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8th Class Science Force and Pressure Introduction


Category : 8th Class

*         Introduction


Have you ever thought, why a camel can run easily in desert? Or an army tank easily runs upon the continuous chain etc. In all of these cases, an object is exerting force on one another object when coming in contact with each other, making the thing to move on. The force applied by an object on another, depends on the area of contact with each other. Thus, the pressure is defined as the forces exerted by an object per unit area of the surface.

If we stand on the loose sand, the force equivalent to the weight of the body acts perpendicular to the surface, equal to the area occupied by our feet. On the other hand, if we lie down on the sand, the force will act on the surface equal to the area of the body in contact with the sand. We therefore see that the effect of same force of equal magnitude is different over different area. Hence, we can say that the pressure acting on the surface depends on the area of contact with the surface. If we press an iron sheet with the help of our finger, our finger does not dip into the sheet as the force of finger is falling on a large area of the sheet and the force per unit area of the sheet is small. The pressure is given by:


Or, \[P=\frac{F}{A}\]


The Sl unit of pressure is Newton per meter, which is also called Pascal (Pa).

1 Pascal = 1 Newton per square meter

Or, 1 pa = 1 N/m2

Pascal is a small unit of pressure. The bigger unit of pressure is kilopascal or cap. One kilopascal is equal to the one thousand Pascal.

i.e. 1 kPa = 1000 pa

In real life, there are many such situations where the pressure is the most important parameter. If you are peeling a potato, pressure is the key factor. If the knife is sharp, the area of contact is small and you can peel with less force on the blade. But if the knife is not sharp, you have to apply more pressure to peel it off. Similarly, if a nurse is giving injection to the patient and the needle is sharp, she has to apply less pressure on the syringe. In such case, the patient will feel less pain, as the sharp needle will have lesser area of contacts and less force is required to push the needle through the skin. It is normally seen that the foundation of the building is kept very wide. This is done in order to provide strength to the building.

Thus, we can say that same force acting on the smaller area exerts larger pressure than the force acting on the large area.






       If an object exerts a force of 45 N over a surface of dimension 15 m x 20 m, what is the pressure exerted by the liquid?

(a) 0.150 Pa

(b) 1.25 Pa

(c) 0.58 Pa

(d) 2.55 Pa

(e) None of these


Answer: (a)


We know that pressure exerted by the liquid at any point within the medium is equal to the force acting per unit area. On calculation it is found to be 0.150 Pa.



          The pressure acting on an object depends on many parameters. Smith took a plastic bucket and immersed in water. The pressure acting on bucket is independent of, which one of the following parameters?

(a) Mass                                                              

(b) Acceleration

(c) Area

(d) Shape

(e) None of these


Answer: (d)


The pressure acting on an object depends on its mass, accelerations and area.

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