Chapter 8: Conservation of Energy & Simple Machines

Conservation of Energy Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

A little Review

Try these questions.

The answers A: PE = 40 J (since the same mass is elevated to 4/5-ths height of the top stair) B: PE = 30 J (since the same mass is elevated to 3/5-ths height of the top stair) C: PE = 20 J (since the same mass is elevated to 2/5-ths height of the top stair) D: PE = 10 J (since the same mass is elevated to 1/5-ths height of the top stair) E and F: PE = 0 J (since the same mass is at the same zero height position as shown for the bottom stair).

Practice Problem

Work Input = Work Output Machine: A device used to multiply forces or simply to change the direction of forces. Work Input = Work Output Fd = fd

Simple Machines

Mechanical Advantage: The ratio of output force to the input force of a machine.

A machine lifts a 575 N mass, but the person using the machine only exerts 25 N. What is the mechanical advantage of the machine?

A lever is used to lift a load weighing 300 N A lever is used to lift a load weighing 300 N. The force needed was 600 N. What is the mechanical advantage of this first class lever?

Lever: A bar that turns around a fixed point(fulcrum). 3 types

Class 1 lever •M.A. = 1, when Load arm = Effort arm, e.g. beam balance Relative positions of load, fulcrum and effort of class I lever •M.A. = 1, when Load arm = Effort arm, e.g. beam balance •M.A. > 1, when Effort arm > Load arm, e.g. Cutting pliers •M.A. < 1, when Effort arm < Load arm, e.g. Garden scissors

Class 2 lever MA is always greater than 1 Force multiplier effort arm is always greater than the load arm

Class 3 lever MA always less than 1 Gain speed, larger movement of load

Pulley: A lever that can change the direction of a force. MA is determined by the strands the support the load.

Efficiency: The ratio of useful work output to total work input.

A machine requires an put of 600 J of energy to lift a 25 N box into the air a distance of 4 meters. Calculate the efficiency of the machine.

A machine operates with an efficiency of 65 % A machine operates with an efficiency of 65 %. How much energy would the machine require if the machine was to lift a 3 kg box into the air 20 m?