MOTION, WORK , AND POWER

MOTION 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 Speed- The rate at which an object moves

Velocity- Speed in a given direction

Since distance is usually measured in meters and time in seconds, the unit for speed is given in meters per second (m/sec)

Speed = Distance Time

MEASURING SPEED If a car travels 500 meters in 20 seconds, what is

its speed? 500 meters 20 seconds

Speed = distance = 500 meters = 25 m/sec time 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 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 Time

DETERMINING ACCELERATION 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 Time

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 Time

Deceleration = 10 m/ sec – 25 m/sec = -15 m/sec = -5 m/sec/sec

3 sec 3 sec

LAWS OF MOTION Newton’s laws of motion describe three

states of motion

Rest Constant motion Accelerated motion

The laws also explain how forces cause all of the states of motion

NEWTON’S FIRST LAW 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 force

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 forward

NEWTON’S SECOND LAW OF MOTION

The second law of motion show how force, mass, and acceleration are related

Force = Mass x AccelerationWhen 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

NEWTON’S THIRD LAW OF MOTION 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 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

MEASURING WORKWork = 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 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 unittime it can also be written

p = (f x d) / t

The unit for power is the newton-meter per second (m-n/sec) or the joule per second (J/Sec)

Large quantities of power are measured in kilowatts (kW) one kilowatt equals 1000 watts

MEASURING POWERpower = 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 wA 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