Force and Pressure

(1) Change in shape — rolling dough, stretching a spring; (2) Change in speed — making a stationary object move or stopping a moving object, making it faster or slower; (3) Change in direction — deflecting a ball, turning a key. A single force can produce more than one effect at the same time (for example, hitting clay changes both its shape and speed).

Contact forces act through physical touch: muscular force (lifting an object), friction (resistance between two surfaces). Non-contact forces act without physical touch: magnetic force (a magnet attracting a key), electrostatic force (a rubbed pen attracting paper), gravitational force (the Earth pulling a falling apple).

Pressure = force ÷ area. Unit: N/m² = pascal (Pa). Same force applied over a smaller area gives greater pressure; applied over a larger area gives less pressure.

Wide feet provide a large area, which means less pressure — so the feet do not sink into the sand (whereas narrow feet would sink in). Similarly, tractors have wide wheels, tanks have wide chains, and buildings have wide pillars — all of these use a large area to reduce pressure so they do not sink into the ground.

Liquid pressure increases with depth — the column of water at the bottom is heavier, so it exerts greater pressure. Therefore, the wall of a dam is made thicker at the bottom to withstand this greater pressure — otherwise it would break.

When you suck through the straw, you remove the air inside it — the pressure inside the straw decreases. The atmospheric pressure outside (which is greater) then pushes the water upward into the straw. The same principle works in a rubber sucker, a syringe, and a pipette.