Thrust and Pressure
Thrust:- The force acting on an object perpendicular to the surface is called thrust.
When we stand on loose sand, the force, that is, the weight of our body is acting on an area equal to area of our feet. When we lie down, the same force acts on an area equal to the contact area of of our whole body, which is larger than the area of our feet. Thus, the effects of forces of the same magnitude on different areas are different. In these cases thrust is the same. But effects are different. Therefore the effect of thrust depends on the area on which it acts.
Pressure:-The effect of thrust on sand is larger while standing than while lying. The thrust on unit area is called pressure. Thus,
Pressure = thrust/ area
Substituting the SI unit of thrust and area so, we get the SI unit of pressure as N/m2 or N m-2.
In honour of scientist Blaise Pascal, the SI unit of pressure is called pascal, denoted as Pa.
Ex-A block of wood is kept on a tabletop. The mass of wooden block is 5kg and its dimensions are 40 cm × 20 cm × 10 cm. Find the pressure exerted by the wooden block on the table top if it is made to lie on the table top with its sides of dimensions (a) 20 cm × 10 cm and (b) 40 cm × 20 cm.
Solution :- Mass of wooden block = 5 kg
The dimensions = 40 cm × 20 cm × 10 cm
Thrust = F = m × g
= 5 kg × 9.8 m s-2
= 49 N
Area of a side = length × breadth
= 20cm × 10cm
200 cm2 = 0.02 m2
Pressure = 49N/0.02 m2
= 2450 N m-2
When the block lies on its side of dimensions 40 cm × 20 cm, it exerts the same thrust.
Area = length × breath
= 40 cm × 20 cm = 800 cm2 = 0.08 m2
Pressure = 49N/ 0.08 =612.5 N m-2
The pressure exerted by the side 20 cm × 10 cm is 2450 N m-2 and by the side 40 cm × 20 cm is 612.5 n m-2.
