Physical Science
Ch. 3: Forces
Newton's 2nd Law of Motion
• If a net force is applied to an object, that
object will accelerate in the direction of the
force.
Force = Mass x Acceleration
(N)
(kg)
(m/s/s)
F=MxA
F=MxA
F=MxA
72 = 6 x 12
72 = 12 x 6
72 = 18 x 4
• Examples of Newton's 2nd law:
-Car engines
-A semi (full or empty)
-Pushing a kid in a swing
• Practice problem:
If a motorcycle has a mass of 212 kg and
the engine produces 3816 N of force, at
what rate will the motorcycle accelerate?
Weight is a measure of gravitational pull, which is a
force. So if F = M x A, then weight also = M x A
More specifically though, the acceleration for weight is
due to gravity, so you can automatically put 9.8 m/s/s
in the equation.
So the equation ends up being:
Weight = Mass x 9.8
Projectiles
A projectile is any object which has been
shot or thrown through the air, with a 1 time
force.
Projectiles follow a perfectly curved path
through the air (in a vacuum).
• This perfectly curved path is called a
parabola.
Is a bottle rocket a projectile?
• A projectile has both horizontal force and vertical
force applied to it. One makes it want to go up
and down, the other wants it to go sideways.
The combination of the 2 forces creates the
curved path.
Regardless of the shape, the path followed
by a projectile is called its trajectory.
Golf Ball Trajectory
• In real life, projectiles are effected by air
resistance (the force which air exerts on a
moving object) and therefore may not follow
a perfectly curved path.
Like the flight of a golf ball.
Examples of air resistance:
• Car aerodynamics
• Spoilers on racecars
• Buses
• Hand surfing
Falling Objects
• A falling object will accelerate
downward at 9.8 m/s/s (in a
vacuum). However, in real life air
resistance will gradually slow it
down.
• Terminal velocity is the top speed
reached by a falling object. At
terminal velocity air resistance is
equal to gravitational pull.
• Which will reach
terminal velocity
first, the feather
or the coin?
• Which will have
the higher
terminal velocity?
The world’s record for the highest
dropped ball that was caught was
2,400 m, dropped from a hot air
balloon. Based on what you know
about falling objects, answer the
following:
What was the downward velocity of
the ball when it was caught, if it fell
for 16 sec. (disregard air resistance)?
In real life, it wouldn’t matter if it fell
for 16 sec. or 60 sec. Why?
• An object thrown into the air will deccelerate
at -9.8 m/s/s on the way up, and accelerate
at +9.8 m/s/s on the way down.
• This effects only vertical velocity, not
horizontal.
• So if a projectile has the same negative
acceleration on the way up as positive
acceleration on the way down (-9.8 and +9.8
m/s/s), answer the following:
• If a baseball is hit and is in the air for 4 seconds total,
how long is it moving upward? Downward?
• What would be it’s vertical speed when it hits the
ground?
• What was it’s vertical speed when it was hit if it took 2
sec. to reach the top of it’s trajectory?
• A punted football
has a hangtime of
4 sec. What was
it's vertical velocity
when it hit the
ground?
1. Larry's car has run out of gas. So while pushing it to the gas station, Larry
applies a force of 845 N which causes the car to accelerate at a rate of
.75 m/s/s. What was the mass of the car?
2. The car being pushed in problem #1 starts to coast down a hill on it's own
and reaches a speed of 5.4 m/s. What was the momentum of the car?
3. A person throws a round projectile through the air. The person applies
59 N of force to the object, which causes it to accelerate at 7.5 m/s/s
through the air. Was the object thrown most likely a bowling ball, a baseball,
or a ping-pong ball? Explain.
4. If a 20 kg object is dropped out of a 10 story window, how much downward
force does gravity exert on the object?
5. When the object in problem #4 reaches terminal velocity, what will be the
exact force of air resistance experienced by the object?
6. A car with a mass of 1,000 kg strikes a car with a mass of 850 kg. If the
first car's rate of acceleration after the collision was 1.5 m/s/s, what was the
second car's rate of acceleration after the collision?
• A falling object is said to be in freefall
when it is effected only by gravity.
• Is a paruchutist experiencing freefall?
Circular Motion
• According to Newton's
1st Law, an object will
move in a straight line
unless a net force
causes it to do
otherwise.
• Therefore, an object
moving in a curved or
circular path, like a
tetherball, must have a
force pulling it in a circle.
• A centripetal force is a
force which pulls an object
toward the center of a
curved or circular path.
• For example, the friction
between the tires and the
pavement will pull the car
through the curve.
• What if the road were icy?
• Tell what is producing the curved path in
each of the following examples:
• The Gravitron
• Motorcycle Cage O' Death
• Rock and sling
• Race track curves
Other examples of centripetal force:
•
•
•
•
A centrifuge
Washing machine
Fighter jets
Hammer throw
• If the mass and/or
acceleration of the
object exceeds the
centripetal force, then
the object will not make
the curve.
• Like swinging a 10 lb
weight around on a
piece of string.
• What will happen to the weight hanger as
the mass is swung faster and faster? Why?
• When an object is pulled in a curved or
circular path, this is called centripetal
acceleration, since it is changing
directions.
• A car has a mass of 1,200 kg and is
accelerating at a rate of 5 m/s/s through a
curve.
• How much centripetal force will the tires need to provide in
order to get the car through the curve?
• If the tires produce 5,000 N of friction force, what exactly
will happen to the car as is goes through the curve?
Newton's 3rd Law of Motion
• For every force, there is a force equal in
size, but opposite in direction.
• Ex.:
-hitting a wall
-rocket propulsion
-the kick of a gun
-an arrow being shot
-hard hit in football
-swimming
-pogo stick
F1 = F2
or
M1 x A1 = M2 x A2
Practice problem:
If Larry kicks a .5 kg ball with 90 N of
force, at what rate will the ball accelerate?
• A 12 gauge shotgun fires a slug which has a mass
of .15 kg and accelerates at 250 m/s/s. What is
the force of the recoil (kick) of the gun when it is
fired?
• Does the shotgun slug experience any centripetal
force, and if so, what is producing that force?
Momentum
• Momentum is inertia in motion.
It is determined by how big an object is
and how fast it is moving.
p=mxv
(kg x m/s)
A stationary object has zero momentum.
• Practice problem:
A car has a mass of 1050 kg and has
5250 kg x m/s of momentum as it drives
down the road. What is the velocity of
the car?
• Is it possible for a little Volkswagon to
have more momentum than a Mack truck?
Explain.
• The Law of Conservation of Momentum
states that the total amount of momentum
of an object or group of objects does not
change (unless acted on by a net force).
• Think of it like a class poker day in Physical Science.
Everybody brings their money, and some of that
money will change hands throughout the course of
the hour. But from beginning to end, the total
amount of money at the beginning of the hour will be
the same as at the end of the hour.
• Examples:
– Newton's Cradle
– Dog pile
– Jar of marbles dropping
– Pool break
– Car wreck
• Satellites are objects in space which orbit
Satellite
Motion
a larger body.
• Man-made satellites are used for
communications, weather, surveillance,
positioning systems, etc.
1. Kenny drops an object out of a window. If gravity exerts a
force of 147 N on the object, what is the mass of the object?
2. A baseball player hits a high fly ball to the outfield. If the
ball is in the air for a total of 6 seconds, how fast was the ball
traveling downward when the outfielder caught it?
3. In problem #3, how fast was the ball traveling upward when
it left the bat? Explain.
4. Two bumper cars collide with each other. The first car has a
mass of 124 kg (car and driver), while the second car has a total
mass of 148 kg. When the cars collide, the first is knocked
backwards with a rate of acceleration of 4.77 m/s/s. At what
rate of acceleration was the other car knocked backwards?
5. Which will have more momentum, a softball (.42 kg) thrown at
45 m/s, or a baseball (.35 kg) thrown at 55 m/s? Show work.
In 1999, Krishna Gopal Shrivestava
of India set an unverified world
record by pulling a ship with a
mass of 244,000 kg (536,800
pounds) with his teeth! His efforts
can teach us about force, mass,
and acceleration.
1. What would happen if the man
were pulling a small rowboat instead
of a large ship?
2. When he stops pulling, will the
ship stop moving? Explain.
3. What direction is the ship moving?
Why?
• There is no up or down in orbit. If not secured, objects
simply float here and there. Planes like NASA’s KC-135,
the “Vomit Comet,” are able to briefly simulate the
conditions of being in orbit.
1.On the plane, how can you tell up from down?
2. If a scale floated by and an astronaut stood on
it, what would the scale say?
• It takes more than one firefighter to aim a fire hose. The
force of the water shooting out of the nozzle causes a
reaction that can be difficult to control.
1. Describe what happens if you step from a canoe or small boat
onto a dock. How is this related to the fire hose?
2. Does a garden hose need more than one person to hold it?
What’s the difference between a garden hose and a fire hose?
3. What would happen if the firefighters dropped the hose?
• The dart is attached to the hoop by a
string fed through an i-screw. So
when the dart is thrown at the hoop,
the string will slacken and the hoop will
drop downward as the dart flies
towards it. Where should you aim in
order to make the dart go through the
middle of the hoop as it is falling?
Eric (mass=70 kg) was canoeing at the lake and
decided to take a swim. When he dove off the front of
the canoe, the canoe moved backward at 4.5 m/s/s. If
the canoe had a mass of 55 kg, at what rate did Eric
accelerate forward when he dove off the canoe?
Which one do you think
moved further, him
forward or the canoe
backwards, and why?
• Jim and Andy both threw a baseball through the
air. Jim’s baseball landed 180 ft. away, while
Andy’s landed 110 ft. away.
• Which ball was in the air for a longer period of time?
• What are the forces which cause the baseballs to
follow a curved path?
• How are these forces related?
• A mortar is fired at a target and it takes the shell 10
seconds to hit the target. (assume no effect of air
resistance)
• What was the shells vertical velocity when it left the mortar?
• If the shell travelled 800 m, what was it’s horizontal velocity?
• Four objects are dropped out of a window: a
bowling ball, a feather, an iron weight, and a
cat. Which one will reach terminal velocity first?
• A dump truck (4,000 kg) is carrying a load of
gravel (6,000 kg)
A. How much momentum does it have if it’s traveling 60
mi/hr (26.7 m/s)
B. What are 2 ways that the momentum figured in A could
be decreased by ½?
• A parachutist jumps from a
plane, and due to air
resistance she accelerates
downward at ½ the normal
rate.
• If her terminal velocity is 78.4
mi/hr, how long will it take for her
to reach that speed?
• What, in general would happen
to her terminal velocity if she
went head first instead of
spread-eagle?
• A sky-diving instructor is taking a tandem
jump with one of his students. What effect, if
any, would this have on the instructors
terminal velocity if he had another person
strapped to his back? Hmmmm………