1. How would the effort exerted by a backpacker over level ground compare to the effort in climbing a steep hill?

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1. How would the effort exerted by a backpacker over level ground compare to the effort in climbing a steep hill? - PowerPoint PPT Presentation

1. How would the effort exerted by a backpacker over level ground compare to the effort in climbing a steep hill? 2. How would the weight of the backpack affect the amount of force needed to move it?. Work and Machines A. Work 1. The transfer of energy when a force makes an object move.

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1. How would the effort exerted by a backpacker over level ground compare to the effort in climbing a steep hill?

2. How would the weight of the backpack affect the amount of force needed to move it?

Work and Machines

A. Work

1. The transfer of energy when a force makes an object move.

2. W = f ·d

(Unit is a joule

or N ·m)

Box being given energy

3. Energy is transferred

between objects when

work is done.

B. Power

1. The rate at which work is done.

2. P = W / t

3. Unit is the watt (1 joule / second)

4. Btu = 1,055 watts

(for heating and cooling units)

5. Horsepower = 746 watts

(for motors and engines)

C. Using Machines

1. A machine makes doing work easier.

2. They may multiply the applied force.

The car may weigh a lot, but you don’t have to use nearly that much force to lift it with a jack.

In this case, the amount of force necessary to push the chair up the ramp was decreased.

4. They may change the direction a force has to be applied.

The nail comes up as the person pulls to the side.

D. Important terms for machines

1. Resistance – the force being moved (FR)

2. Effort – the force being used to move a resistance (FE)

3. Effort Distance – the distance the effort force moves through (de)

4. Resistance Distance – how far the resistance moves (dr)

E. Work Calculations

1. Work Input – the amount of work done on a simple machine.

2. Win = FE • de

3. Work output – the amount of work the machine actually does.

4. Wout = FR • dr

F. Conservation of Energy

1. No machine can create energy, so Wout can never be greater than Win

2. In reality, Wout is alwaysless than Win because of friction producing heat; the heat had to come from the energy put into the machine.

3. An ideal machine is theoretical; it does not take friction into account.

4. Ideal machine: Win = Wout

1. MA is the number of times a machine multiplies the effort force.

2. MA = FR / FE & MA = de / dr

H. Efficiency

1. Measures how much of the work input is changed into useful output

2. Efficiency = (Wout / Win) x 100%

3. Lubricants (such as oiland graphite) reduce friction & increase efficiency.

Oil fills the space between surfaces so high spots don’t rub against each other.

I. The Simple Machines

1. Levers

a. 1st class:

b. 2nd class:

c. 3rd class:

d. IMA = effort arm / resistance arm

FE

FR

FE

FR

FR

FE

2. Pulleys

a. Fixed pulley

1) changes only the direction of a force

2) always has IMA = 1

b. Movable pulley

1) attached to object

2) IMA = 2

c. Block and Tackle

This strand does not count toward the IMA

1) system of fixed and movable pulleys

2) IMA = number of strands supporting the resistance

3. Wheel and Axle

a. IMA = rw / ra

b. Gears are modified forms of the wheel and axle

4. Inclined Planes: IMA = Ls / Lh

5. The Screw

a. Modified inclined plane wrapped around a cylinder

b. The pitch of the threading determines the IMA

6. The Wedge: two inclined planes back-to-back