Class 9 Science

Thrust and Pressure

Thrust: Force exerted by an object perpendicular to the surface is called thrust.

Pressure: Force exerted by any object per unit area is called pressure.

Pressure = Force ÷ Area
Or, Pressure = Thrust ÷ Area
Or, `p=F/A` ---(i)

Where, p is pressure, F is thrust or force and A is the surface area of the object.


See Answer

1: (a) Force ÷ Area, 2: (b) Nm-2, 3: (c) Point of needle, 4: (d) Battle tank, 5: (a) To reduce pressure, 6: (b) Upward thrust, 7: (c) Directly proportional, 8: (a) Mass ÷ Volume, 9: (d) Kg m-3, 10: (c) Water

Since, pressure is indirectly proportional to the surface area of the object, thus, pressure increases with decrease in surface area and decreases with increase in surface area.

SI unit of Pressure

The unit of force is newton (N) and unit of area is square meter

Therefore, by substituting the unit of force and are in equation (i) we get

`p=N/m^2 or Nm^(-2)`

Therefore, unit of pressure is `Nm^(-2)`

In the honour of Blaise Pascal, the SI unit of pressure is known as pascal.

One pascal is equal to the 1 newton per square meter.

Or, 1 pascal = N m – 2

Thrust and pressure in everyday life

Pressure exerted by a brick

A stretched brick kept on the ground exerts less pressure than a brick kept on the ground in standing position. This happens because the surface area of contact of brick in stretched position is more in comparison to the brick in standing position. And we know that pressure increases with decrease in surface area and vice versa. Thus, in the case of stretched position a brick exerts less thrust over the ground in comparison to the brick kept in standing position.

Camel can run easily over the sand

The feet of a camel are large. Larger feet mean larger area which results in low pressure. Due to this cam can easily walk on sand without sinking its feet.

Battle Tank can easily run over sand and mud

Battle tanks are fitted with caterpillar tracks, around their wheels. Caterpillar tracks, surrounding the wheels provide a larger surface area. Due to this, a tank can move on any terrain without sinking.

Tractor has broader tyres

Tractors are made mainly for agriculture purpose. Because of broader tyres, a tractor exerts less pressure over the ground as pressure decreases with increase in surface area or contact and hence easily runs over the muddy field.

One end of a nail is pointed

When a nail is pushed in a wall or wood, it is pushed from pointed side. Because of pointed shape, nail exerts more pressure over the wall or wood and is easily pushed by hammering.

The straps of school bag are broader

Since larger surface area exerts less pressure, therefore school bags with broader straps exert less pressure over the shoulder of a student while being slung over the shoulder and a student feels it easy to carry even a heavy bag with more books.

Knife with sharp edge cuts easily compare to one with blunt edge

Knife with sharp edge exerts more pressure because of less area in contact with an object, such as vegetable and hence it cuts more easily than a knife with blunt edge.

Pressure in Fluids

Any solid object exerts pressure because of weight. Similarly, since fluids also have weight, thus they exert pressure.

Gas and liquid both are considered as fluids. Fluids exert pressure in all directions over the inner walls of a container in which they are kept.