Force
Force is the action of one body on another; it can set a body in motion, change its speed or direction, or change its shape. A force has a size and a direction; its unit is the newton (N) and a spring balance measures it. Muscle force comes from muscle contraction, mechanical force from machines. Friction acts when a body slides or tries to slide over another surface and it opposes the motion; it arises from the roughness of the surfaces, so it is small on smooth ice and large on a rough road. Friction can be useful (walking, brakes) or harmful (wear of parts); it also produces heat (rubbing palms warms them). The Earth pulls every body – this is the force of gravity (weight). A body of mass 1 kg is pulled with about 10 N (more exactly 9.8 N): F = m × g, g ≈ 10 N/kg.
Test friction: place a thick book on the desk, hook a rubber band to it and pull gently, measuring the band’s stretch with a ruler. Then put 3 pencils (rollers) under the book and repeat: the stretch is smaller for rolling. Safety: do not overstretch the band (it can snap back at faces); do not put heavy books near a table edge.