Exam practice

The state in which a body changes its position compared with the objects around it is the state of motion.

The push or pull that changes the state of rest or motion of a body is called force.

The SI unit of force is the newton (N).

Force is needed to bring a body at rest into motion.

The motion of a bullet fired from a gun is linear motion.

A simple machine makes our work easy (easier) and faster.
The word “simple machine” is not needed, and “force” fills two blanks.

Whether a body is at rest or in motion depends on what we compare it with. The same body can be at rest compared with one thing and in motion compared with another. So rest and motion are relative.
Example: A passenger sitting in a moving bus is at rest compared with the bus and the other passengers, because the passenger stays in the same seat. But the same passenger is in motion compared with the trees and houses by the road, because their position keeps changing.
Another example: a student sitting on a sack pulled along the ground is at rest compared with the sack, but in motion compared with the ground.

A force is a push or a pull that changes, or tries to change, the state of rest or motion of a body. Its SI unit is the newton (N).

A force can:

1. make a body at rest move (kicking a ball);
2. bring a moving body to rest (a goalkeeper stopping the ball);
3. change the speed of a moving body (pushing a swing harder);
4. change the direction of a moving body (a bat hitting a cricket ball);
5. change the shape of a body (rolling dough into a roti);
6. change the size of a body (blowing up a balloon).

The passing of a force from one place to another is called transformation of force (or transfer of force).
Examples:

1. When we cut wood with an axe, we push on the handle, and the force is passed to the sharp edge, which cuts the wood.
2. When we press a bicycle’s brake lever, the force is carried along the cable to the brake pads at the wheel, which stop it.
3. When we push the piston of one syringe, the water in the tube passes the force to the piston of another syringe joined to it.
4. When we pull down a rope over a pulley, the force is passed along the rope and lifts the bucket.

A force can change a body from rest to motion, or from motion to rest.
Example: A football lying on the ground is at rest. When a player kicks it, the force of the kick makes it move: its state changes from rest to motion. When the goalkeeper catches it, the force of the goalkeeper’s hands brings it to rest again: its state changes from motion to rest. So a force can change the state of a body.

An axe, a khukuri, a sickle (hasiya), a spade (kodalo), a kuto, a hoe (chande), a wedge, a knife, a pulley, a broom, a bicycle’s brakes and a bicycle pump all transfer force from where we push to where the work is done.

A spring balance has a spring inside a case, a hook at its lower end, a pointer fixed to the spring, and a scale marked in newtons.

1. Hold the balance by its top ring, or hang it from a hook.
2. Hang the thing on its hook (or hook it onto the thing and pull).
3. The force stretches the spring. The bigger the force, the more the spring stretches, and the farther the pointer moves down the scale.
4. When the pointer stops, read the scale at the pointer. That is the force, in newtons.

For example, a stone that stretches the spring to the 6 N mark has a weight of 6 N.

When you sweep, you hold the top of the handle with one hand and push the middle of the handle sideways with the other. The force of your hands is passed down the handle to the bristles at the bottom. The bristles press on the floor and push the dust and dirt along. So the force applied at the handle is transferred to the bristles, where the work (sweeping) is done.

A simple machine is a tool of simple structure that helps us do work easily, quickly and conveniently, using our own effort and no fuel. Examples: scissors, a pulley, a crowbar, an axe, a slanting plank.

1. A heavy load can be lifted or moved with a small effort (a crowbar rolls a big stone).
2. More work can be done in less time (an axe splits wood quickly).
3. Effort can be applied in a convenient way or direction (a pulley lets us pull down to lift a bucket up).

A pulley is used because it lets us lift the bucket by pulling the rope down instead of lifting it straight up.
It helps in these ways:

1. Pulling down is easier: we can use our own weight and keep our back straight.
2. The rope runs smoothly over the turning wheel, so it doesn’t rub against the edge of the well; the rope lasts longer and the work is lighter.
3. We can stand to one side of the well, safely, while the bucket comes up straight from the middle.
4. The water comes up quickly and with less tiredness.

Draw each one large and clear, and label its parts.