Work and Energy

Work Done by a Constant Force

Work Done by a Constant Force

Work Done by a Constant Force

Work is done when a force F pushes a car through a displacement s.

Work Done by a Constant Force

Work is done when a force F pushes a car through a displacement s.

Work = Force X Distance.

Question

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 against the wall they each do. 

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.

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)

Question

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

Bench Pressing

During bench-pressing work is done against gravity

Kinetic Energy

SI Unit of Kinetic Energy: joule (J)

6.3 Gravitational Potential Energy

6.3 Gravitational Potential Energy

The gravitational potential energy PE 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:

6.3 Gravitational Potential Energy

The gravitational potential energy PE 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:

6.3 Gravitational Potential Energy

The gravitational potential energy PE 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)

Pile Driver

Gravitational potential energy of the hammer relative to the ground is,

A Gymnast on a Trampoline

A gymnast springs vertically upward from a trampoline. The gymnast leaves the trampoline at a height of 1.20 m and reaches a maximum height of 4.80 m before falling back down. All heights are measured with respect to the ground. Ignoring air resistance, determine the initial speed v0 with which the gymnast leaves the trampoline.

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.

Roller Coaster (Ideal)

The tallest and fastest roller coaster in the world is now the Steel Dragon in Mie, Japan (Figure 6.20). The ride includes a vertical drop of 93.5 m. The coaster has a speed of 3.0 m/s at the top of the drop. Neglect friction and find the speed of the riders at the bottom.

PowerThe 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

Erg/s

BE/USC pound (lb)

foot (ft) foot·pound (ft·lb)

Ft.lb/s

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?

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 the 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).

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:

The CONSERVATION OF ENERGY

Energy can neither be created nor destroyed, but can only be converted from one form to another.

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:

The CONSERVATION OF ENERGY

Energy can neither be created nor destroyed, but can only be converted from one form to another.

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:

Learning how to convert energy from one form to another more efficiently is one of the main goals of modern science and technology.