Work and Machines Chapter 5

What machines do you use in your life to help you do some type of work?

Work - Transfer of energy that occurs when a force makes an object move. - For work to occur, an object must move - the motion of an object must be in the same direction as the applied force on the object

Work and Energy Work and energy are related, since energy is always transferred from the object doing the work to the object on which the work is done. Work is done on an object only when the force is being applied to the object and the object moves. Calculating work- Work equals force multiplied by distance Work(in Joules)= applied force(Newtons) X distance(meters) W=Fd

1.A force of 75N is exerted on a 45kg couch and the couch is moved 5m. How much work is done in moving the couch? 2.A lawn mower is pushed with a force of 80N. If 12,000J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed?

POWER The amount of work done in a certain amount of time; rate at which work is done - Calculating power- Power equals work divided by time Power(watts)=work(joules) time(seconds) P=W t 1.To lift a baby from a crib 50j of work is done. How much power is neede if the baby is lifted in 0.5s? 2.If a runner’s power is 130W, how much work is done by the runner in 10 minutes?

USING MACHINES A device that makes doing work easier is a MACHINE. Machines increase applied force and/or change direction of applied force to make work easier. -Same amount of work can be done by applying a small force over a long distance as can be done by applying a large force over a short distance, since work equals force times distance. -Increasing distance reduces the amount of force needed to do the work -Some machines change the direction of the applied force to do the work.

Machines can help move things that resist being moved. - Force applied to machine is INPUT FORCE -OUTPUT FORCE – force applied by a machine to overcome resistance. -Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine a. some energy transferred is changed to heat due to friction b. An ideal machine with no friction would have the same input work and output work.

Input and output force

Mechanical Advantage (MA) Is the number of times a machine multiplies the effort force. It is calculated by: MA= resistance force divided by effort force

EFFICENCY Measure of how much of the work put into a machine is changed into useful output work by the machine. -Calculating efficiency- output work divided by input work times 100% - Efficiency of a machine is always less than 100% - Lubricants can make machines more efficient by reducing friction.

SIMPLE MACHINES A machine that does work with only ONE motion LEVER- a bar that is free to pivot about a fixed point called the fulcrum. -Input arm is part of the lever on which effort force is applied. -Output arm is part of the lever that exerts the resistance force.

Three classes of levers based on positions of effort, force, and fulcrum 1. FIRST CLASS LEVER- fulcrum is located between the effort and resistance forces; multiplies and changes direction of force

Second class lever- resistance force is located between the effort force and fulcrum; always multiplies force.

Third class lever- effort force is between the resistance force and fulcrum; doesn’t multiply force but does not increase distance over which force is applied.

Calculating ideal Mechanical Advantage (IMA) of a lever- IMA equals the length of the input arm divided by the length of output arm

PULLEY Grooved wheel with a rope, simple chain, or cable running aalong a groove is a pulley, which is a modified first class lever. 1. A FIXED PULLEY- is attached to something that doesn’t move; force is not multiplied but direction is changed; IMA=1

A MOVEABLE PULLEY- pulley has one end of the rope fixed and the wheel free to move; multiplies force; IMA=2

BLOCK AND TACKLE- system of pulleys consisting of fixed and moveable pulleys; IMA= number of ropes supporting resisting weight

Wheel and Axle- machine with two wheels of different sizes rotating together; modified lever form IMA=radius of wheel divided by the radius of the axle Gears are a modified form of the wheel and axle

INCLINED PLANE- sloping surface that reduces the amount of force required to do work. IMA= length of slope(effort distance) divided by height of slope(resistance distance) Less force required if a ramp is longer and less steep

Screw- inclined plane wrapped in a spiral around a CYNDRICAL POST Wedge-an inclined plane with one or two sloping sides

Compound Machine- uses a combination of two or more simple machines