momentum, a 5kg toy
truck moving at 1m/s or a 10 kg toy car
moving 2m/s?
Momentum = mass x velocity
Quiz 6.2
1.
2.
3.
Newton's first law of motion applies :
a.
To moving objects
b.
To objects that are not moving
c.
To objects that are accelerating
d.
Both a and b
A golf ball and a bowling ball are moving with the
same velocity. Which has more momentum?
____________ is the path that a thrown object
follows. (free fall, projectile motion)
Quiz 6.2 Cont.
4.
Name three action reaction force pairs
involved in doing your homework. Name
which object is exerting the force and
which is receiving the force.
5.
Which has more momentum, a mouse
running 1m/s north or an elephant walking
3m/s east? Explain your answer.
Momentum = Mass x Velocity