Chapter 6 – Work and Machines

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 the object must be in the same direction as the applied force on the object. 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 a force is applied to the object and the object moves. Calculating work: Work = Force (in Newtons) x distance

POWER Power – the amount of work done in a certain amount of time; rate at which work is done. POWER = work/time Power is measured in watts (W) Since energy and work are related, power also can be calculated. POWER = energy/time

Using Machines A device that makes work easier is a machine.

Machines increase applied force and/or change direction of applied force to make work easier. 1. The same amount of work can be done when applying a small force over a long distance or applying a larger force over a short distance, since work = force x distance.

Machines help move things that resist being moved. 2. Increasing distance reduces the amount of force needed to do work. 3. Some machines change the direction of the applied force to do the work. (pulley) Machines help move things that resist being moved. 1. Force applied to a machine is input force. 2. Output force – force applied by the machine.

3. The amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine. Some energy transferred is changed to heat due to friction. An ideal machine with no friction would have the same input work and output work.

Mechanical Advantage Mechanical advantage (MA) is the number of times a machine multiplies the input force. MA = output force/input force.

Efficiency Measure of how much work is put into a machine is changed into useful output work by the machine. Efficiency = output work/input work x 100 The efficiency of a machine is always less than 100%. WHY? Lubricants can make machines more efficient by reducing friction.

Simple Machines A machine that does work with only one movement is a simple machine. Lever – bar that is free to pivot about a fixed point called a fulcrum.

Three classes of levers based on positions of input force, output force, and fulcrum. First Class Lever – fulcrum is located between the input and output force; multiplies and changes the direction of the force.

Second Class Lever – output force is located between the input force and fulcrum; always multiplies force.

Third Class Lever – input force is between the output force and fulcrum; doesn’t multiply force but does increase distance over which force is applied.

Ideal Mechanical Advantage Calculating ideal mechanical advantage (IMA) of a lever: IMA = length of the input arm / length of the output arm.

Pulleys A grooved wheel with a rope, simple chain, or cable running along the groove is a pulley. A fixed pulley is attached to something that doesn’t move. Force is not multiplied but direction is changed. IMA=1.

A movable pulley has one end of the rope fixed and the wheel free to move. It multiplies force; IMA = 2.

Block and tackle – system of pulleys consisting of fixed and movable pulleys; IMA = # of ropes supporting the weight.

Wheel and Axle Machine with 2 wheels of different sizes rotating together; modified lever form. IMA = radius of wheel / radius of axle. Gears are a modified form of wheel and axle.

Inclined Plane Sloping surface that reduces the amount of force required to do work. IMA = length of the slope (input distance) / height of the slope (output distance). Less force is required if a ramp is longer and less steep.

Screw – inclined plane wrapped in a spiral around a cylindrical post. Inclined plane with one or two sloping sides is a wedge. Compound Machine – uses a combination of 2 or more simple machines.

How would you calculate the IMA of this machine?