THINK ABOUT IT….
If Wile E. Coyote and a boulder fall off a cliff at the same time,
which do you think will hit the ground first? Would it matter
if the cliff were very high or particularly low? How could Mr.
Coyote slow down his fall?
GRAVITY AND MOTION
Gravity and Falling Objects
Gravity and Acceleration
Demo 1: Which will hit the ground first, second, third?
Basketball, baseball, ping pong ball?
Were you correct?
Demo 2:
Prediction and Observation 1:
Prediction and Observation 2:
ACCELERATION DUE TO GRAVITY
VELOCITY OF FALLING OBJECTS
Equation:
V=gxt
Example Problems:
EX 1: A penny at rest is dropped from the top of a tall stairwell.
What is the penny’s velocity after it has fallen for 2 s?
EX 2: The same penny hits the ground in 4.5s. What is the
penny’s velocity as it hits the ground?
VELOCITY OF FALLING OBJECTS
EX 3: A marble at rest is dropped from a tall building. The
marble hits the ground with a velocity of 98 m/s. How long
was the marble in the air?
EX 4: An acorn at rest falls from an oak tree. The acorn hits
the ground with a velocity of 14.6 m/s. How long did it take
the acorn to land?
AIR RESISTANCE AND FALLING OBJECTS
Demo1: Which falls faster?
Crumpled paper or flat sheet of paper?
Why did this happen?
AIR RESISTANCE AND FALLING OBJECTS
Air Resistance - force that opposes the motion of objects
through air
ACCELERATION STOPS AT TERMINAL VELOCITY
As the speed of a falling object increases, air resistance
increases. The upward force of air resistance continues to
increase until it is equal to the downward force of gravity.
The object then falls at a constant velocity called the
terminal velocity.
Is terminal velocity a good thing or bad thing?
FREE FALL OCCURS WHEN THERE IS NO AIR
RESISTANCE
An object is in free fall only if gravity is pulling it down and no
other forces are acting on it.
What are the 2 places that have no air and free is possible?
Vacuum and space
ORBITING OBJECTS ARE IN FREE FALL
Two Motions Combine to Cause Orbiting
A space shuttle moves forward at a constant speed and it
is in constant free fall towards the earth causing orbiting
The unbalanced force that causes objects to move in a
circular path is centripetal force (ex. Moon orbiting the
earth, earth around sun.)
Why does the shuttle not fall to earth if gravity is pulling
down on it?
Because it is traveling forward
What would happen if the shuttle started moving much
faster or slower?
Faster – out of orbit
Slower – fall towards earth
PROJECTILE MOTION AND GRAVITY
Projectile Motion and Gravity
Projectile Motion- the curved path that an object follows
when thrown, launched, or otherwise projected near the
surface of earth.
Horizontal Motion - When you throw a ball, the forward
force your hand exerts on the ball
Vertical Motion- Pulled down towards the earth by gravity
NEWTON’S LAWS OF MOTION
Newton’s First Law of Motion
An object at rest remains at rest, and an object in motion
remains in motion at constant speed and in a straight line unless
acted on by an unbalanced force.
What does this mean?
This means that there is a natural tendency of objects to keep on
doing what they're doing. All objects resist changes in their state
of motion. In the absence of an unbalanced force, an object in
motion will maintain this state of motion.
NEWTON’S FIRST LAW OF MOTION
What is the motion in this picture? Forward motion – rolling
What is the unbalanced force in this picture? Rock
What happened to the skater in this picture? Fall off the skateboard
What other force do you know that can stop an object from moving?
Friction, gravity
NEWTON’S FIRST LAW OF MOTION
Newton’s 1st Law is also the
reason you wear a seatbelt!
Newton’s 1st Law is also known as inertia
Inertia – the tendency of an object to resist being moved, or if the object is moving to
resist a change in speed or direction until an outside force acts on it
Does mass have an affect on inertia? Would you rather catch a baseball or bowling
ball?
Yes mass has an affect on inertia
NEWTON’S SECOND LAW OF MOTION
Acceleration is produced when a force acts on a mass. The greater the mass
of the object being accelerated the greater the amount of force needed
to accelerate the object.
What does this mean?
Everyone knows the Second Law. The heavier objects require more force
to move the same distance as lighter objects
NEWTON’S 2ND LAW OF MOTION
The 2nd Law gives us an exact relationship between force, mass,
and acceleration. It can be expressed as a mathematical
equation:
or
FORCE = MASS times ACCELERATION
SI Unit for force = Newtons (N)
NEWTON’S 2ND LAW OF MOTION
Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to
push the car to a gas station, and he makes the car go 0.05
m/s2. Using Newton's Second Law, you can compute how much
force Mike is applying to the car.
NEWTON’S 2ND LAW OF MOTION
What is the acceleration of a 3 Kg mass if a force of 14.4 N is used
to move the mass?
What force is necessary to accelerate a 1,250 Kg car at a rate of 40
m/s2?
NEWTON’S THIRD LAW
For every action there is an equal and opposite re-action.
What does this mean?
This means that for every force there is a reaction force
that is equal in size, but opposite in direction. That is to say
that whenever an object pushes another object it gets
pushed back in the opposite direction equally hard
NEWTON’S THIRD LAW
Let's study how a rocket works to understand Newton's 3rd Law.
The rocket's action is to push down on the ground with the force of its
powerful engines, and the reaction is that the ground pushes the
rocket upwards with an equal force.
Newton’s Cradle
MOMENTUM
Momentum - Depends on the object’s mass and velocity. The more
momentum an object has, the harder it is to stop the object or
change directions.
What would have more momentum?
A bowling ball or a basketball rolled at the same velocity?
Bowling ball b/c it has more mass
A tractor trailer or a Honda civic at the same velocity?
Tractor trailer
MOMENTUM
Equation:
Momentum = mass x velocity
p=
m x v
(Kg m/s) (Kg)
(m/s)
Example:
A 100 Kg car falls off a cliff from rest and hits the ground with a velocity of 35
m/s. What is the car’s momentum when it hits the ground?
A 35 Kg bowling ball was thrown at Tommy at a velocity of 15 m/s. What was the
bowling ball’s momentum?
MOMENTUM PROBLEMS
What is the momentum of an ostrich with a mass of 120 Kg that runs with a
velocity of 16 m/s north?
What is the momentum of a 6 Kg cat that is moving at 10 m/s down the alley
toward the mouse?
An 85 Kg man is jogging with a velocity of 2.6 m/s to the north. Nearby, a 65 Kg
person is skateboarding and is traveling with a velocity of 3 m/s north. Which
person has a greater momentum? Show your work.
LAW OF CONSERVATION OF MOMENTUM
Any time objects collide, the total amount of momentum stays
the same. This is true for all collisions if no other forces act
on the colliding objects
Conservation of Momentum can be explained by
Newton’s third law of motion.
Action Force
5
Reaction Force