Work, Power, and Simple Machines

Work = Force x Distance W = Fd Work: the amount of force applied to an object to move it a distance Units of work is the Joule (J) 1 Joule ( J ) = 1 Nm W = Fd

Work Work is done on an object when the object moves in the same direction as the applied force. -No work without motion. -Work must be in the same direction as the motion of the object.

How much work does it take to move a 500 N car a distance of 20 m? Example1 How much work does it take to move a 500 N car a distance of 20 m? F = 500 N d = 20 m W = Fd W = (500 N)(20 m) = 10,000 J

Power Power = = Units of power is the Watt ( W ) = 1 Work Time Power: the rate at which work is done. Power = = P = = Units of power is the Watt ( W ) = 1 *1 horsepower = 746 watts Work Time Force x Distance Time W t Fd t Joule Sec

Example 2 How much power does a person weighing 750 Newtons need if he takes 20 seconds to walk up a staircase 10 meters high? F = 750 N d = 10 m t = 20 s P = Fd t P = (750 N)(10 m) = 375 W 20 s

Machines Changes at least one of three factors Forces – Input or Output Distance the force acts Direction the force acts Simple machines change input forces, they do not create force. A motor produces kinetic energy output from a fuel source.

Mechanical Advantage The number of times a machine increases a input force. Mechanical Advantage = Work (Force x Distance) is not changed. Output force Input force

Ramp A flat sloped surface Less input force over a longer distance Inclined Plane Ramp A flat sloped surface Less input force over a longer distance Ramp

Wedge Starts as a thin edge and tapers to a thick end Two inclined planes back to back Changes the direction of the force and uses less input force over a longer distance knife

Screw An inclined plane wrapped around a cylinder Changes the direction of the force and uses less input force over a longer distance

Lever - First Class A rigid bar that can rotate around a fulcrum. - Changes direction - Less Force over a longer distance Fulcrum is between forces 4:1 ratio Crowbar, pliers Opposite is light switch, jaw

Lever - Second Class A rigid bar that can rotate around a fulcrum. - Less Force over a longer distance (as shown) 5:1 ratio can opener, nut cracker, wheelbarrow, ball of your foot muscle in calf

Lever - Third Class A rigid bar that can rotate around a fulcrum. - More Force over a shorter distance 1:5 ratio bat, tennis racket, ax, shovel, fore arm elbow is the fulcrum

Wheel and Axle Two cylinders or circular objects attached to each other - The wheel has the larger radius - The axle has the smaller radius A lever that can turn 360o, always working Changes distance and force Screw driver Mechanical advantage = output radius/input radius

Pulley A rope over a grooved wheel - A single pulley changes direction A pulley system (block and tackle) - Can change direction of force - Changes force and direction Fixed pulleys are attached to a structure, Moveable is attached to the object that is being moved

A combination of two or more machines Compound Machines A combination of two or more machines

Efficiency of Machines Efficiency = x 100% If there were no friction, the efficiency would be 100%, but as a machine becomes more complex or moves further, friction is increased. Output work Input work

Machines Changes at least one of three factors Forces – Input or Output Distance the force acts Direction the force acts