Work Physics Definition

Work Physics Definition. Pay attention to the arrow that goes off from the archer's bow. The event involves several concepts of physics. Force (in the pulling) is done by the hand of the athlete on the bow in such a way that the arrow could go off from the archer's bow.

When a force interacts with a matter (body) so that moves, then the force said does work on the body. What does it mean by work? Does the meaning of work equal to the one used in daily lives?

A. The Meaning Of Work

In daily lives, the term "work" is used for all efforts or activities done to reach certain objectives. For example, to gain knowledge, someone does the study activity. Then, what is the meaning of work in physics?

In Physics, work always involves force and displacement. Work exists if the force done to the body results from displacement. Although a huge force is applied to a body, but if it does not move, then there is no work done.

Work done by the constant force F is the dot product between force and displacement of s, which is mathematically written as

W = F.s

In SI, the unit of work is joule, abbreviated by J. If the unit of force is Newton and the unit of displacement is a meter, the one joule equals to one newton-meter (Nm).
1 Joule = 1 newton meter = 10⁷ erg
Base on the relationship, we can conclude that:

The one Joule work is work done by one newton of force to displace a body one matter in the direction of the force. 

1. Work of Constant Force 

The work done by force F in figure 3.4 is the product of the component of force in the direction of the displacement. Mathematically it can be written as follows.

W = (F cosӨ) (s),

Where Ө is the angle between the force and the displacement.

The value of cos Ө can be positive, negative, or zero; thus W can be positive, negative, or zero as well. Work is positive if the force that caused the displacement has the same direction with the displacement. If the force is perpendicular to the displacement, the work is zero.

2. Work of several Forces

Work is a scalar quantity. If several forces do work on a body each with the amount of W₁, W₂, W₃,…., Wn then the total work done by the forces equals to the sum of all works done by each force; that is:

W₁+ W₂ + W₃ +….+ W_n  ....... [3.3]

In a system shown in Figure 3.5, the working forces are F and, thus the total work is

W = (F - fk).s  ....... [3.4]

Figure 3.7 shows the graph of constants force F, which works on a body so that it moves/displaces with the distance of s. The works done by the force equals to the area of Fs under the graph.

Example 3.1

1.  A body with the mass of 2 kg, is placed on a slippery level floor. The force done to the body is 40 N, which direction forms an angle of 60^o toward the horizontal line. If the displacement is 5 m, then determine the work done by the force.
Answer:
W = (F cosӨ) (s)
    = (40 N) (cos 60^o ) ( 5 m)
    = 100 joules
Thus, the work done by the force to the body is 100 joules.

Exercise 

1. A Horizontal force of 100 N is used to push a box of 20 kg on the surface of a rough floor. The box is moving with constant velocity as far as 10 m. What is the work done by the force?
2. A body of 4 kg mass is placed on a smooth level plane. If a pulling force of 60 Newton, which makes an angle of 60 toward horizontal, work on the body for 2 seconds; then determine the work done by the force.

B. Energy

Energy is the ability to do work. We will learn two kinds of energy, potential, and kinetic energies. The potential energy we will learn first is the gravitational potential energy.

1. Gravitational Potential Energy

Gravitational potential energy is the energy stored in a body due to its position. Gravitational potential energy (Ep) owned by a body with mass m and situated in a height of meters above the earth's surface can be calculated by using the equation:

Ep = mgh

where :
Ep = potential energy (Joule)
m  = mass (kg)
g   = acceleration due to gravity (m/s2)
h   =  height  (m)

2.  Kinetic energy

Kinetic energy id the energy owned by moving object. Kinetic energy (Ek) of a body with m, which moves at speed of v, can be calculated by using the equation:

Ek  =  1/2 mv2

Where

Ek   = kinetic energy  (Joule)

m    = mass (kg)

v     = speed or velocity   (m/s)

Example 3.2

1.  A mango with mass 500 g is hanging 7 m high above the ground (g = 10 m/s2). Calculate the potential energy stored in the mango.

Answer :

Ep  =  m g h

      = (0,5 kg) (10 m /s2) ( 7 m)

      = 35 joules

2.  A car with mass of 2,000 kg moves at speed of 72 km/hour. Calculate the kinetic energy owned by the car.

v = 72 km/hour   = 20 m/s

Ek   = 1/2  mv2

       = (1/2)  (2,000 kg) (20 m/s2)

       =  4 x 105 joules

The kinetic energy owned by the car is 4 x 105 J

3.  Conservative Force

When we throw a ball vertically upward, the kinetic energy of the ball will decrease, but its potential energy will increase. The kinetic energy of the ball gradually changes into potential energy. When the ball reaches its highest potion, its kinetic energy equals to zero and its potential energy s maximum. On the contrary, when the ball moves downward, its kinetic energy ill increase, but its potential energy will decrease. On downward motion, the potential energy gradually changes into kinetic energy.

On the case above, the force that works on the ball is gravitational force. Gravitational force is a conservative force. If a system is subjected only to a conservative force, then the law of mechanical energy conservation will be applied to the system.

Exercise

1. A boy with the mass of 25 kg is able to climb a stair of 4 m in 15 seconds. If g = 10 m/s2, what is the power of the boy?
2. A runner engages the average power of 200 W when he covers the distance of 100 m in 80 seconds. What is the average force needed?
3. A waterfall with the height of meters flows 100,000 kg of water every second. If g = 10 m/s2, what is the force produced by the waterfall?

Multiple Choice

1. From this unit, the one that is not the unit of work energy is ............

a.  watt

b.  watt hours

c.  kilowatt hours (kWh)

d. newton meter

e. joule

2.  A body with mass of 2 kg, which moves with velocity of v, has kinetic energy of 400 J. The magnitude of velocity v is ....... m/s

a.  8                       d.  35

b.  20                     e.  37

c.  31

3. The energy kinetic possessed kinetic by a moving body is proportional to ..............

a.  the gravitational acceleration

b.  the square of velocity

c.  the square root of velocity

d.  the velocity

e.  the square root of mass

4.  A mango with the mass of 0.2 kg is falling down from its branch at the height of 5 meters. If the acceleration is 10 m/s², the kinetic energy of the mango when it hits the ground is.......

a.  16 J              d.  7 J

b.  15 J              e.  5 J

c.  10 J

5.  A bullet, which mass is 200 gram, is fired at the elevation angle of 30^o and with an initial velocity of 10 m/s. If the earth gravitation acceleration 10 m/s², the potential energy of the bullet at the highest point is....

a.  3.5 J

b.  3.0 J

c.  2.75 J

d.  2.5 J

e.  1.5 J

6. A block of 4 kg mass is initially moving with a velocity of 2 m/s. In order to stop the block at a distance of 5 meters measured from the initial position, the magnitude of friction force that should be provided is....

a.  1.2 N               d.  1.8 N

b.  1.4 N               e.  2.0 N

c.  1.6 N

7.  Below is the unit of power, except...........

a.  kWh                    

b.  watt

c.  J/s
d.  kW

e.  hp

8.  A loaded lift has a mass of 2,000 kg. The power needed to lift up the loaded lift by 50 m in 20 seconds is....

a.  1,000 kW

b.  200 kW

c.  100 kW

d.  50  kW

e.  40  kW

9.  A body with a mass of 2 kg is moving with velocity of 2 m/s. A short time later the body is moving with a velocity of 5 m/s. The total work applied to the body is....

a.  25 J                d.  22  J 

b.  24 J                e.  21  J

c.  23 J

10. The engine of an airplane is able to move the airplane with a force of 15,000 Newtons. When the airplane is moving with constant speed of 300 m/s, the power delivered by the engine is...........

a.  4,500,000 kW

b.  450,000 kW

c.  450,000 W

d.  4,500 kW

e.  4,500 W

work and energy Work Physics Definition

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