Thrust and Pressure

The force acting on an object perpendicular to the surface is known as thrust. The force acting on a unit area of a surface is known as pressure.

`"Pressure" = "Force" / "Area"`

The pressure is defined as force per unit area. The SI unit of thrust is N/m². It is called pascal (Pa). Since pressure is indirectly proportional to the surface area of the object, pressure increases with a decrease in surface area and decreases with an increase in surface area. Pressure applied to a fluid is transmitted in all directions.

The pressure is defined as force per unit area.

`"Pressure" ="Force" / "Area"`

• The SI unit of force is Newton (N). Area is measured in m². Therefore, the SI unit of pressure is N/m². It is known as Pascal (Pa). 1 Pa = N/m² and In the CGS system, the unit of pressure is dyne/cm².
• In atmospheric science, the unit for pressure is bar, where 1 bar = 10⁵ Pa.
• Pressure is a scalar quantity. If the area increases, the pressure decreases for the same force. If the area decreases, the pressure increases for the same force.

For example:

• Consider a very sharp needle that has a small surface area and consider a pencil whose back is very blunt and has more surface area than the needle.
• If we poke a needle in our palm, it will hurt as the needle gets pierced inside our skin. Whereas if we poke the blunt side of the pencil into our hand, it won’t hurt so much.
• This is because the area of contact between the palm and the needle is very small; therefore, the pressure is large.
• Whereas the area of contact between the pencil and the palm is greater, therefore the pressure is less.

Conclusion: Two factors that determine the magnitude of the pressure are:

Force: the greater the force, the greater the pressure, and vice versa.

Coverage area: the greater the area, the less the pressure, and vice versa.

Other Example:

• Consider a balloon kept on the bed of nails, and an external force is applied to it, which means there are large numbers of nails on any rectangular slab. All the nails are identical and equal in height.
• We can see that the balloon does not burst. This is because there is a large number of nails, and all the nails are closely spaced with each other.
• All the small pointed nails make a large surface area; therefore, the weight of the balloon is compensated by the entire area of all the nails. The surface area increases; therefore, pressure is reduced.
• But even if one nail is greater than the others, then the balloon would burst. Because then the surface area will be less, and as a result, pressure will be higher.

1. Aim: To demonstrate how pressure is affected by the area over which force is applied.

2. Requirements: sharp-pointed nails, a hammer, a wooden plank, drawing pins, and ordinary pins.

3. Procedure

I. Nail into Wooden Plank

• Hammer sharp-pointed nails into a wooden plank from the pointed end and observe how easily they penetrate.
• Place a nail on the plank with its flat head touching the surface and try hammering it from the pointed end. Notice how difficult it is to drive it in.

II. Pressing Pins into Drawing Board

• Press a drawing pin into a board using your thumb and observe how easily it goes in.
• Try pressing an ordinary pin into the board with your thumb and note the discomfort or pain.

4. Conclusion: The experiment shows that a sharp-pointed object exerts higher pressure on a smaller area, making it easier to penetrate surfaces. When force is spread over a larger area, pressure decreases, making it harder to push the object in. This illustrates that reducing the area of contact increases pressure for the same force.