1. Motion, Work , and Power

2. 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
Motion

3. Speed Speed = Distance Time 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

4. 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 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

5. 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

6. 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

7. Newton’s second law of motion
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

8. 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

9. 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

10. Work = force x distance (w = f x d )
Measuring work
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 N grain of sugar. How far does the ant drag the sugar?
distance = work / force = 1 N-m / N = 500 m

11. 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 unit
time 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

12. Measuring Power 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 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