5.4 Non-Contact Forces
Must two objects touch for a force to act between them? A magnet picking up a pin from a small gap away says no. Forces that act without contact are called non-contact forces. There are three to know: magnetic, electrostatic, and gravitational.
Magnetic force
Activity 5.5 — A floating magnet Slide a ring magnet onto a vertical wooden stick, then add a second ring magnet above it with like poles facing. The top magnet floats in the air, held up without touching the one below! Press it down gently and you feel it pushing back. The two magnets exert a force on each other across a gap.
Magnetic Force The force a magnet exerts on another magnet or a magnetic material is the magnetic force. Like poles repel; unlike poles attract — and because the magnet acts from a distance, it is a non-contact force.
Electrostatic force
Activities 5.6 & 5.7 — Charges that pull and push Rub a plastic scale with polythene and bring it near tiny paper pieces (without touching): the paper jumps up and sticks to it. Now rub two balloons with wool and hang them close — they swing apart (repel). But bring the woollen cloth near a rubbed balloon and it is attracted.
Rubbing certain materials together makes static charges build up on their surfaces — the object is then “charged.” There are two kinds of charge, positive and negative. From the balloon experiment we learn the rule: like charges repel, unlike charges attract (the two balloons got the same charge and pushed apart; the balloon and cloth got opposite charges and pulled together).
Electrostatic Force The force exerted by a charged body on another charged or uncharged body is the electrostatic force. It too acts without contact, so it is a non-contact force. (When such charges actually flow, they make an electric current — the very current that ran the lamps, magnets and heaters of Chapter 4.)
Gravitational force
Activity 5.8 — What goes up… Throw a ball straight up. It rises, slows, stops for an instant, then falls back to the ground. Throw it harder — still it comes back down. Whatever you toss, in whatever direction, the ground always wins.
Gravitational Force Because all objects fall toward the Earth, the Earth must be pulling them. The force with which the Earth attracts objects toward itself is the gravitational force (or gravity). It acts without contact, so it is a non-contact force — and unlike magnetic or electrostatic forces, gravity is always attractive: it only ever pulls, never pushes.
This is the answer to Sonali’s downhill mystery — gravity was pulling the bicycles down the slope.
Now sort the examples below by whether the force needs contact or acts from a distance.