# What is pressure? [duplicate]

When do we use the word pressure instead of force? I think pressure and force are same. Please explain without including the formula. In the example of nail penetrating into the wall whereas if we invert the nail and then it will be hard to penetrate this example doesn't help me. I think here there is only requirement of force nothing else.

I'll give you a short but better example.

Consider a plain circle made of clay. It will have a certain weight, whatever it is. Weight is the force that pushes it downwards.

If you put that circle on a water surface, it might float.

Now, take the circle and convert it into a sphere with your hands. Now the sphere does sink.

You will agree that the force is the same, because there is the same amount of mass. The mass has only been redistributed, but the amount is the same, so the weight is the same, same force.

However, now it sinks, because that force is being divided in a much smaller area.

The force is the same in both cases. The pressure is bigger in the second case, because pressure is the ratio of force divided by area.

• Can we say pressure means penetration – user230507 May 29 '19 at 5:45
• If a body penetrate more than the other body then we can say it has high pressure .is this assumption correct about what is pressure?? – user230507 May 29 '19 at 5:46

Pressure is the amount of force per unit area $$P = \frac{F}{A}.$$ Let's say you push on a balloon with a lot of force, it might be that the balloon is very flexible and does not pop. If you push with your hand the force you push with is spread out over the surface of your hand.

If you now push equally hard holding a nail in your hand, the same amount of force is now concentrated in the tip of the nail. Since the tip of the nail has a much smaller area than your hand, the same amount of force now gives rise to a much larger pressure at the point where the tip of the nail touches the balloon. Though the force is the same in both cases, the pressure will now be much larger, enough to pop the balloon.

Simply Force is the what tries to push or pull a certain thing. Preassure is the force that acts per unit area.

In regard to the "nail example", or any other example involving bending,breaking, or penetration for that matter- will require a certain minimum pressure to occur. This is because strain i.e. deformation(from applied force) per unit original dimension is proportional to stress(=force per unit area) and not force. This nature of deformation --strain's proportionality to stress (to first order) is why pressure is a useful quantity (in fact this is how moduli of elsticity are defined).

For the same amount of force the stress produced on a surface by the nail's tip would be much larger than that by its head- hence more strain i.e. higher penetration.

Stress is another name for pressure in this context. Pressure is defined as the magnitude of force normal to an area per unit area. From this definition some key differences are clear:

1. Pressure is a scalar--its always defined normally to a surface--a tangential non-zero force produces zero pressure.

2. Pressure is not completely defined by force alone--the area element on which the force acts is also needed-a small force in a small area produces no less a pressure than a larger force in a larger area

What about pressure in fluids? In a(n) (isotropic)fluid any arbitrary point experiences a pressure that is same from all directions--in another words if a differential area dA was to be placed at that point, then regardless of its direction it would experience the same force.

What about gasses? There the pressure is defined by finding the average force imparted to the walls of a container per unit area--the same definition just the computation is different.( given the velocities with which the gas particles hit the container have a distribution and so each individual hit doesn't produce the same pressure.)

Pressure as in atmospheric pressure? Its just the weight of the mass of an infinite(in theory) air column above sea --again--per unit area.

There are countless examples where pressure--and not just force--plays the deciding role not the least important of which is the threshold in safety release valves or the nature of sound.

Pressure doesn't depend only on force, it also depends on area and it is inversely proportional to area.

In your example, in the first case, the area is small and hence for a given force, the pressure is higher than in the second case, where the blunt head has a larger area (thus a low pressure and no penetration).

• Can we say pressure means penetration – user230507 May 5 '19 at 7:21
• If a body penetrate more than the other body then we can say it has high pressure .is this assumption correct about what is pressure?? – user230507 May 5 '19 at 7:23
• It doesn't necessarily mean penetration. For example, water exerts fluid(hydraulic) pressure , air exerts atmospheric pressure but these do not cause penetration, rather they just increase the force acting across a particular area. – Ak. May 5 '19 at 7:32
• Then in case of cutting of vegetable with sharp knife require minimum force and more pressure as per your answer there is no requirement of more force? – user230507 May 5 '19 at 14:02
• How to interpret pressure as I was interpreting like penetration please say which will be applicable for all the three cases solids liquids and gases? – user230507 May 5 '19 at 14:04