Newton’s
st
1
Law of Motion
Have you ever been riding in a car when the driver stopped
suddenly? How did your body move as the car came to a stop? Did it
feel like your body was moving forward?
When you felt this happening you experienced Newton's first law of
motion. Newton‘s first law of motion says that an object in motion
will stay in motion and an object at rest will stay at rest unless acted
on by an unbalanced force. In the car your body was in motion,
traveling at the same speed as the car. When the car stopped, your
body stayed in motion. If you were not wearing a seatbelt and you
were traveling very fast, your body could continue to move forward
through the windshield!
This idea is called inertia.
Explain why your body feels like it is being pushed back when the
car starts back up again:
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Newton’s 2nd Law of Motion
If a ping pong ball and a basketball were both dropped at the
same time from the roof of our school, which would hit the
ground with a greater force? Common sense tells us that the
basketball ball would. The difference in forces would be
caused by the different masses of the balls. Newton stated this
relationship in his second law, the force of an object is equal
to its mass times its acceleration.
List two other situations where Newton's 2nd Law may apply.
Forces
A. Newton’s 2nd Law
1. A net force acting on an object
causes the object to accelerate in
the direction of the net force.
2. a = FNET / mass or a = F / m
Using F = m × a
1.Your bicycle has a mass of 9.1 kilograms. You
accelerate at a rate of 1.79 m/s2. Calculate the net
force that is accelerating the bicycle.
2.The Space Shuttle has a liftoff mass of 2,041,000 kg
and accelerates at a rate of 16 m/s2. Calculate the
force (thrust) that is accelerating the Space Shuttle.
3. A rocket accelerates at 56 m/s2. It has a mass of
800,000 kg. Calculate the force (thrust) that the rocket
engines must supply.
Using a = F ÷ m
1.A runner has a mass of 89 kilograms. He produces a
force of 84 Newtons between the ground and his running
shoes. How fast does he accelerate?
2.Calculate the acceleration of a car if the force on the
car is 450 Newtons and the mass is 1300 kilograms.
3. Calculate the acceleration of a jet car racing on the
Bonneville Salt Flats if the force on it (the “Thrust”) is
500,000 Newtons and the mass is 2,100 kilograms.
Newton’s 3rd Law of Motion
Imagine a rocket is being launched from the earth.
Hot gases are pushed out from the bottom of the
rocket as the rocket is pushed upward. The force of
the gases pushing against the surface of the earth is
equal and opposite to the force with which the
rocket moves upward. The motion of the rocket can
be explained by Newton's third law, for every
action there is an equal and opposite reaction. In
other words, when one object exerts a force on
another object, the second object exerts a force of
equal strength in the opposite direction on the first
object.
Practice Problems
1. Sam pushes on the wall with a force of 25
Newton (N) to relieve some stress. What
force is being exerted back by the wall if the
wall does not move?
2. Sara pushes against Adan with a force of 9 N,
but he doesn’t move. What force is Adan
pushing back with?
Momentum
• Product of an object’s mass and its velocity.
• An object has a large momentum if the product
of its mass and velocity is large.
• Momentum for any object at rest is 0.
• You can calculate momentum by multiplying an
object’s mass (in kilograms) and its velocity (in
meters per second). Momentum=Mass x
Velocity
Practice Problems
• Which has more momentum, a 0.046kg golf
ball with a speed of 60.0m/s, or a 7.0kg
bowling ball with a speed of 6.0m/s?
• What is the momentum of an 80kg runner
moving at the speed of 2.5m/s?
B. Friction
1. Force that opposes motion
between two surfaces in contact.
2. Amount depends on:
a. Kinds of surfaces in
contact.
b. Amount of force pressing
surfaces together. Something that
weighs more will have greater
friction.
3. Friction is caused by microwelds
4. Types of friction:
a. Static (usually the greatest)
b. Sliding
c. Rolling (usually the least)
C. Air resistance (drag force)
1. Force that opposes
motion of objects through air
2. Pushes up on falling
objects
3. Affected by object’s
speed, size, shape
4. Without drag force, all objects
fall at the same rate
5. Terminal velocity is the max
speed at which an object can fall
D. Gravity
1. Attraction between objects
2. Weakest force in universe
3. Farthest range
4. Directly proportional to the
masses of the objects
5. Inversely proportional to the
squares of the distance between
E. Gravitational Acceleration
1. g = 9.8 m/s/s on Earth
2. FWEIGHT = m x g
3. All objects fall with the
same g
4. Weight is NOT the same
as mass
Practice Problems
1. Sara has a mass of 11.22g. How much
does she weigh?
2. A ball has a weight of 10 pounds.
What is the mass of the ball?
3. A chair has a weight of 7 lbs. What is
the mass of the chair?
F. Free Fall (Weightlessness)
1. As long as an object is free falling,
nothing exerts an upward force
2. With no upward force, FW = 0 N
G. Projectile and Circular Motion
1. Projectile motion
a. Follow a curved path
b. Two types of motion are
independent of one another:
1) Horizontal (based on initial
velocity and inertia)
2) Vertical (based on gravity)
c. An object launched horizontally
will land at the same time as an
object simply dropped from the
same height
2. Circular Motion
a. Objects moving in circular paths
accelerate toward the center
b. Centripetal acceleration
c. Centripetal force (FC = m x aC)
d. Centrifugal force is imaginary
e. Weightlessness in orbit exists
because objects are constantly falling
toward Earth, but have enough
forward velocity to keep them in orbit