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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.
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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?
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.
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)
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.
The nail comes up as the person pulls to the side.
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)
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
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
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.
a. 1st class:
b. 2nd class:
c. 3rd class:
d. IMA = effort arm / resistance arm
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
This strand does not count toward the IMA
1) system of fixed and movable pulleys
2) IMA = number of strands supporting the resistance
a. IMA = rw / ra
b. Gears are modified forms of the wheel and axle
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