Loading...

PPT – Chapter 15 PowerPoint presentation | free to download - id: 748504-MmUwN

The Adobe Flash plugin is needed to view this content

Chapter 15 Work, Power Simple Machines

- Essential Questions
- I. What is Work? (In Physics Terms!)
- II. What is Power? (In Physics Terms!)
- III. How do machines make work easier and how

efficient are they? - IV. What are the 5 types of simple machines?
- V. What are compound machines?

15-1 What is Work?

- Work
- Def. Work is done when a force acts on an

object along the parallel direction the object

moves - In order for work to be done, a force must be

exerted over a distance. - Ex you can push on a wall for hours, youll be

real tired, but you havent done any work in

the scientific sense, anyway

15-1 Work

- Work
- The amount of work done in moving an object is

equal to the force applied to the object along

the direction the object moves times the distance

through which the object moves

- Units
- Force is measured in Newtons, Distance is

measured in meters. So, the unit is Newton X

meters. A Newtonmeter is known as a Joule (J)

15-1 Work

- A 700 N person climbs a 50 m cliff. How much

work did she perform?

GIVEN W F d F 700 N d 50 m

WORK W F d W (700 N) (50 m) W 35,000 J

15-1 Work

- An object weighing 200 N is lifted 0.5 m. How

much work was required?

GIVEN W F d F 200 N d 0.5 m

WORK W F d W (200 N) (0.5 m) W 100 J

15-1 Work

- A dog does 50 N-m (Joules) of work dragging a

0.05 N bone. How far did the bone move?

GIVEN W F d W 50 J F 0.05 N

WORK W F d d W F d (50 J)

(0.05 N) d 1,000 m

15-1 Work

- Mrs. OGormans superhuman strength allows her to

lift a pickup truck 2.0 m above the ground. How

much force was required if 25.0 Joules (J) of

work was done?

GIVEN W F d W 25.0 J d 2.0 m

WORK W F d F W d F 25.0 J 2.0 m F

12.5 N

15-2 Power

- Power
- Def The rate at which work is done, or the

amount of work per unit time. - Power tells you how fast work is being done so

it is a rate similar to the way speed, velocity

and acceleration are rates. Power is work per

unit time. - Any measurement per unit time is a rate!!
- Formula

15-2 Power

- Power
- rate at which work is done
- measured in watts (W)

P power (W) W work (J) t time (s)

15-2 Power

- Formula
- Since works formula is force X Distance, the

formula for Power may ALSO be written as

15-2 Power

- Units
- Work is measured in Joules (J), So, the unit for

Power is a Joule per second (J/s). - The short way to write a J/s is a Watt (W).

15-2 Power

- When do we use Watts in our Daily Lives?
- They are used to express electrical power.
- Electric appliances and lightbulbs are rated in

Watts. - Ex A 100 Watt light bulb does twice the work in

one second as a 50 Watt lightbulb.

15-2 Power

- A small motor does 4000 J of work in 20 sec.

What the power of the motor in Watts?

GIVEN W 4000 J T 20 sec P ?

WORK P W t P 4000 J 20 s P 200 J

s So P 200 W

15-2 Power

- An engine moves a remote control car by

performing 120,000 J of work. The power rating

of the car is 2400 W. How long does it take to

move the car?

GIVEN P 2400 W W 120,000 J T ?

WORK t W P t 120,000 J 2400 W t 50 sec

15-2 Power

- A figure skater lift his partner who weighs 450

N, 1.5 m in 3.0 sec. How much power is required?

GIVEN P ? F 450 N d 1.5 m t 3.0 sec

WORK

P F x d t P 450 N x 1.5 m 3.0

sec P 625 J (Nm) 3.0 sec

P 225 W

15-2 Power

- A sumo wrestler lifts his competitor, who weighs

300 N, 2.0 m using 300 Watts of power. How long

did it take him to accomplish this show of

strength?

GIVEN F 300 N d 2.0 m P 300 W t ?

WORK P W t W F x d W (300 N)(2.0 m)

600 J t 600 J 300 W t 2.0 s

15-3 Machines

- Machine def. Any device that changes the size

of a force, or its direction, is called a

machine. - Machines can be anything from a pair of tweezers

to a bus.

15-3 Machines

- There are always 2 types of work involved when

using a machine - Work Input - The work that goes into it.
- Work Output - The work that comes out of it.
- The work output can NEVER be greater than the

work input!!!

15-3 Machines

- So, if machines do not increase the work we put

into them, how do they help us? - Machines make work easier because they change

either the size or the direction of the force put

into the machine.

15-3 Machines

- Lets analyze this
- Machines can not increase the amount of work, so

work either stays the same or decreases. - The formula for work is
- Work force x distance

15-3 Machines

- Again, the formula for work is
- Work force x distance
- So, mathematically speaking, to end up with the

same or less work - If the machine increases the force then the

distance must decrease. - If the machine increases the distance, then the

force must decrease.

15-3 Machines

- Why is it that machines cant have more work

output than input? Where does all the work

disappear to? - A machine loses some of the input work to the

force of friction that is created when the

machine is used. - Part of the input work is used to overcome the

force of friction. - There is no machine that people have made that is

100 efficient

15-3 Machines

- If machines make our work easier, how much easier

do they make it? - The ratio of how much work output there is to the

amount of work input is called a machines

efficiency. - Efficiency is usually expressed as a percentage

().

15-3 Machines

- Efficiency
- measure of how completely work input is converted

to work output

- It is always less than 100 due to the opposing

force of friction.

15-3 Machines

- A worker exerts a force of 500 N to push a 1500 N

sofa 4.0 m along a ramp that is 1.0 m high. What

is the ramps efficiency?

GIVEN Fi 500 N di 4.0 m Fo 1500 N do 1.0

m

WORK Win (500N)(4.0m) 2000 J Wout

(1500N)(1.0m) 1500 J E 1500 J 100

2000 J E 75

15-3 Machines

- Mechanical Advantage is another way of expressing

how efficient a machine is. - Mechanical advantage is the ratio of resistance

force to the effort force.

15-3 Machines

- A worker exerts a force of 500 N to push a 1500 N

sofa 4.0 m along a ramp that is 1.0 m high. What

is the mechanical advantage of the ramp?

GIVEN Fe 500 N Fr 1500 N

WORK MA F resistance F effort MA

1500N 500 N MA 3

15-4 Simple Compound Machines

- Simple Machines
- There are six types of simple machines. They are

the - 1 - Inclined plane
- 2 - Wedge
- 3 - Screw
- 4 - Lever
- 5 - Pulley
- 6 - Wheel and axle

15-4 Simple Compound Machines

- 1 - Inclined Plane
- Def - A slanted surface used to raise an object.
- The force needed to lift the object decreases

because the distance through which the object

moves increases.

15-4 Simple Compound Machines

- 2 - Wedge - Inclined Plane Type 1
- Def an inclined plane that moves in order to

push things apart. - Tines of a fork, axe, knife.

15-4 Simple Compound Machines

- 3 - Screw - Inclined Plane Type 2 -
- Def - An inclined plane wrapped around a central

bar or cylinder, to form a spiral. - Ex screw duh!!!

15-4 Simple Compound Machines

- 4 - Lever
- Def - A rigid bar that is free to pivot, or move

around a fixed point called a fulcrum. - Ex see saw
- There are three main types (classes) of levers.

15-4 Simple Compound Machines

- 3 classes of levers
- First-class levers have the fulcrum placed

between the load and the effort, as in the

seesaw, crowbar, and balance scale. - Ex - a see-saw or scissors

15-4 Simple Compound Machines

- 3 classes of levers
- Second-class levers have the load between the

effort and the fulcrum. - Ex - a wheel barrow

15-4 Simple Compound Machines

- 3 classes of levers
- Third-class levers have the effort placed between

the load and the fulcrum. The effort always

travels a shorter distance and must be greater

than the load. - Ex - a hammer or tweezer

15-4 Simple Compound Machines

- 5 - Pulley
- Def - A rope, chain or belt wrapped around a

grooved wheel. - It can change the direction of force or the

amount of force needed to move an object.

15-4 Simple Compound Machines

- To calculate how much mechanical advantage a

pulley system creates Count the number of ropes

that are attached to the MOVEABLE pulley that

is your mechanical advantage!!!

15-4 Simple Compound Machines

- 6 - Wheel Axle
- Def - Made of 2 circular objects of different

sizes attached together to rotate around the same

axis.

15-4 Simple Compound Machines

- Compound Machine
- Def - A combination of 2 or more simple machines