Energy Physics

Energy Energy may be the most familiar concept in science; however, it is the most difficult to define. Energy is a concept that has been developed in the last 150 years. Persons, places, and things all have energy but it is observable only when it is being transferred or transformed

Energy In general for Physics… Energy is the ability of an object to produce a change in itself or it’s environment It is a measure of the capability to produce change.

Energy and Work Work transfers energy… …but what is work?

Work In physics, work is defined as a force acting upon an object which causes it to move some distance. Work = force x distance Ex. A weight lifter holding a barbell above his head is doing no work. Work is only done when he lifts it. Work Atlas Not Work Sisyphus

Is Work being done? No… Why? No… There is a force, but no displacement Situation Answer A teacher applies a force to a wall and becomes exhausted. A book falls off a table and free falls to the ground. A waiter carries a tray full of meals above his head by one arm across the room. A rocket accelerates through space. No… There is a force, but no displacement Yes… Weight is the force and the table height is the displacement No… Why? Yes… There is force and displacement

Don’t Forget Units… Work = Force x distance Work = Fd Work Units = Newtons times Meters Work Units = N-m Work (and Energy) expressed in Joules The Joule is the standard unit of work 1 Joule = 1 N-m In other words, one Joule is equivalent to one Newton of force causing a displacement of one meter

Examples If you push a box with a force of 20N a distance of 3m how much work have you done? If you lift a 30kg bag from the ground up 2m how much work is done? 20N x 3m = 60Nm or 60 J 30 kg x 9.8m/s2 = 294N 294N x 2m = 588 J

Power Power deals with how fast work is done. Power is the rate at which work is done. Power = work done/time interval Units for power = joules/s = watt (W) In the metric system, engines are rated in kilowatts In the US, we rate electricity in W and kW but engines in horsepower. Horsepower (hp) is another measure of power 1hp =0.75kW 164 hp 700 hp

Examples John carries a heavy box up a set of stairs. Joe carries a similar box but it takes him half the time. Who does more work? Who exerts more power? They do the same amount of work. Joe exerts twice as much power.

Mechanical Energy Energy enables an object to do work. The most common forms of mechanical energy are kinetic and potential. Kinetic Energy – energy due to the movement of something Potential Energy – energy due to the position of something.

Potential Energy (PE) Energy that an object has stored in readiness that has the potential for doing work. Work is required to lift anything in the Earth’s gravity. Energy due to an elevated position is called gravitational potential energy. PE = mass x gravity x height or mgh

Example How much work is done to push a 100N boulder with a force of 100N, 10m across the room? How much PE did it gain? How much PE is gained by lifting the boulder 5m? W = Fd 100N x 10m =1000J 0J because it didn’t change height PE=mgh 100N x 5m = 499.8J

Kinetic Energy (KE) Energy of motion KE = ½ mv2 The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest. Net force x distance = kinetic energy or Fd= ½ mv2

Law of Conservation of Energy Energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy never changes. KE + PE for an object is the same throughout a transformation. KEi + PEi = KEf + PEf

Example The sum of the woman’s kinetic (KE) and potential energy (PE) remains the same throughout her fall. The sum (TME) of the rollercoaster’s kinetic (KE) and potential energy (PE) remains the same throughout the ride.

Machines A machine is a device used to multiply force or change the direction of forces. There are 6 types of simple machines Lever, inclined plane, pulley, screw, wedge, axle and wheel.

Mechanical Advantage Every machine has a mechanical advantage. Mechanical advantage is the ratio of output force to input force (output force/input force) Neglecting friction, can also be determined by the ratio input distance/output distance

Efficiency Efficiency is the ratio of useful work output to total work input. Or actual mechanical advantage/ theoretical mechanical advantage. Efficiency is most often expressed as a percent. An ideal machine would have an efficiency of 100% Why can’t we have 100% efficient machines?

Examples Using a lever, Jenny tries to move a boulder. She exerts a force of 4N and a force of 60N is exerted on the boulder what is the mechanical advantage? 15 Using a pulley system, Craig must exert 50J of work to get 35J out of the pulleys. What is the efficiency of the pulley system? 35J/50J = .70 or 70%

Review Energy - the ability of an object to produce a change in itself or it’s environment Work = force x distance Power = Work/time Potential energy - Energy due to an elevated position. PE = mgh Kinetic energy - Energy of motion. KE = ½ mv2 Law of Conservation of Energy - Energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy never changes. KEi + PEi = KEf + PEf Mechanical advantage – output force/input force Efficiency – (actual mechanical advantage/theoretical mechanical advantage) x 100%