Motion and Forces
3
Inertia and Mass
•  Inertia (ih NUR shuh) is the tendency of an
object to resist any change in its motion.
•  If an object is moving, it will have uniform
motion.
•  It will keep moving at the same speed and in
the same direction unless an unbalanced force
acts on it.
Motion and Forces
3
Inertia and Mass
•  The velocity of the object remains constant
unless a force changes it.
•  If an object is at rest, it tends to remain at
rest. Its velocity is zero unless a force makes
it move.
•  The inertia of an object is related to its mass.
The greater the mass of an object is, the
greater its inertia.
Motion and Forces
3
Newton’s Laws of Motion
•  The British scientist Sir Isaac Newton
(1642–1727) was able to state rules that
describe the effects of forces on the motion
of objects.
•  These rules are known as Newton’s laws of
motion.
Motion and Forces
3
Newton’s First Law of Motion
•  Newton’s first law of motion states that an
object moving at a constant velocity keeps
moving at that velocity unless an unbalanced
net force acts on it.
•  If an object is at rest, it stays at rest unless
an unbalanced net force acts on it.
•  This law is sometimes called the law of
inertia.
Motion and Forces
3
What happens in a crash?
•  The law of inertia can explain what happens
in a car crash.
•  When a car traveling
about 50 km/h
collides head-on with
something solid, the
car crumples, slows
down, and stops
within approximately
0.1 s.
Motion and Forces
3
What happens in a crash?
•  Any passenger not wearing a safety belt
continues to move forward at the same speed
the car was traveling.
•  Within about 0.02 s (1/50 of a second) after
the car stops, unbelted passengers slam into
the dashboard, steering wheel, windshield, or
the backs of the front seats.
Motion and Forces
3
Safety Belts
•  The force needed to slow a person from 50
km/h to zero in 0.1 s is equal to 14 times the
force that gravity exerts on the person.
•  The belt loosens a little as it restrains the
person, increasing the time it takes to slow
the person down.
Motion and Forces
3
Safety Belts
•  This reduces the force exerted on the person.
•  The safety belt also prevents the person from
being thrown out of the car.
Motion and Forces
3
Safety Belts
•  Air bags also reduce injuries in car crashes by
providing a cushion that reduces the force on
the car’s occupants.
•  When impact occurs, a chemical reaction
occurs in the air bag that produces nitrogen
gas.
•  The air bag expands rapidly and then deflates
just as quickly as the nitrogen gas escapes out
of tiny holes in the bag.
Newton’s Second Law
1
Force, Mass, and Acceleration
•  Newton’s first law of motion states that the
motion of an object changes only if an
unbalanced force acts on the object.
•  Newton’s second law of motion describes
how the forces exerted on an object, its mass,
and its acceleration are related.
Newton’s 2nd Law
•  Newton’s Second Law states: An object
acted upon by an unbalanced force will
accelerate in the direction of the force.
•  If you kick the
ball, it starts
moving.
•  The ball
accelerates only
while your foot is
in contact with the
ball.
Newton’s Second Law
…can be written as a formula
Don’t write this
version of the
Law. There is
another more
common
version coming
up…
•  In this equation, a is the acceleration, m is the mass,
and Fnet is the net force.
•  If both sides of the above equation are
multiplied by the mass, the equation can be
written this way:
This is the more common way to write Newton’s Second Law:
Newton’s Second Law
force = mass x acceleration
  Force is measured in newtons (N).
  Mass is measured in kilograms (kg).
  Acceleration is measured in meters per second per
second (m/s2).
13 of 8
© Boardworks Ltd 2008
Example
A book with a mass of 2.0 kg is pushed along a
table. The acceleration of the book is 0.5 m/s2.
What is the net force on the book?
Cover the quantity that you are trying to work out, which
gives the rearranged formula needed for the calculation.
So to find force (f),
cover up f…
14 of 8
© Boardworks Ltd 2008
Newton’s 2nd Law
(Example 2)
•  Newton’s second law of motion can be used
to calculate acceleration.
•  For example, suppose you pull a 10-kg sled
so that the net force on the sled is 5 N.
•  The acceleration can be found as follows:
Gravity & Weight
•  Gravity is the force of attraction that exists
between any two objects that have mass.
•  The force of
gravity depends
on the mass of
the objects and
the distance
between them.
Gravity
2
•  Gravity is an attractive force between
any two objects that depends on the
masses of the objects and the distance
between them.
If the mass of either of
the objects increases, the
force between them
increases .
If the objects are closer
together, the gravitational
force between them
increases
Gravity
2
The Range of Gravity
•  No matter how far apart two objects are, the
gravitational force between them never
completely goes to zero.
•  Because the gravitational force between two
objects never disappears, gravity is called a
long-range force.
Gravity & Weight
•  Weight is a force, like the push of your hand
is a force, and is measured in Newtons.
•  Your weight on Earth is the gravitational
force between you and Earth.
Gravity & Weight
•  The force of gravity causes all objects near
Earth’s surface to fall with an acceleration
of 9.8 m/s².
•  This acceleration is given the symbol g and is
sometimes called the acceleration of gravity.
Gravity
2
Earth’s Gravitational Acceleration
•  By Newton’s second law of motion, the force
of Earth’s gravity on an object (i.e., its
weight) is the object’s mass times the
acceleration of gravity.
Gravity
2
Weight
Recall Newton’s Second Law when calculating weight:
force = mass x acceleration
Gravity
2
Weight and Mass
•  Weight and mass are not the same.
•  Weight is a force, and mass is a measure of
the amount of matter an object contains.
•  Weight and mass are related. Weight
increases as mass increases.
Gravity
2
Weight and Mass
•  The weight of an object can change,
depending on the gravitational force
on the object.
Gravity
2
Weight and Mass
•  The table shows how various weights on
Earth would be different on the Moon and
some of the planets.
Gravity
2
Weightlessness and Free Fall
•  You’ve probably seen pictures of astronauts
and equipment floating inside the space
shuttle.
•  They are said to be experiencing the
sensation of weightlessness.
Do not attempt to write an explanation of weightlessness in your
outline until the teacher directs you to do so at the end of this set of
slides….
Gravity
2
Weightlessness and Free Fall
•  However, for a typical mission, the shuttle
orbits Earth at an altitude of about 400 km.
•  According to the law of universal gravitation,
at 400-km altitude the force of Earth’s gravity
is about 90 percent as strong as it is at Earth’s
surface.
•  So an astronaut with a mass of 80 kg still
would weigh about 700 N in orbit, compared
with a weight of about 780 N at Earth’s
surface.
…So he is NOT weightless in
space!
Gravity
2
Floating in Space
•  So what does it mean to say that something
is weightless in orbit?
•  When you stand on a
scale, you are at rest
and the net force on you
is zero.
•  The scale supports you
and balances your
weight by exerting an
upward force.
Gravity
2
Floating in Space
•  The dial on the scale shows the upward force
exerted by the scale, which is your weight.
•  Now suppose you
stand on the scale
in an elevator that
is falling.
Gravity
2
Floating in Space
•  If you and the scale were in free fall, then you
no longer would push down on the scale at all.
•  The scale dial
would say you
have zero weight,
even though the
force of gravity on
you hasn’t
changed!
Gravity
2
Floating in Space
•  Similarly, a space shuttle in orbit is in free
fall, but it is falling around Earth, rather
than straight downward.
•  Everything in the orbiting space shuttle is
falling around Earth at the same rate, in the
same way you and the scale were falling in
the elevator.
•  Objects in the shuttle seem to be floating
because they are all falling with the same
acceleration.
You may now write an explanation of
weightlessness in your outline.
PROJECTILE MOTION
Gravity
2
Projectile Motion
•  If you’ve tossed a ball to someone, you’ve
probably noticed that thrown objects don’t
always travel in straight lines. They curve
downward.
•  Earth’s gravity causes projectiles to follow
a curved path.
Gravity
2
Horizontal and Vertical Motions
•  When you throw a ball, the force exerted by
your hand pushes the ball forward.
•  This force gives the ball horizontal motion.
•  No FORCE
accelerates it forward
once it’s in the air, so
its horizontal velocity
is constant, if you
ignore air resistance.
Gravity
2
Horizontal and Vertical Motions
•  However, once you let go of the ball, gravity
can pull it downward, giving it vertical
acceleration.
•  The ball has constant horizontal velocity
but increasing vertical velocity.
Gravity
2
Horizontal and Vertical Motions
•  Gravity exerts an unbalanced force on the
ball, changing the direction of its path from
only forward to forward and downward.
•  The result of these two motions is that the
ball appears to travel in a curve.
Gravity
2
Horizontal and Vertical Distance
•  If you were to throw a ball as hard as
you could from shoulder height in a
perfectly horizontal direction, would it
take longer to reach the ground than if
you dropped a ball from the same
height?
Gravity
2
Horizontal and Vertical Distance
•  Surprisingly, it
wouldn’t.
•  Both balls travel the
same vertical
distance in the same
amount of time.
Circular Motion- Centripetal Force
Gravity
2
Centripetal Force and Acceleration
•  When a ball enters a curve, even if its speed
does not change, it is accelerating because its
___________
is changing.
direction
•  When a ball goes around a curve, the change
in the direction of the velocity is toward the
center of the curve.
The word centripetal means “center seeking”
Circular Motion
•  If you are constantly accelerating, there must
be a force acting on you the entire time.
•  The force exerted is the centripetal force and
always points toward the center of the circle.
Gravity
2
Centripetal Force and Acceleration
•  Acceleration
toward the
center of a
curved or
circular path
is called
centripetal
acceleration.
Gravity
2
Gravity Can Be a Centripetal Force
•  Imagine whirling an object tied to a string
above your head.
•  The string exerts a centripetal force on the
object that keeps it moving in a circular path.
Gravity
2
Gravity Can Be a Centripetal Force
•  In the same way, Earth’s gravity exerts a
centripetal force on the Moon that keeps it
moving in a nearly circular orbit.