Recently, the students have been complaining that physics has been putting too pressure on them.  But what is pressure?

In physics we can define pressure on a solid as a force distributed over the contact surfaces of the two objects.

P = F/A

where P is pressure, F is force, and A is the area between the two surfaces in contact.

A nice way to think about variations in pressure is the way a book can be laid on a surface.

If these books all have the same weight, they will have different pressures because a different area is in contact with the surface.

The common metric unit for pressure is the Pascal (Pa) which is a Pa = N/m2

However, when you inflate a bicycle tire, more likely you will see the units for pressure in  lb/in2

The conversion is 1 lb/in2 = 6895 Pa

We commonly also use atmospheres as a unit of pressure, also known as a bar

One Atmosphere (atm) =  1.01 x 105 Pa = 1 bar

Sometimes you will see millibar as a unit on vacuum pumps.

In weather reports, you will often see pressure presented in units of inches of Mercury = inHg, as this is the units that you would read on a barometer. This is the barometer from my grandfather’s liquor store, Silk City Liquors.

When I was a pilot, I use to take the altimeter settings on my airplane in inHg. You can see the small calibration window on this altimeter between the 2 and 3 is set for 29.9 inHg.


atm = 29.92 inHg = 760 mmHg = 760 Torr

In metric units this would be millimeters of Mercury (Hg)  Another name for the millimeter of Mercury would be the Torr in honor of Evangelista Torricelli who

Torricelli, one of Galileo’s students, is know historically as taking over Galileo’s academic posts.   He did a lot of work with vacuum pumps and built an accurate barometer using Mercury (as opposed to those who had done it earlier with water), which solved many of the problems of using the barometer over a wide temperature range.