Motion
Unit 1: Motion
Chapter 3: Laws of Motion
 3.1
Newton's First Law
 3.2
Acceleration and Newton's Second Law
 3.3
Gravity and Free Fall
3.1 Investigation: Law of Inertia
Key Question:
Why are heavier objects harder
to start or stop moving?
Objectives:

Apply an understanding of Newton’s first law to justify
experimental data and observations.

Explain the meaning of inertia.

Distinguish between mass and weight.
Force changes motion

A force is a push or pull, or any action that is able
to change motion.
Law of inertia

Newton’s first law says that objects continue the
motion they already have unless they are acted
on by a net force.

If the net force is zero, an object at rest will stay
at rest.

If an object is acted upon by unbalanced forces,
its motion will change.
Net force
 Newton’s
first law is
often written in terms of
the net force:
 “An
object at rest will
stay at rest and an
object in motion will
continue in motion at
constant velocity
UNLESS there is a net
force.”
According to these vectors, in what
direction is the net force?
Force changes motion

Forces can be used to increase or decrease the
speed of an object, or to change the direction an
object is moving.
Force, mass, and inertia
 Some
objects resist changes in motion more than
others.
 Inertia
is the property of an object that resists
changes in its motion.
 The
greater an object’s inertia, the greater the force
needed to change its motion.
 A bowling
ball has more inertia than a golf ball.
Force, mass, and inertia
 Inertia
comes from mass.
 Objects
with more mass have more inertia and are
more resistant to changes in their motion.
 A 5-kilogram
bowling ball is 100 times as massive as
a 50 gram golf ball, so it has 100 times the inertia.
Force, mass, and inertia
 For
small amounts of mass, the kilogram is too large
a unit to be convenient.
 A dollar
bill has a mass of about a gram, and a liter
of soda is 1,000 g. or 1 kg.
Units of force
 The
pound is a unit of force
commonly used in the United
States.
 For
smaller amounts, pounds are
divided into ounces (oz.).
 There
are 16 ounces in 1 pound.
Newtons
 Although
we use pounds all
the time in our everyday life,
scientists prefer to measure
forces in newtons.
 The
newton (N) is a metric
unit of force.
Unit conversions
 The
newton (N) is a smaller unit of force than the
pound (lb).
 If
one pound of force equals 4.448 newtons, then a
100 lb person weighs 444.8 newtons.
The net force
 The
motion of objects changes in response to the
total force acting on the object, including gravity and
any other forces that are present.
The net force in the horizontal direction
 The
term net force is used to describe the total of all
forces acting on an object.
 When
used this way, the word net means “total.”
The net force in vertical direction
 Gravity
 The
exerts a force downward on the box.
floor exerts an equal and opposite force upward
on the box.
The net force in vertical direction
 The
net force on the box in
the “up-down” direction is
zero.
 When
equal forces applied
to the same object are in
opposite directions they
cancel.
Applications of Newton’s First Law
 Two
very important safety
features of automobiles are
designed with Newton’s first
law in mind: seat belts and air
bags.
 Both
supply a restraining force
to counteract your inertia and
to slow your body down.
Applications of Newton’s First Law
 Prior
to the invention of cup
holders, drink containers left
on the dash obeyed the first
law of motion and made quite
a mess.
 Can
you think of other
applications of Newton’s first
law?
Unit 1: Motion
Chapter 3: Laws of Motion
 3.1
Newton's First Law
 3.2
Acceleration and Newton's Second Law
 3.3
Gravity and Free Fall
3.2 Investigation: The Second Law: Force,
Mass, and Acceleration
Key Question:
What is the relationship between force, mass,
and acceleration?
Objectives:
Measure the acceleration for an Atwood’s apparatus of
fixed total mass.
 Create a graph of force versus acceleration for the
Atwood’s machine.
 Determine the slope and y-intercept of the graph and then
relate them to Newton’s second law.

Newton’s second law

Newton’s first law tells us that motion cannot
change without a net force.

According to Newton’s second law, the
amount of acceleration depends on both the
force and the mass.
Acceleration and force
 The
second law says that
acceleration is
proportional to force.
 If
force is increased or
decreased, acceleration
will be increased or
decreased by the same
factor.
 The
stronger the force
on an object, the
greater its
acceleration.
—
—
Force is directly
proportional to
acceleration.
If twice the force is
applied, the
acceleration is twice
as great.
 The
greater the
mass, the smaller the
acceleration for a
given force.
—
—
Mass is inversely
related to force.
An object with twice
the mass will have
half the acceleration if
the same force is
applied.
Applying the second law

Keep the following important
ideas in mind:
1. The net force is what causes
acceleration.
2. If there is no acceleration, the
net force must be zero.
3. If there is acceleration, there
must also be a net force.
4. The force unit of newtons is
based on kilograms, meters,
and seconds.
Using units in calculations
 In
terms of solving physics problems, use the
following units when using force in newtons:
— mass in kilograms (kg)
— distance or position in meters (m)
— time in seconds (s)
— velocity in meters per second (m/s)
— acceleration in meters per second per second
(m/s2)
Three forms of the second law
 When
using the second law, the force that appears is
the net force.
 Consider
all the forces that are acting and add them
up to find the net force before calculating any
accelerations.
Using Newton’s second law
A car has a mass of 1,000 kg. If a net
force of 2,000 N is exerted on the
car, what is its acceleration?
1.
2.
3.
4.
Looking for: … the car’s acceleration.
Given: …car’s mass (m= 1,000 kg) and the net force (Fnet
= 2,000N).
Relationship: Use: a = F
m
Solution: a = 2,000N = 2 kg• m/s2 = 2 m/s2
1,000 kg
kg
Force and energy
 Forces
are created any time there
is a difference in energy.
 A stretched
rubber band has more
energy than a relaxed rubber
band.
 The
forces can transfer energy
from one object to another.
Unit 1: Motion
Chapter 3: Laws of Motion
 3.1
Newton's First Law
 3.2
Acceleration and Newton's Second Law
 3.3
Gravity and Free Fall
3.3 Investigation: Free Fall
Key Question:
What kind of motion is falling?
Objectives:

Explain the meaning of falling is a physics sense.

Determine an equation for the velocity in free fall.

Use the equation to make predictions.
Gravity and Free fall
 An
object is in free fall
if it is accelerating due
to the force of gravity
and no other forces
are acting on it.
 A ball
thrown upward
is also in free fall after
it leaves your hand.
Free fall
 Falling
objects increase their speed by 9.8 m/s every
second, or 9.8 m/s2
Upward launches

If you throw a ball upward, the ball
will slow down as it moves
upward, come to a stop for an
instant, and then fall back down.

As it moves upward, the speed
decreases by 9.8 m/s every
second until it reaches zero.

The ball then reverses direction
and starts falling down.

As it falls downward, the speed
increases by 9.8 m/s every
second.
Changes in velocity
 Recall
 The
that velocity is speed with direction.
positive sign means upward and the negative
sign means downward.
Changes in velocity
 In
free fall and other situations when there is
constant acceleration, the average velocity is the
average of the starting or initial velocity (vi ) and the
final velocity (vf )
Using average velocity
A rock falls off a cliff and splashes into a river 5 seconds
later.What was the rock’s average velocity?
1.
Looking for: … for average velocity in m/s.
2.
Given: …the time (5s) and assume the rock was at rest on
the cliff, so it’s vi = 0.
Relationships: Use: v = gt and vavg = vi – vf
2
3.
4.
Solution: v = (9.8 m/s2) (5 s) = 49 m/s
vavg = 0 – 49 m/s = 24.5 m/s
2
Calculating distance
 Using
the average velocity to calculate the distance
traveled by an object in free fall requires multiple
steps.
Calculating distance
1.
If the initial velocity is zero and the object falls for t seconds,
then the final velocity is gt.
2.
The average velocity is half the final velocity or 1/2 gt.
3.
The distance is the average velocity multiplied by the time or
1/2 gt2.
The general formula is therefore:
d = 1 gt2
2
*This formula only works when the object starts at rest and is in
free fall.
Using average velocity
A skydiver falls for 6s before opening her parachute.
Calculate her actual velocity at the 6-second mark and
the distance she has fallen in this time.
1.
Looking for: … velocity in m/s after 6 seconds and
distance fallen.
2.
Given: … the time (6s) and assume skydiver was at rest, so
it’s vi = 0.
Relationships: Use: v = gt ; vavg = vi – vf and d = vavgt
2
3.
Solution: v = (9.8 m/s2) (6 s) = 58.8 m/s
vavg = 0 – 58.8 m/s = 29.4 m/s
d = (29.4 m/s) (6 s) = 176 m
2
4.
Gravity and Weight

The force of gravity on an object is called
weight (Fw).
 At
Earth’s surface, gravity exerts a force of 9.8 N
on every kilogram of mass.
Weight vs. mass

Weight and mass are not the same.

Mass is a fundamental property of matter measured in
kilograms (kg).

Weight is a force measured in newtons (N).

Weight depends on mass and gravity.
Weight depends on mass and gravity
A 10-kilogram rock has the same mass no matter where it
is in the universe. On Earth, the10 kg. rock weighs 98 N..
On the moon, the same rock only weighs 16 N.
Weight and mass
Legend says that about 1587, Galileo dropped two balls from the
Leaning Tower of Pisa to see which would fall faster. Suppose the balls
had masses of 1.0 kg and 10 kg.
a. Use the equation for weight to calculate the force of gravity on each ball.
b. Use your answers from part a and Newton’s second law to calculate each ball’s
acceleration.
1. Looking for: … the force due to gravity (Fw)
3.
and the acceleration for each ball
Given: … one ball’s mass = 1.0 kg.
Relationships: Use: Fw = mg and a = F ÷ m
4.
Solution: For the 1.0 kg ball:
2.


a) Fw = (1.0 kg)(9.8 m/s2) = 9.8 N
b) a = (9.8 N) ÷ (1.0 kg) = 9.8 m/s2
Weight and mass
a. Use the equation for weight to calculate the force of gravity on each ball.
b. Use your answers from part a and Newton’s second law to calculate each ball’s
acceleration.
2.
3.
4.
Given: …the other ball’s mass = 10 kg.
Relationships: Use: Fw = mg and a = F ÷ m
Solution: For the 10 kg ball:


a) Fw = (10 kg)(9.8 m/s2) = 98 N
b) a = (98 N) ÷ (10 kg) = 9.8 m/s2
Summary: Both balls have the same acceleration!
Air resistance
 When
something falls through air, the air exerts an
additional force.
 This
force, called air resistance, acts opposite to the
direction of the object’s motion.
Terminal velocity
 Objects
only accelerate until the force of air
resistance equals the force of gravity.
 The
net force then becomes zero and the object
reaches a constant velocity called the terminal
velocity.
 The
terminal velocity depends on the ratio of an
object’s weight to its air resistance.
Parabolic Flights

NASA has been conducting
parabolic flights since the 1950s to
train astronauts.

Scientists and college students
have also gone on parabolic flights
to perform a wide variety of
chemistry, biology, and physics
experiments.

ZERO-G flights contain three types
of parabolas: Martian gravity (1/3
Earth gravity), Lunar gravity (1/6
Earth gravity), and zero gravity.