Presentation on theme: "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."— Presentation transcript:
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 Box being given energy 3. Energy is transferred between objects when work is done. (Unit is a joule or N ·m)
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 dont have to use nearly that much force to lift it with a jack.
3. They may increase the distance over which a force is applied. 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 (F R ) 2. Effort – the force being used to move a resistance (F E ) 3. Effort Distance – the distance the effort force moves through (d e ) 4. Resistance Distance – how far the resistance moves (d r )
E. Work Calculations 1. Work Input – the amount of work done on a simple machine. 2. W in = F E d e 3. Work output – the amount of work the machine actually does. 4. W out = F R d r
F. Conservation of Energy 1. No machine can create energy, so W out can never be greater than W in 2. In reality, W out is always less than W in 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: W in = W out
G. Mechanical Advantage 1. MA is the number of times a machine multiplies the effort force. 2. MA = FR FR / F E & MA = de de / drdr H. Efficiency 1. Measures how much of the work input is changed into useful output 2. Efficiency = (W out / W in ) x 100%
3. Lubricants (such as oil oil and graphite graphite) reduce friction & increase efficiency. Oil fills the space between surfaces so high spots dont rub against each other.
I. The Simple Machines 1. Levers a. 1 st class: b. 2 nd class: c. 3 rd class: d. IMA = effort arm / resistance arm FRFR FEFE FRFR FEFE FRFR FEFE
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 1) system of fixed and movable pulleys 2) IMA = number of strands supporting the resistance This strand does not count toward the IMA
3. Wheel and Axle a. IMA = rw rw / rara b. Gears are modified forms of the wheel and axle
4. Inclined Planes: IMA = Ls Ls / LhLh 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