Chapter 5 — Exploring Forces

A force is a push or pull on an object that results from the object's interaction with another object. At least two objects must interact for a force to come into play. The SI unit of force is newton, written with a small 'n' and symbolised as N.

Applying force on an object can: make it move from rest, change the speed of a moving object, change the direction of motion of a moving object, bring about a change in the shape of an object, or cause some or all of these effects together.

Contact forces act only when there is physical contact (direct or indirect) between objects — examples include muscular force and friction. Non-contact forces can act even when objects are not touching — examples are magnetic force, electrostatic force, and gravitational force.

Muscular force is the force produced by the action of muscles. It occurs when muscles contract and elongate during activities like walking, running, lifting, pushing, jumping, or stretching. Animals, birds, fish, and insects all use muscular forces for movement and survival. Inside the body, muscular force helps in chewing food, pushing food through the alimentary canal, and in the expansion and contraction of the heart to circulate blood.

Friction is the force that comes into play when an object moves or tries to move over another surface. It always acts in a direction opposite to the direction of motion. Friction arises because even apparently smooth surfaces have minute irregularities; when two surfaces are in contact, these irregularities lock into each other and oppose any effort to move one surface over the other. Friction is greater on rough surfaces.

Gravitational force is the force with which the Earth attracts objects towards itself. It is always an attractive force — unlike magnetic force or electrostatic force, which can be either attractive or repulsive. Because it acts without physical contact with the object it attracts, it is classified as a non-contact force.

Mass is the amount of matter in an object, measured in grams (g) or kilograms (kg); its value remains the same at every place. Weight is the gravitational force with which the Earth (or another planet) pulls an object; it is measured in newtons (N). Since gravitational force varies slightly from place to place on Earth and differs greatly on other planets, weight can change but mass does not. For example, an object of 1 kg mass weighs 10 N on Earth but only 1.6 N on the Moon.

A spring balance consists of a spring fixed at one end with a hook at the other. When an object is hung from the hook, the spring stretches; the amount of stretching gives the weight of the object. A scale on the balance shows weight in newtons (N) and may also show a corresponding mass scale in grams. The range and smallest division of the balance must be checked before use — for example, a 0–10 N spring balance with 5 divisions per newton can read a minimum of 0.2 N.

When two objects of certain materials are rubbed together, electrical charges build up on their surfaces — these are called static charges. The force exerted by a charged body on another charged body or on an uncharged body is called electrostatic force. It is a non-contact force. Like (similar) charges repel each other, and unlike (opposite) charges attract each other. The two kinds of static charge are called positive and negative.

When an object is placed in a liquid, the liquid applies a force on the object in the upward direction known as upthrust or buoyant force. At the same time, the Earth's gravitational force pulls the object downward. If the gravitational force on the object is greater than the buoyant force, the object sinks; if the two forces are equal, the object floats. According to Archimedes' Principle, the upward buoyant force equals the weight of the liquid displaced by the object.

When you stop pedalling, no driving force is applied, but friction continues to act between the bicycle tyres and the road surface in a direction opposite to motion. This frictional force gradually reduces the bicycle's speed until it comes to a stop. On a rougher road the bicycle stops sooner, because friction is greater on rough surfaces.

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