Summary of the Laws of Motion First Law: If no net force acts on it, then an object remains at rest OR an object in motion remains in motion Second Law: F = ma Third Law: Action force is equal to reaction force

Lecture 5 Work and Energy Chapter 3.1  3.7 Outline Work and Power Kinetic Energy Potential Energy Conservation of Energy

Work W = work F=force d=distance The work done by a force acting on an object is equal to the force magnitude times the distance through which the force acts. A force is any influence that can change the speed or direction of motion of an object W = work F=force d=distance W = F d [W] = joule, 1 joule (J) = 1 N x 1 m Raising an apple from a table to your mouth takes ~1 J of work: m=0.1 kg, w=mg=1 N, d ~ 1m  W~1 N x 1 m ~ 1 J

An average manpower is ~75 W Power is defined as a rate of doing the work P = power W = work t = time W P = ---- t [P] = watt, 1 watt = 1 joule/ 1 s (J/s) 1 Horsepower (hp) = 746 W = 0.746 kW An average manpower is ~75 W

Kinetic Energy Energy is a property that enables something to do work Energy has various forms (kinetic, potential, rest energy) Kinetic energy is the energy of a moving object mv2 KE = --------- 2 It takes ~1000 N (or ~ 225 lbs) to hit a nail and drive it 5 mm into a surface.

Potential Energy Potential energy is a capacity of doing some work Potential energy is the energy of position Gravitational Potential Energy W = Fd = mgh = PE (potential energy) PE of a 1000-kg car at the top of a 50-m multilevel parking lot is: mgh= (1000 kg)(9.8 m/s2)(50 m)=490 kJ PE is relative!

Energy Transformations Many mechanical processes involve interchanges between KE, PE, and work. Energy exists in some other forms: chemical energy, heat energy, radiant energy, etc.

Conservation of Energy Energy cannot be created or destroyed. It can only be changed from one form to another. The above statement is called the law of conservation of energy

Summary Any object, which has mass, contains energy. Energy enables objects to do work Power is the rate of doing work