Science Text Book 44-45 / Formula of Work

Science for Junior High School

2nd Semester Grade VIII

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Science Text Book Page 44-45 

Formula of Work in terms of Physics

A. Work

*Highlighted parts = Important 

In terms of physics, work means a force that causes an object move. However, if a force doesn't make an object move, it isn't considered as work.

1. What is work?

         Work is a force that causes an object move as explained above. To be stated as work in terms of physics, it has several requirements. Firstly, it needs a force that causes displacement of the object. Additionally  the direction of the force has to be the same as the direction of the displacement. 

         Examples of Work in sports : 

Dwight Howard superman dunk

Basketball players produces force to lift the ball up high. Since the ball is moved from player's hand to the higher part, basketball player is considered as doing a work.

Weight lifting world champion Lu Xiaojun

Athlete of weight lifting is probably the job that requires the most work. Lu Xiaojun who is world record holder for weight lifting has great muscles to provide the work. When an athlete is lifting a weight from the floor and holds it above his head, he is considered as doing work. However, if an athlete is holding up the weight and stay still (with no displacement) in physics it is considered as not doing work. Although the athlete is producing muscular force, the weight is still being kept in her hands and not moving anywhere. 

We can conclude that If a force doesn't cause any displacement, then the work is zero. If a force doesn't have the same direction as the displacement, then the work is also zero. 

This is another example of a force that doesn't have the same direction as the displacement so the work is zero.

Waiters holding up the tray is a good example for the force that doesn't have any work. Waiter is giving a force upwards to the tray while moving. So the tray is moving the right side as the white arrow represent, and the waiter is giving a force upwards as the black arrow represent. The tray is having a displacement without giving an external force on it. We could conclude that work is zero in this example.

2. Formula of Work 

The work done by a constant force of magnitude F on a point that moves a distance d in the direction of the force is the product. 

W= F x s

Work = Force x Displacement

If there are two forces act on one object, the force should be decreased or added. 

-Units-

F = Force, unit of measurement is N (Newtons)

s = Displacement, unit of measurement is m (Meters)

W = Work, unit of measurement is J (Joules)

Example of finding a Work in proper way

1. What is the work needed to push a box with a 100 N force, so that the box moves for 276m?

Solution: 

Force F= 100N

Displacement s = 276m

W = Fs

    = 100N x 276m = 27600 Joules

Tug of war is also a great example for how several forces act. If left side produces 150N of force and opposite side produces 100N. 

Solution of finding the greater force:

Force F= <-150N-100N->

Left side has greater force, so conclusively its <-50N by decreasing each other 150N-100N = 50N