Motion, Work , and Power

Motion- A change in position in a certain
amount of time
 In order to describe motion, you need a frame of
reference to compare it with
Speed- The rate at which an object moves
Velocity- Speed in a given direction
Speed = Distance
Since distance is usually measured in meters and
time in seconds, the unit for speed is given in meters
per second (m/sec)
If a car travels 500 meters in 20 seconds, what is its
500 meters
20 seconds
Speed = distance = 500 meters = 25 m/sec
20 seconds
What is the speed of a jet plane that travels 7200 km
in 9 hours?
Speed = 7200km / 9 hr = 800 km/hr
The speed of a cruise ship is 50km/hr. How far will
the ship travel in 14 hours?
Distance = Speed x time
= 50km/hr x 14 hrs
= 700 km
Acceleration- The rate in change of velocity
If something is accelerating, it is speeding up,
slowing down, or changing direction
 The acceleration of an object is equal to its
change in velocity divided by the time in
which the change occurs
Acceleration = Final velocity – Original Velocity
A rollercoaster has a velocity of 10 m/sec at the top of a hill. Two
seconds later it reaches the bottom of the hill with a velocity of 20
m/sec. What is the acceleration of the rollercoaster?
Acceleration = Final velocity – Original Velocity
Acceleration = 20 m/ sec – 10 m/sec = 10 m/sec = 5 m/sec/sec
2 sec
2 sec
A rollercoaster is moving at 25 m/sec at the bottom of a hill. Three
seconds later it reaches the top of the next hill, moving at 10 m/sec.
What is the deceleration of the rollercoaster?
Deceleration = Final velocity – Original Velocity
Deceleration = 10 m/ sec – 25 m/sec = -15 m/sec = -5 m/sec/sec
3 sec
3 sec
Newton’s laws of motion describe three states of
 Constant motion
 Accelerated motion
The laws also explain how forces cause all of the
states of motion
The first law states that an object at rest will remain
at rest and an object in motion will remain in motion
at a constant velocity unless acted upon by an outside
Since constant velocity means the same speed and
direction, in order for an object to change velocity, or
accelerate, a force must act on it
The concept of inertia forms the basis for this law
Inertia- the property of matter that tends to resist any
change in motion
 The inertia of an object is related to its mass
 The more massive an object, the more inertia it has and
the more difficult it will be to move
You feel the effects of inertia when you are riding in a
car that stops suddenly while you keep moving
The second law of motion show how force, mass,
and acceleration are related
Force = Mass x Acceleration
When mass is measured in kilograms and
acceleration in meters/sec/sec, force is
measured in Newtons (N)
Tells us that a greater force is require to
accelerate an object with a greater inertia (mass)
Explains one reason why smaller cars get better
gas mileage than larger ones
States that for every action, there is an equal and
opposite reaction
 According to the third law:
Every force must have an equal and opposite force
 All forces come in pairs
You demonstrate the third law when you walk
Your feet push against the floor
 The floor pushes with an equal but opposite force
against your feet
 You move forward
The reaction engine utilizes the third law
Work- a force acting through a distance
 In order for work to be done on an object, a force
must move it
 Work is the amount of force applied to an object
times the distance the object moves in the
direction of the force
If an object doesn’t move, no work is done
 If an object doesn’t move in the direction of the force,
no work is done
Work = force x distance (w = f x d )
Force (f) is measured in Newtons
Distance (d) is measured in meters
Work (w) is measured in newton-meters (n-m) or joules (J)
A force of 10,000 N is applied to a stationary wall. How much
work is performed?
Work = f x d = 10,000 N x 0 m = 0 N-m or 0 J
A 950 N skydiver jumps from as altitude of 3000 m. What is the total work
performed on the skydiver?
Work = f x d = 950 N x 3000 m = 2,850,000 N-m or 2,850,000 J
An ant does 1 N-m of work in dragging a 0.002 N grain of sugar. How far does
the ant drag the sugar?
distance = work / force = 1 N-m / 0.002 N = 500 m
Power- how fast work is done
The rate at which work is done, or the amount of work per
unit time
power = work / time (p = w / t)
Since power is the amount of work done per unit
time it can also be written
p = (f x d) / t
The unit for power is the newton-meter per second (mn/sec) or the joule per second (J/Sec)
Large quantities of power are measured in kilowatts (kW)
one kilowatt equals 1000 watts
power = work / time (p = w / t) or p = (f x d) / t
A horse performs 15,000 J of work pulling a wagon
for 20 seconds. What is the horse’s power?
power = 15,000 J x 20 seconds = 300,000 w
A 750 N pole vaulter lifts himself 5.0 m in 2.5
seconds. What is his power?
power = 750 N x 5.0 m / 2.5 sec = 1500 w
A tow truck pulls a car out of a ditch in 6.5 sec. If
6000 watts of power is used, how much work is
performed by the truck?
work = p x t = 6000 watts x 6.5 sec = 39,000 J