Force and Pressure
ICSE Class 8 · Physics · Chapter 3
What this chapter covers
- Force: a quick revision
- Turning effect of a force
- Factors on which the turning effect depends
- Moment of force
- Pressure
- Pressure in daily life
- Pressure exerted by liquids
- Pressure exerted by gases
- Atmospheric pressure
Key definitions
Moment of force is the turning effect of a force about a pivot, measured as the product of the force and the perpendicular distance of its line of action from the pivot.
Pressure is the force acting on a unit area of a surface.
Atmospheric pressure is the pressure exerted by the air of the atmosphere on every surface in contact with it.
Key points
- A force is a push or a pull that can change an object's state of rest or motion, its direction or its shape; its SI unit is the newton (N).
- A force can make a body turn about a fixed point or line called the pivot.
- The turning effect of a force depends on the size of the force and on its perpendicular distance from the pivot.
- Moment of force = force × perpendicular distance from the pivot (M = F × d); its SI unit is the newton metre (N m).
- Moments can be clockwise or anticlockwise; a see-saw balances when the two are equal.
- Pressure is the force acting on a unit area: P = F ÷ A; its SI unit is the pascal (1 Pa = 1 N/m²).
- The same force gives a larger pressure on a smaller area: sharp knives and pointed nails use this, while broad straps, wide tyres and foundations reduce pressure.
- Liquids exert pressure on the bottom and sides of their container, equally in all directions at the same depth, and the pressure increases with depth.
- Gases exert pressure because their moving particles keep striking the walls of the container.
- Atmospheric pressure is the pressure of the air around the Earth; it acts in all directions and decreases with height.
- Suckers, straws, droppers and syringes all work because of atmospheric pressure.
Questions answered
Define moment of force. Write its SI unit.
Moment of force is the turning effect of a force about a pivot, measured as the product of the force and the perpendicular distance of its line of action from the pivot. Its SI unit is the newton metre (N m).
Why is the handle of a door fixed at the edge farthest from the hinges?
The farther the force acts from the pivot (the hinges), the greater its moment. A handle far from the hinges lets a small force produce a large turning effect, so the door opens easily.
A force of 20 N is applied at right angles to the end of a spanner 0.3 m long. Find the moment of force.
M = F × d = 20 N × 0.3 m = 6 N m.
A tap needs a moment of 2 N m to turn. If the handle is 0.05 m long, what force must be applied at its end?
F = M ÷ d = 2 N m ÷ 0.05 m = 40 N.
Define pressure and write its SI unit.
Pressure is the force acting on a unit area of a surface. Its SI unit is the pascal (Pa), the pressure exerted by a force of 1 N acting at right angles on an area of 1 m², so 1 Pa = 1 N/m².
A girl weighing 450 N stands on both feet. The total area of her soles is 0.03 m². Find the pressure she exerts on the floor.
P = F ÷ A = 450 N ÷ 0.03 m2 = 15 000 Pa.
Why is it easier to cut an apple with a sharp knife than with a blunt one?
A sharp edge has a very small area, so the same force produces a much larger pressure, which cuts through the apple easily.
Why are the walls of a dam made thicker at the bottom?
The pressure of a liquid increases with depth, so the water presses hardest on the lowest part of the wall. A thicker base can withstand this greater pressure.