Unbalanced Forces and
Unit: 06
Lesson: 01
Reviewing Motion Notes
• a. What is motion?
• Motion - The act or process of changing position or
• Examples - moving vehicles, swinging a bat, ball
flying through the air.
• b. What is speed?
• Speed - distance traveled per unit time.
• Examples – 2 meters per second, 55 miles per hour,
22 kilometers per hour.
• c. What is displacement?
• Displacement – the distance and direction of an
objects change in position from the starting point.
Force and Motion
• d. What is distance?
• Distance – how far an object has moved: uses units such
as meters, feet, inches, centimeters, kilometers etc.
• e. What is constant speed?
• Constant speed - when an object in motion neither speeds
up or slows down.
• f. What is average speed?
• Average speed – the total distance traveled divided by the
total time of travel.
• g. What is velocity?
• Velocity – includes the speed of an object AND its
• Examples – race car traveling at 55 mph due north, boat
traveling southeast at 22 km/h.
Review Newton’s 1st Law
• What is the relationship between mass and
As mass increases, force increases
• Inertia- the tendency of a body at rest to remain
at rest or a body in motion to remain in motion
unless acted upon by an outside force.
• Unbalanced forces- forces that act upon an
object that cause the object to move in the
direction of the greater force
Newton’s 1st Law
This law is often called "the law of inertia".
• What does this mean?
• This means that there is
a natural tendency of
objects to keep on
doing what they're
• 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
Newton's 2nd Law F=ma
• Acceleration is
produced when a
force acts on a
• The greater the
mass (of the object
being accelerated)
the greater the
amount of force
needed (to
accelerate the
Newton’s Second Law
F = ma
What does the equation mean?
F = force (unit is Newton’s)
m = mass (unit is Kilograms)
a = acceleration (unit is meters/second2)
Remember that velocity is the speed and direction of an
• Remember that speed is the rate of change of an objects
• Acceleration is the rate of change of an object’s velocity.
• Force is defined as a push or pull that one object exerts on
another object.
• Mass is the amount of matter contained in an object.
• 1. You are pushing a friend on a sled. You
push with a force of 20 N. Your friend and
the sled together have a mass of 60 kg.
Ignoring friction, what is the acceleration of
your friend on the sled?
F = 20 N
m = 60 kg
a = 0.33 m/s2
• 2. A student pedaling a bicycle applies a
net force of 400 N. The mass of the rider
and the bicycle is 25 kg. What is the
acceleration of the bicycle and the rider?
• F = 400N m = 25 kg
• a = 16 m/s2
• 3. How much force will a tennis racket
need to exert on a tennis ball, with a mass
of 0.67 kg, to make it accelerate at a rate
of 5,600 m/s2?
• m = 0.67kg a = 5,600 m/s2
• F = 3752 N
Newton’s 3rd Law
• For every action
there is an equal
and opposite reaction.
• This means that for
every force there is
a reaction force that
is equal in size, but
opposite in
Other examples of Newton’s Third
• The baseball forces
the bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
3rd Law
• Consider the motion
of a car on the way to
school. A car is
equipped with wheels
which spin
backwards. As the
wheels spin
backwards, they grip
the road and push the
road backwards.
3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
A What?
What does a tribologist study in relation to force
and motion.
Tribology comes from the Greek “tribo” , meaning
“to rub”, and “logos” meaning “principle or logic.”
Is a scientist that studies the principle of friction,
lubrication, and wear (of interacting surfaces that
are in relative motion).
Explain the results from the shoe drag
1. What is happening to the shoe before it
begins to move?
It is experiencing static friction between it and the
floor. Static friction is the force which keeps a
motionless object from being pushed or pulled
across a surface.
Friction (cont.)
2. Why is the spring not stretched as far once the
shoe begins to move?
Once the shoe starts moving, the spring is not
stretched as far as when the shoe was sitting
still because when the shoe was sitting still, it
had to overcome the static friction, as well as
inertia, to get going. Once the shoe was going, it
experiences kinetic friction. Kinetic friction acts
to resist the motion of an object sliding across a
Friction (cont.)
3. Why is there less friction to resist its
movement once the shoe starts moving?
Kinetic friction is less than static friction, if
everything else stays the same.
4. What happened to the shoe when the
rocks were added?
When rocks were added, the increased mass
increased inertia. The aluminum foil and wax
paper also had different effects on the amount of
friction that was present.
Friction (cont.)
5. What has an impact on the amount of
friction in a system?
The kind of material, weight, roughness and
lubricants can all have an impact on the amount
of friction in a system.
Which of the following examples
show how Newton’s First Law of
Motion occurs in everyday life?
• A car suddenly stops and you strain against the
seat belt.
• While riding a skateboard, you fly off the board
when hitting a curb, a rock, or another object,
which abruptly halts the motion of the
• A magician pulls the tablecloth out from under a
table full of dishes and the don’t fall off the table.
• The car turns left and you lean or slide to the
right side of the car.
Newton’s 1st Law of Motion= Inertia
Newton's First Law of Motion= Inertia
• Why is Newton’s First
Law of Motion called
the law of inertia?
States that an object at rest
tends to stay at rest, and an
object in motion tends to stay
in motion with the same
speed and in the same
direction unless acted upon
by an unbalanced force.
Newton's First Law of Motion= Inertia
• What is an unbalanced
Since these two forces are of
equal magnitude and in
opposite directions, they
balance each other. The
book is said to be at
equilibrium. There is no
unbalanced force acting
upon the book and thus, the
book maintains its state of
No motion
Gravity Pulling Down on all objects
Force applied to the book by your hand
Table pushes upward on the book
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