Work is the energy transferred when a force makes an object move In order for work to take place, a force must be exerted through a distance. The amount of work done depends on two things: the amount of force exerted and the distance over which the force is applied. There are two factors to keep in mind when deciding when work is being done: something has to move and the motion must be in the direction of the applied force. Work can be calculated by using the following formula: Work=force x distance Work Formula Work can be calculated by using the following formula: Work=force x distance Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally. 1
Power Power is the amount of work done in one second Power Formula Power(in watts)=work(in joules)/time(in seconds) P = W/t Power(in watts)=energy transfer(in joules)/time(in seconds) P = E/t Machine A machine is a device that makes doing work easier Work and Machines Machines make doing work easier Machines increase the force that can be applied to an object. Example: A screwdriver increases the force you apply to a screw. 2
Machines can also increase the distance over which a force is applied Example: a leaf rake Machines can make work easier by changing the direction of an applied force Example: A pulley changes a downward force to an upward force Mechanical advantage The force that is applied to the machine is the input force (F in ) The force thatis applied by the machine is called the output force (F out ) Mechanical advantage The ratio of output force to the input force of a machine is called mechanical advantage ***Machines make the OUTPUT force greater than the INPUT, making work easier 3
Ideal Machines Theoretical machine where there is no friction W in = W out Mechanical Advantage Formula MA = Output force (in Newtons) / Input force (in Newtons) MA = F out /F in Efficiency Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine Machines can be made more efficient by reducing friction Efficiency Formula efficiency(%) = output work(in joules/input work(in joules) x100% efficiency = W out /W in x 100% 4
Simple Machines A simple machine is a machine that does work with only one movement of the machine Levers A lever is a bar that is free to pivot or turn around a fixed point called a fulcrum First-Class Lever Third-Class Lever The fulcum is between the input force and the output force -a screwdriver is an example The input force is between the fulcrum and the output force -a bat is an example 5
Ideal mechanical advantage of a lever IMA = length of input arm (m)/length of the output arm(m) IMA = L in /L out Pulley A pulley is a grooved wheel with a rope, chain or cable running along the groove Fixed pulleys The pulley is attached to something that does not move Changes only the direction of force IMA is 1 Movable Pulleys A pulley in which one end of the rope is fixed and the wheel is free to move is called a movable pulley Multiplies the force IMA is 2 6
Block and Tackle a system of pulleys consisting of fixed and movable pulleys is called a block and tackle IMA is equal to the number of rope segments that support the weight Wheel and Axle A wheel and axle is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together Doorknobs, screwdrivers, and faucet handles are examples Usually the input force is applied to the wheel and the output force is exerted by the axle IMA of a wheel and axle IMA = radius of wheel (m)/radius of axle (m) IMA = r w /r a Gears A gear is a wheel and axle with the wheel having teeth around its rim When the teeth of two gears interlock, the turning of one gear causes the other gear to turn When gears of different sizes are interlocked they rotate at different rates 7
Inclined Planes An inclined plane is a sloping surface, such as a ramp, that reduces the amount of force required to do work. IMA of an inclined plane IMA = length of slope (m)/height of slope (m) IMA = I/h Screw A screw is an inclined plane wrapped in a spiral around a cylindrical post IMA of a screw is related to the spacing of the threads IMA is larger, when the threads are closer together A wedge is an inclined plane with one or two sloping sides Changes the direction of the input force Wedge 8
Compound machines Two or more simple machines that work together form a compound machine 9