Work and Simple Machines

Work and Power Force- a push or pull on an object Did you know that if you pushed on a wall all day, you would be very tired, but you would not have done work…..... UNLESS, the wall moved!!!! We tend to think of work as cleaning house, doing chores on the farm, going to an office or factory, or even homework. But work can be turning a doorknob, raising the window blinds, picking up your pencil, or walking.

What is work? 2 conditions must be met: 1- you must apply a force Work- a force causes an object to move in the same direction that the force is applied. Exerting a force does not mean you have worked. 2 conditions must be met: 1- you must apply a force 2- object must move in same direction as applied force

Work

Energy Energy is the ability to produce change, OR Energy is the ability to do work. Work converts energy from one form to another. Work-energy theorem: When work is done on an object, the object’s kinetic energy changes. The work done is equal to the change in kinetic energy. (kinetic energy is energy of motion)

Energy and Work The unit we use to measure work and energy is the Joule (J), named for the physicist James Prescott Joule.

Calculating Work More work is done when the force is increased or the object is moved a greater distance. So the equation is: Work (J) = force (N) X distance (m) W= Fd

Power Power- How quickly work is done Two weightlifters may lift the same amount of weight the same vertical distance, doing the same amount of work. But, if one does it in less time, he/she has more power. Power (watts) = work (J) time (s) We also measure Power in Horsepowers, power required for 33,000 lbs to be moved 1 ft in 1 minute. 746 watts = 1 horsepower

Work and Energy When pushing an object a distance, work is done, kinetic energy is increased. When an object is lifted, it’s potential energy increases because it is higher above Earth’s surface.

Power and Energy Since energy can never be created or destroyed (law of conservation of energy), if one object gains energy, another must lose energy (energy is transferred). Power is the amount of energy transferred in a certain amount of time.

Using Machines Machine- a device that makes doing work easier. We often think of machines as cars, coffee makers, dryers, etc. but something as simple as a knife or a broom is a machine. Machine- a device that makes doing work easier. Machines change the way in which you do work, but they do not reduce the amount of work you need to do.

Machines don’t work by themselves! You must exert a force on a rake or lift the handles of the wheelbarrow. The work you do on the machine is equal to input forces X distance the machine moves. Input force- the work you do on the machine

Machines don’t work by themselves! The machine exerts force to move an object over a distance. Ex. Rake on the leaves. Output force- the work the machine does Sometimes called “resistance force” b/c machine is overcoming some resistance.

Remember this!!! Output work can NEVER be bigger than input work. So what’s the advantage of using a machine?

Machines make work easier. Machines make work easier by: 1- changing the amount of force needed 2- changing the distance over which the force is exerted 3- changing the direction in which the force is exerted

Mechanical Advantage Mechanical Advantage= the number of times a machine increases the input force. No units as it is a ratio. MA - Output force (N) Input force (N)

Example To pry the lid off a paint can, you apply a force of 50 N to the handle of the screwdriver. What is the mechanical advantage of the screwdriver if it applies a force of 500N to the lid? MA = force output 500 N force input 50 N MA = 10 The screwdriver increased the input force (effort) by 10 times.

Efficiency Efficiency of a machine is the ratio of the output work to the input work. Efficiency (%) = output work (J) input work (J) X 100% Efficiency is decreased if friction is increased. Machines are never perfectly efficient because of friction changes some work into heat energy.

Efficiency examples You do 100 J of work in pulling out a nail with a claw hammer. If the hammer does 70J of work, what is the hammer’s efficiency? You do 150 J of work pushing a box up a ramp. If the ramp does 105 J of work, what is the efficiency of the ramp?