#### Transcript Work and Power - Todd County Schools

Physics Work, Energy, and Simple Machines Work and Energy • Work is defined as the product of the force exerted on an object and the distance the object moves in the direction of the force. • W = Fd • Work is a scalar quantity; it has no direction. • The SI unit of work is the joule (J). 1 J = 1N.m • The joule is named for James Prescott Joule, a 19th century English physicist and brewer. Work Done • *work is done only if the object moves in the direction of the force. • Example: A student lifts a box of books that weighs 185 N. The box is lifted 0.800 m. How much work does the student do? Solution • • • • Known: Unknown: F = mg = 185 N Work = ? d = 0.800 m W = Fd W = 185 N x 0.800 m • Work = 148 J Work and Direction of Force • Work is done only if a force is exerted in the direction of motion. • For work done at an angle, only the movement that is in the direction of the force is considered work. • The work you do when you exert a force at an angle to a motion is equal to the component of the force in the direction of the motion times the distance moved. • For work done at an angle: • W = F(cos q)d or Fd(cos q) • Negative work indicates that work is being done on an object; positive work indicates that you are doing work. Energy • The effect of doing work is that you give an object energy. • Energy is defined as the ability to do work or the ability to produce a change in itself or its surroundings. • By doing work on an object, you have transferred energy from your body to the object. • Therefore, Work is the transfer of energy by mechanical means. Power • Power is the rate of doing work or the rate at which energy is transferred. • Power is measured in watts (W). One watt is one Joule of energy transferred in one second. • Because a watt is so small, power is often measured in kilowatts (kW). • P = W/t Power = work divided by time Example Problem • An electric motor lifts an elevator that weighs 1.20 x 104 N a distance of 9.00 m in 15.0 s. • A. What is the power of the motor in watts? • B. What is power in kilowatts? Solution • • • • • • • Given: Unknown: F = 1.20 x 104 N power = ? d = 9.00 m t = 15.0 s P = (1.20 x 104 N)9.00 m / 15.0 s P = 7.20 x 103 w P = 7.20 x 103 w (1 kw/1000w) = 7.20 kW Machines • A machine eases the load either by changing the magnitude or the direction of a force, but does not change the amount of work done. • The force you exert on a machine is called the effort force, Fe. • The force exerted by the machine is called the resistance force, Fr. • The ratio of resistance force to effort force is called the mechanical advantage of the machine: • MA = Fr/Fe Simple Machines • Simple machines include lever, pulley, wheel-and-axle, inclined plane, wedge, and screw. Compound Machines • A compound machine consists of 2 or more simple machines linked so that the resistance force of 1 machine becomes the effort force of the second. • For example, in the bicycle, the pedal and sprocket act like a wheel-and-axle. The effort force is the force you exert on the pedal; the resistance is the force the sprocket exerts on the chain. Examples of Compound Machines