Moments: unit Nm (Triple Physics Only)

A force or a system of forces may cause an object to rotate.

The turning effect of a force is called the moment of the force. The size of the moment is defined by the equation:

moment of a force = force × distance

The turning effect of a force

moment of a force = force × distance

moment of a force = 1400 x 0.3

The moment of the rock is 420 Newtons

Law of Moments

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

Learn this definition

A simple lever and a simple gear system can both be used to transmit the rotational effects of forces.

total clockwise moment about a pivot = the total anticlockwise moment

300 x 2.0 = mass of second person x 1.5

mass of second person = 600 / 1.5

The mass of the second person is 400 Newtons

Pressure & Fluids (Triple Physics Only)

A fluid can be either a liquid or a gas.

The pressure in fluids causes a force normal (at right angles) to any surface. Pressure is measured in Pascals, Pa.

The pressure at the surface of a fluid can be calculated using the equation:

pressure = force normal to the surface / area of the surface

pressure = force normal to the surface / area of the surface

pressure = 27 / 0.009

The pressure is 3000 Pascals

The pressure due to a column of liquid can be calculated using the equation:

pressure = height of the column × density of the liquid × gravitational field strength

or

pressure change = change in depth × density of the liquid × gravitational field strength

pressure change = change in depth × density of the liquid × gravitational field strength

pressure change = (20 – 8) x 1000 x 9.8

pressure change is 117600 Pascal

rate of flow of water less

because pressure is less

In a liquid, pressure at a point increases:

  • with the height of the column of liquid above that point; because there is more weight acting caused by the liquid above the point
  • with the density of the liquid; if the density increases the weight of the liquid above the point increases

Calculate the differences in pressure at different depths in a liquid.

A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.

An object will float if the weight of the object is less than the upthrust acting on it.

The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.

Air molecules colliding with a surface create atmospheric pressure.

The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.

air molecules colliding with a surface create pressure

at increasing altitude fewer molecules

so always less weight of air than below