Work & Energy

Work The product of the force exerted on an object and the distance the object moves in the direction of the force. W = F • d J = N • m

Work Work is done only if the object moves. Work is done only if the direction of the force and the direction the object moves are the same. The area under the curve of a force-displacement graph is the work done.

Work If the force applied to an object is at an angle, then the horizontal component of the force is used to calculate the work done horizontally and the vertical component of the force is used to calculate the work vertically. Wx= (Fa • cos Q) • dx Wy= (Fa • sin Q) • dy

Incline If the object is on an incline, the parallel component of weight will be the used in determining the net force along the incline. Fn = Fwsin Q + Fa + Ff

Energy Energy is the ability to do work or cause a change If you do work on an object you will cause a change or give it energy or take it away.

Power Power is the rate of change of work

Machine a device that makes work easier by changing the size or the direction of the applied force A machine does NOT change the amount of work done

Input & Output WI = FE • dE WO = FR • dR Input Output

Mechanical Advantage The ratio of the amount of force needed (FE) to the amount of weight moved (FR). Large mechanical advantages show a low force is required to move a heavy object. Mechanical advantages can be less than one (but not less than zero).

No Friction If there is no friction, then the work input can become work output. Then, WI = WO FE • dE = FR • dR If we divide both sides by FE & dR, we find that the ratio (FR/ FE) = (dE/ dR). This is ideal mechanical advantage (IMA).

Efficiency The ratio of the amount of work output (WO) to the work input (WI). Efficiency is expressed as a percent; multiply the ratio by 100. Efficiency can never be more than 100%! Friction always reduces efficiency.

Compound Machines Compound machines have more friction, therefore have lower efficiencies. The efficiency of a compound machine is found by multiplying the individual efficiencies together. The MA of a compound machine is found by multiplying the individual mechanical advantages together.

MA > 1 What is the advantage of a machine that has a MA > 1? Less force is required to move a heavy object. The downside is more distance is needed.

MA < 1 What is the advantage of a machine that has a MA < 1? The input will be a shorter distance. However, more effort will be required.

MA = 1 What is the advantage of a machine that has a MA = 1? The same force will be needed to move the object. The advantage is that the direction of the force will change.

Classes of Levers 1st Class FE FR 2nd Class FR FE FE FR 3rd Class

Classes of Levers FE FR 1st Class The MA can be less than one or greater than one depending on the length of the effort arm and the length of the resistance arm.

Pulleys Pulleys will be studied in more detail in the lab.

Classes of Levers FR FE 2nd Class The MA is always more than one because the length of the effort arm is always longer than the resistance arm.

Classes of Levers FE FR 3rd Class The MA is always less than one because the length of the effort arm is always shorter than the resistance arm.