Non-contact forces

A non-contact force is a force exerted by one object on another without physical contact between them. This sets it apart from contact forces (muscular force, friction, the normal push of a surface), which stop working the instant the objects are separated. A non-contact force keeps acting even with empty space in between — the size of the gap changes how strong the force feels, but it does not switch the force off. This chapter studies exactly three non-contact forces, each covered in full in its own article:

• Magnetic force — the push or pull a magnet exerts on another magnet, or on a magnetic material such as iron, nickel, or cobalt. Two like poles (N-N or S-S) always repel; two unlike poles (N-S) always attract. A compass needle swinging to point north is magnetic force at work, mediated by the Earth's own magnetism.
• Electrostatic force — the push or pull between charged objects, most easily produced in the classroom by rubbing two materials together (a plastic comb on dry hair, a balloon on a sweater), which leaves both surfaces carrying static charge. Like charges repel; unlike charges attract; and a charged object can even attract a neutral one, which is why a charged balloon sticks to a bare wall.
• Gravitational force — the pull with which the Earth attracts every object towards its centre. As studied at this stage, gravitational force is treated as always attractive: it only ever pulls objects down, and it never pushes anything away. This is the one clear difference from the other two non-contact forces, which can go either way.

Despite acting without touching, all three are still forces in the full sense of the word — they are pushes or pulls arising from the interaction between two objects, and they can start, stop, speed up, slow down, or change the direction of motion just as any contact force can. The "non-contact" label describes only how the force reaches its target, not whether it counts as a real force.

There is no single formula at this stage for any of the three forces — the mechanism behind each is developed later (magnetism through fields, electrostatics through charge, gravitation through Newton's law of gravitation). What Class 8 needs instead is the reasoning that lets you correctly sort any everyday example into contact or non-contact, and then into the right one of the three non-contact forces. The test for "contact or non-contact" is simple: ask whether the force would vanish the instant you pulled the two objects a centimetre apart. Muscular force, friction, and the normal push of a table all disappear immediately — remove the touching and the force is gone, so they are contact forces. Magnetic force, electrostatic force, and gravitational force do not disappear; a magnet still pulls a pin from a few centimetres away, a charged balloon still tugs at paper from across a small gap, and the Earth's pull does not switch off the moment your feet leave the ground. That persistence across a gap is the defining signature of a non-contact force. Once you know a force is non-contact, sort it by asking two further questions. First, is a magnet or a magnetic material involved, or is a charged object (usually created by rubbing) involved? If yes to the first, it is magnetic force; if yes to the second, it is electrostatic force. Second, if neither magnetism nor charging is mentioned and the force in question is simply pulling something towards the Earth, it is gravitational force. Then check whether the situation could go either way (attract or repel) — only magnetic and electrostatic force can, because both depend on which poles or which charges are facing each other, while gravitational force in this chapter is only ever a pull towards the Earth.

Question: Classify each of the following as a contact force or a non-contact force, and where it is non-contact, name the specific force involved: (a) an apple falling from a tree; (b) an inflated balloon, rubbed on a sweater, sticking to a wall; (c) two bar magnets facing each other by their north poles, pushing apart before you let go of them; (d) a shopping trolley being pushed forward by hand.

Step 1. Look at (a). The apple and the Earth are never in contact as the apple falls, yet the Earth clearly pulls the apple downward the whole time. Since the force persists across the gap between apple and ground, this is a non-contact force — specifically gravitational force.

Step 2. Look at (b). Rubbing the balloon on the sweater gives it a static electric charge. The charged balloon then sticks to the wall purely because of that charge attracting the (essentially neutral) wall surface — no pressing or touching force is needed to hold it there. This is a non-contact force — specifically electrostatic force.

Step 3. Look at (c). The two north poles face each other and push apart while there is still a visible gap between the magnets — like poles repel, and the repulsion is felt before the magnets ever touch. This is a non-contact force — specifically magnetic force.

Step 4. Look at (d). The instant your hand loses contact with the trolley handle, the pushing force stops acting on the trolley (it only keeps moving afterwards because of inertia, not because the force is still there). Since the force vanishes without contact, this is a contact force — muscular force.

Step 5. Collect the answers: (a) non-contact, gravitational force; (b) non-contact, electrostatic force; (c) non-contact, magnetic force; (d) contact force, muscular force. In every non-contact case the force kept acting across a gap; in the contact case it stopped the moment contact ended.

Final answer: (a) gravitational, (b) electrostatic, (c) magnetic — all non-contact; (d) muscular — contact.

Competitive stretch

A tricky variant asks you to classify a case where both a contact and a non-contact force act together, such as a fridge magnet held flat against a steel door by hand and then released. While your hand presses it, both muscular force (contact) and magnetic force (non-contact) act on it; the moment you let go, only the magnetic force remains, and it alone is what keeps the magnet stuck to the door. The lesson: more than one type of force can act on the same object at the same time, and only checking which forces survive the removal of contact tells you which ones were non-contact all along.

• Assuming every force needs touching — magnetic, electrostatic, and gravitational force all clearly act across a gap; the "non-contact" name describes exactly this.
• Treating non-contact forces as weaker or somehow less "real" than contact forces — they are genuine pushes and pulls with the same ability to start or stop motion.
• Forgetting that gravitational force (at this stage) is always attractive, while magnetic and electrostatic force can be either attractive or repulsive depending on poles or charges.
• Naming the general category ("non-contact force") when the question asks for the specific force ("magnetic force," "electrostatic force," or "gravitational force") — always give the specific name once you have identified it.

Ask the tutor: want to go deeper into how magnetic, electrostatic, or gravitational force actually works? Ask the tutor to open up any one of the three in its own article.