WORK AND ENERGYIII-Galileo

Presentors:

Modanza, Kent Noreen G. Garrido, Nurissa M. Sanchez, Ellen Jane A. Buray, Leri Maree Axela C. Olvis, Hazel Lynn S. Caduan, Joereth Anne P.

Definition of TermsWork-

In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. The term work was first coined in 1826 by the French mathematician Gaspard-Gustave Coriolis.

Source: http://en.wikipedia.org/wiki/Work_(physics)

Force-In physics, a force is any influence

that causes an object to undergo a change in speed, a change in direction, or a change in shape. Force can also be described by intuitive concepts such as a push or pull.

A force has both: magnitude and direction….

making it a vector quantity.

http://en.wikipedia.org/wiki/Force

Source :

Motion- In physics, motion is a change in position of an object with respect to

time. Change in action is the result of an unbalanced force. Source:http://en.wikipedia.org/wiki/Motion_(physics)

Power-In physics, power is the rate at which energy is transferred, used, or

transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit time.

Source:http://en.wikipedia.org/wiki/Motion_(physics)

Force is necessary to do work.

Work is the use of force to move an object Work = (F) (d)

Joule is the unit for work Work depends on force and distance

• WORK involves the application of a force over a distance.

• Therefore, we measure work in terms of a:

• FORCE (F) acting over a straight line and DISTANCE (d).

• W=Fd

Mathematically,

W=F(cos ᶿ) d

Where F= force, d= displacement, and the angle (theta) is defined as:

-the angle between the force and the displacement vector, which means it is not just any angle, but rather a very specific angle.

There are three key words in this definition-

force, displacement, and cause.

In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement. In other words..in this case…if there is no force or displacement, no work.

The standard metric unit for work (and also energy) is the joule (abbreviated “J”). One joule is equivalent to one newton of force causing a displacement of one meter. In other words:

1 Joule = 1 newton * 1 meter1 J = 1 N * m

Non-standard units of work include the following:

ft-pound kg*m/s 2 *m kg*m2/s2

Notice that when analyzed, each set of units is equivalent to a force unit times a displacement unit.

Summary:

Work is a force acting upon an object to cause a displacement.

Three quantities must be known in order to calculate the amount of work. These are: force, displacement and the angle between the force and the displacement.

Problem 1:

Work or No Work??

Click me!!!

F = 10 N

1 meter

Click me!!!

The force, a push along the ground (10 N), and the distance moved by the crate (1 meter) are in the same direction, so work (10 joules) has been done. …..(The Force and the displacement are parallel so the angle ᶿ between them is zero.)

W = F * cos ᶿ * d= 10 N * cos 0° * 1 m= 10 Joules

since cos 0° = 1

SO, WORK IS DONE.

Force is necessary to do work.

Most people say they are working when they do anything that requires a physical or mental effort. In scientific terms, you do work only when you exert a force on an object and move it. According to this definition of work, reading this page is not doing work. Turning the page, however, would be work because you are lifting the page.

What if I just read my book? IS there any work done?

YES and NO.

(Kindly listen to the explanation of the reporters……)

Force,

Displa

cem

ent, and

the A

ngle ᶿ

in b

etween

Force is a vector. This is just like getting the –component of the vector, where the x-axis represents the displacement.

Force is a vector. This is just like getting the –component of the vector, where the x-axis represents the displacement.

ᶿ

F

displacement

is important because even if there is force and displacement on a body, the angle ᶿ will also determine whether there is work, maximum work or zero work.

ᶿ

Maximum work

We can get the maximum possible work if =0°,cos 0° = 1.

ᶿ

=0°ᶿ F displacement

Work is maximized when the force applied is completely along the direction of the displacement. There are no other components that would reduce the value of force.

W = Fd

Zero workLet us take a look at the formula again.

W= F(cos ᶿ ) d

When will W be zero if F and d are non-zero? When cos ᶿ = 0. This will only be zero if ᶿ = 90°, so

ᶿ =90°displacement

F

The angle between the force and the displacement is 90°

Work done against Gravity

The work needed to lift an object of mass m against gravity is easy. The force of gravity on the object is simply its weight w = mg. That will be our F. Our displacement is simply the height to which the object is raised.

W = F x d= mg x h

So, to lift an object of mass m to the height H requires the work mgh.

Force, Motion, and Work

Work is done only when an object that is being pushed or pulled actually moves. If you lift a book, you exert a force and do work. What if you simply hold the book in front of you?

Work is done only by the part of the applied force that acts in the same direction as the motion of an object. Suppose you need to pull a heavy suitcase on wheels. You pull the handle up at an angle as you pull the suitcase forward. Only the part of the force pulling the suitcase forward is doing work. The force with which you pull upward on the handle is not doing work because the suitcase is not moving upward-unless you are going uphill.

*Give two examples of when you are applying a force but not doing work.

WORK CHANGES POTENTIAL AND KINETIC ENERGY.

When you throw a ball, you transfer energy to it and it moves. By doing work on the ball, you can give it kinetic energy (kuh-NEHT-ihk).

When you do work to lift a ball from the ground, you give the ball a different type of energy, called potential energy. (kindly listen to the reporter’s explanation…)

You can also give some objects potential energy by changing their shape. (example: spring. Explain)

KEY CONCEPTS1.If you push very hard on an object but does not

move, have you done work? Explain.2.What two factors do you need to know to

calculate how much work was done in any situation?

3.Was work done on a book that fell from a desk to the floor? If so, what force was involved?

CRITICAL THINKING4. Synthesize Work is done on a ball when a

soccer player kicks it. Is the player still doing work on the ball as it rolls across the ground? Explain.

5. Calculate Tina lifted a box 0.5 m. The box weighed 25N. How much work did Tina do on the box?