11 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B very short answer (2 marks) or short answer (3 marks)

Force, balanced forces and Newton's first law

A force is a push or a pull. Only an unbalanced force changes rest or motion. Every body resists such change: this is inertia.

ForceA push or a pull on a body; SI unit newton (N)

Balanced forcesEqual and opposite forces on one body; the net force is zero, so rest or motion does not change

InertiaThe tendency of a body to stay at rest or keep moving with the same velocity; mass is its measure

Newton's first lawA body stays at rest, or in uniform motion in a straight line, unless an unbalanced force acts on it

What a force can do

Start a body moving from rest, like kicking a football lying on the ground.
Change the speed of a moving body, like pressing the brakes of a bicycle.
Change the direction of motion, like a batter turning the ball with the bat.
Change the shape of a body, like pressing a balloon or rolling dough into a chapati.

Balanced vs unbalanced forces

Balanced forcesEqual in size and opposite in direction, on the same body. The net force is zero. They do not change rest or motion, but they can change shape. Example: two equal teams in a tug of war; the rope stays still.

Unbalanced forcesNot equal, so a net force is left over. The body starts moving, or changes its speed or direction, towards the larger force. Example: one team pulls harder and the rope moves to its side.

A bus moving at a steady speed on a level road also has balanced forces on it: the push from the engine just equals friction.

Inertia and mass

Inertia is the tendency of a body to resist a change in its state of rest or motion. All bodies have inertia, but not equally. An empty carton is easier to push than one full of books. A cricket ball is harder to stop than a table tennis ball at the same speed. So a heavier body has more inertia. Mass is the measure of inertia.

Newton's first law of motion

Newton's first law says: a body stays in its state of rest or of uniform motion in a straight line, unless an unbalanced force acts on it. So bodies resist change on their own. That is why the first law is also called the law of inertia. It also tells us what a force is: the cause that changes rest or motion.

Inertia in daily life

A bus starts suddenly: your feet move with the bus, but your upper body stays at rest (inertia of rest). So you fall backward.
A moving bus stops suddenly: your feet stop, but your upper body keeps moving (inertia of motion). So you fall forward.
A hanging carpet is beaten with a stick: the carpet moves, but the dust stays at rest and falls off.
A mango branch is shaken hard: the branch moves, the fruits tend to stay at rest and drop.
A sprinter runs on for a few steps after the finish line, due to inertia of motion.
Luggage on a bus roof is tied down. Otherwise inertia could throw it off when the bus starts, stops or turns.

Activity: pull a paper strip from under a stack

This shows inertia of rest. You need a paper strip, a table, and five coins or a glass of water.

1
Lay a smooth paper strip (about 30 cm long) on a table, with part of it hanging over the edge.
2
Place a small stack of coins, or a glass half full of water, on the strip near the edge.
3
Pull the strip slowly. The stack moves along with the paper.
4
Put the stack back. Now pull the strip out with a quick, sharp jerk.
5★
Observe: the paper comes out, but the stack stays where it was.
6
Record your conclusion: the stack stays at rest because of its inertia. The quick jerk gives friction too little time to move it.

★ key step

In real life

Seat belts in cars and buses hold you back when the vehicle stops suddenly. Without them, inertia of motion would throw you forward into the seat or the windscreen.

Exam angle

1 marks: “Name the property of a body that makes it resist a change in its state of rest or motion. What is its measure?” Inertia. Its measure is the mass of the body.

2 marks: “Two children push a toy cart with equal forces from opposite sides. What kind of forces are these, and what happens to the cart?” They are balanced forces: equal and opposite, so the net force is zero. The cart's state of rest does not change. Balanced forces can only change shape, for example squeezing a soft toy.

3 marks: “State Newton's first law of motion. A coin is placed on a card over a glass, and the card is flicked away quickly. Use the law to explain what happens to the coin.” Law: a body stays at rest or in uniform motion in a straight line unless an unbalanced force acts on it. The flick moves only the card. The coin's inertia of rest keeps it in place. With the card gone, gravity pulls it straight down into the glass.

Remember it

Rest stays rest and motion stays motion, until an unbalanced force says otherwise.