Machines and Efficiency What are machines A device

Machines and Efficiency

What are machines? • A device for multiplying forces or simply changing the direction of the force. – ↑ force – Changes direction

• Conservation of Energy Work input = work output – The amount of output can only be less than or equal to the amount of input.

Conservation of Energy • Work = force x distance • (Force x distance)input = (force x distance)output

Example Problem 1: • A lever uses 50 N of force pushed 6 m down to move 150 N book up. How far was the book moved?

Answer: (Force x distance)input = (force x distance)output (50 N x 6 m) = (150 N x distance) 300 J = 150 N x distance 2 m = distance

Sample Problem 2: • To lift a boulder 3 m requires 350 N. A ramp requires 9 m to reach the same height. How much force is needed?

Answer: (Force x distance)input = (force x distance)output (350 N x 3 m) = (force x 9 m) 1050 J = 9 m x force 116. 7 N = force

Efficiency • • In an ideal machine, the work input = the work output. In most machines, some energy is lost so the work output is less.

Formula for Efficiency • Efficiency = work ÷ energy used x 100 – Expressed as a percent

Sample Problem: • A person uses 70 J of energy to move a 15 N desk 4 meters. How efficient is this movement?

Answer: Work = force x distance Work = 15 N x 4 m = 60 J of work Efficiency = work ÷ energy x 100 Efficiency = 60 J ÷ 70 J x 100 =85. 7%