Forces of Motion
Chapter 2
Gravity and Motion
• Aristotle 400 B.C the
rate at which and
object falls depends
on its mass
All object fall with the same
acceleration
• Galileo questioned Aristotle’s idea.
• He proved mass does not affect the rate of
falling objects
Acceleration Due to Gravity
• The same for all
objects
• While gravity is
greater b/t objects
with large mass
• Larger mass object
require more force to
fall at higher speeds
Accelerating at a Constant Rate
• All falling object have the same
acceleration
• Thus gravity= g
• agravity = g = 9.8m/s2 = 9.8 m/s/s
• It is important to point out that 9.8m/s2
down is NOT gravity, it is the acceleration
DUE TO GRAVITY
Math Break
• What is the penny’s velocity after it has
fallen for 2 seconds?
Solution
v = gt
9.8 m/s/s x 2s = 19.6 m/s
Math
• The Penny hits the ground in 4.5 seconds.
What is its final velocity?
Solution
• 9.8 m/s x 4.5 s = 44.1 m/s
Air Resistance Slows Down
Acceleration
• Drop two Sheet of Paper one balled up
and one flat
• What happened?
• Why?
Answer
• Fluid friction that opposes motion in air
– *Remember Fluid friction includes gases
• This fluid friction is called air resistance
– Occurs b/t the surface of the falling object and
the air that surrounds it
Elephant and a Feather
• Who will fall the
fastest and why?
Air Resistance Depends
• Size
• Shape
The force of air
resistance
The net
force Not 0
the object
accelerates
down
– The force of gravity
pulls the object
downward as the force
of air resistance
pushes it upward
The force of gravity rate
would be 9.8 m/s w/o air
resistance
Acceleration Stops At terminal
Velocity
• Terminal Velocity- the constant velocity of
a falling object when the size of the
upward force of air resistance matches the
size of downward force of gravity ( net = 0)
•
http://wsgfl2.westsussex.gov.uk/aplaws/intergames/sec_science/ParachuteGame.swf
• Terminal velocity can be helpful how
Free fall Occurs When there is No
Air Resistance
• Free Fall- gravity is the only force acting
• Example in a vacuum
Orbiting Objects Are in a Free Fall
• No Objects are weightless why?
• Because as long as an object has mass
then some gravitational pull (force) will
exist
Orbiting
• Moving Forward
• Ex: When Space Ship
orbits the Earth
• Freefall
Projectile Motion and Gravity
Newton’s 1st Law
• Objects remain at rest or constant speed
unless acted on by an unbalanced force
• Object at Rest
• Object in Motion
Inertia
• The tendency to resist change
Friction is often the opposing Force
• Friction gets in the way of Newton's law
Newton’s 2nd Law
• The acceleration of an object depends on
mass and force applied
• a=F/m or F=ma
Math Break 1
• What is the acceleration of a 7 kg mass if
a force of 68.6 N is used to move it toward
the Earth? (1 N = 1Kg x m/s/s)
Solution
• a= f / m
• 68.6 N / 7 kg =9.8 m/s2
Math Break 2
• What force is necessary to accelerate a
1,250 kg car at a rate of 40 m/s/s?
Solution 2
• F=ma
• 1,250kg x 40 m/s/s = 50,000 N
2nd Law
• Mass
• Force
• acceleration
• Acceleration
Newton’s 3rd Law
• For every action, there is an equal and
opposite reaction.
– Forces come in pairs
Break
• 1. While driving down the road, a firefly
strikes the windshield of a bus and makes
a quite obvious mess in front of the face of
the driver. This is a clear case of Newton's
third law of motion. The firefly hit the bus
and the bus hits the firefly. Which of the
two forces is greater: the force on the
firefly or the force on the bus?
Answer
• Trick Question! Each force is the same
size. For every action, there is an equal ...
(equal!). The fact that the firefly splatters
only means that with its smaller mass, it is
less able to withstand the larger
acceleration resulting from the interaction.
Besides, fireflies have guts and bug guts
have a tendency to be splatterable.
Windshields don't have guts. There you
have it.
• 4. In the top picture (below), Kent is pulling upon a rope which
is attached to a wall. In the bottom picture, the Kent is pulling
upon a rope which is attached to an elephant. In each case, the
force scale reads 500 Newtons. Kent is pulling ...
•
• a. with more force when the rope is attached to the wall.
• b. with more force when the rope is attached to the elephant.
• c. the same force in each case.
• Kent is pulling with 500 N of force in each
case. The rope transmits the force from
Kent to the wall (or to the elephant) and
vice versa. Since the force of Kent pulling
on the wall and the wall pulling on Kent
are action-reaction force pairs, they must
have equal magnitudes. Inanimate objects
such as walls can push and pull.
Momentum Depends
• Mass and Velocity
• Property of moving objects
• Ex: Which is harder to stop a train or car
Law of conservation of momentum
• Any time two or more objects interact, they
my exchange momentum, but the total
amount of momentum stays the same
• Ex: Bowling
• When bowling the bowling balls
momentum is transferred to the pins
Calculating
•
•
•
•
Momentum (p)
Mass (m)
Velocity (v)
The units of momentum are kg·m/s
Question
• Calculate the momentum of an 80kg
basketball player driving to the basket with
a constant velocity of 8 m/s
Solution
• p=mv
• p = (80kg) (8 m/s)