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