Work and machines Machines make jobs easier

Machine – a device that helps do work by either changing the size of a force or changing the direction of the force

Making work easier Machines Change the amount of force you exert Do change the amount of work done Input force output force

Making work easier Machines Change the distance over which you exert force Do change the amount of work done Output force Input force

Making work easier Machines Change the direction in which you exert force Do change the amount of work done Output force Input force

Total work does not change Work = Force X Distance Force and distance have an inverse (opposite) relationship If a machine allows you to apply a smaller force the distance will increase If a machine reduces the distance the force will increase If a machine allows you to apply a force in a different direction it doesn’t change how much force you have to apply or how far you move it

Work Input The work done on a machine W = F x d

Work Output The work done by a machine W = F x d

Mechanical advantage The number of times a force exerted on a machine is multiplied by the machine Mechanical ad. = Output force Input force If M.A. > 1 the machine output force is greater than the input force – helpful for lifting heavy objects If M.A. < 1 the machine reduces force needed but distance increases (fishing pole)

Calculating Mechanical Advantage Problem A mover uses a handcart to lift a refrigerator. He applies an input force of 50 N. The handcart applies an output force of 350N on the refrigerator. What is the mechanical advantage of the handcart?

Write the equation MA = Output force Input force

Fill in the values from the problem MA = 350 N 50 N

Solve the problem MA = 350 N 50 N MA = 7 The output force of the handcart is 7 times the input force of the man The output force is applied through a distance 1/7 the distance of the input force

Efficiency Efficiency compares the output work to the input work (%) Cannot be greater than 100%

Machines are not 100% efficient because of friction Friction A force that opposes motion – will slow machines down Occurs when 2 surfaces touch or rub against each other May cause heating of machine The less work that the machine has to do to overcome friction, the more efficient it is Work output is always less than work input No work is lost Work output + work to overcome friction = work input

There can’t be an ideal machine An ideal machine would have 100% efficiency Since any time 2 surfaces rub together there is friction, this is impossible Technologies exist to increase efficiency by decreasing friction Ideas?

Increasing Efficiency