Work Done by a Constant Force Work is

Work Done by a Constant Force Work is done when a force F pushes a car through a displacement s. Work = Force X Distance. Work is a scalar. Unit: N. m = joule

Q 1: Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5 min longer. Compare the work they do on the wall. a. Joel does 50% more work than Jerry. b. Jerry does 50% more work than Joel. c. Joel does 75% more work than Jerry. d. Neither of them do any work. Q 2: A 102 kg man climbs a 5. 0 meter high stair case at constant speed. How much work does he do against gravity? a. 510 J b. 49 J c. 5000 J d. 2500 J

Units System Force Distance Work SI newton (N) meter (m) N·m = joule (J) CGS dyne cm dyn·cm = erg BE/USC pound (lb) foot (ft) foot·pound (ft·lb)

Kinetic Energy SI Unit of Kinetic Energy: joule (J) Kinetic energy is a scalar.

Gravitational Potential Energy The gravitational potential energy, GPE is the energy that an object of mass m has by virtue of its position relative to the surface of the earth. That position is measured by the height h of the object relative to an arbitrary zero level: SI Unit of Gravitational Potential Energy: joule (J) GPE is a scalar.

Energy

The Conservation of Mechanical Energy

THE PRINCIPLE OF CONSERVATION OF MECHANICAL ENERGY The total mechanical energy (E = KE + PE) of an object remains constant as the object moves, provided that the net work done by external nonconservative forces is zero.

Conservation of Mechanical Energy If friction and wind resistance are ignored, a bobsled run illustrates how kinetic and potential energy can be interconverted, while the total mechanical energy remains constant.

Power The idea of power incorporates both the concepts of work and time. Power is work done per unit time. Average power, P is the average rate at which work W is done, and it is obtained by dividing W by the time t required to perform the work:

Units System Force Distance Work Power SI newton (N) meter (m) N·m = joule (J) J/s = Watt (W) CGS dyne cm dyn·cm = Erg/s erg foot (ft) foot·pound Ft. lb/s (ft·lb) BE/USC pound (lb) Horsepower, hp: 1 hp = 550 ft. lb/s = 746 W

Metabolic Rates for a young 70 -kg male Activity Metabolic Rate (W) Running (15 km/h) 1340 Skiing 1050 Biking 530 Walking (5 km/h) 280 Sleeping 77

Forms of Energy So far we have considered the following forms of energy: Kinetic energy, Gravitational potential energy, and Mechanical energy. Some of the other forms of energy are: Electrical energy, Chemical energy, Nuclear energy, Thermal energy, and Radiant energy.

Energy Transformations Q: Give an example where gravitational potential energy is converted into kinetic energy? A: Falling object.

Energy Transformations

Energy Transformations in the Human body Part of the chemical energy stored in food is transformed into the kinetic energy of physical activities and into thermal energy needed to keep our bodies at a temperature near 98. 6 °F. Energy Transformations in an Automobile In an automobile chemical energy of gasoline is converted into kinetic energy, as well as electrical energy (to operate the radio, headlights, and air conditioner), and heat (to warm the car during the winter).

Energy Transformations in a Nuclear Power Station

The CONSERVATION OF ENERGY Whenever energy is transformed from one form to another, it is found that no energy is gained or lost in the process; the total of all the energies before the process is equal to the total of the energies after the process. This observation leads to the conservation of energy: Energy can neither be created nor destroyed, but can only be converted from one form to another. Learning how to convert energy from one form to another more efficiently is one of the main goals of modern science and technology.