Kick a football and it flies across the ground. Your friend stops it with a foot. A swing goes higher each time you push it. A lump of dough becomes a flat roti under a rolling pin. In each of these, something pushes or pulls, and something changes.
We walk, run, play, dig and carry loads every day. None of it can be done without a push or a pull, that is, without force. In this unit you will find out what it means for a thing to be moving or still, what a force can do, how a force can be passed from one place to another, and how simple tools like a sickle, a pulley and an iron bar make hard work easy.
Remember?
6.1 Motion and its types
While you push the book, it keeps changing its position. We say it is in motion.
Flowing water, a flying bird, a moving bus, a walking man and a moving bicycle are all in motion. When you walk to school, you are in motion too: your position keeps changing compared with the houses and trees you pass.
The state in which a body keeps changing its position compared with the objects around it is called motion.
The tables, chairs, benches and desks in your classroom are not changing their position. They are at rest. A house, a bridge and an electric pole are also at rest.
The state in which a body does not change its position compared with the objects around it is called rest.
So whether a thing is “moving” or “at rest” depends on what we compare it with. We say that rest and motion are relative. The thing we compare with is called the reference. Usually we compare with the ground, the trees and the buildings around us, so we just say “the bus is moving”.
Try it
Who is moving?
Sita sits in the bus. Gopal stands by the road. Choose where you watch from, then press Drive.
Watch from
Sita (in the bus)
?
Gopal (by the road)
?
the trees
?
From the roadside, press Drive and watch who changes place.
Seen from the roadside, the bus and Sita moved; Gopal and the trees stayed at rest.
From Sita’s seat, press Drive and watch who changes place.
Seen from Sita’s seat, Sita stayed at rest beside her window, and Gopal and the trees moved backwards. Both views are right!
Trees: W. Heubach (d. 1923) in O. Schmeil, Lehrbuch der Botanik (1911), No restrictions, via Wikimedia Commons
A book lies on the seat of a moving bus. Compared with the bus, the book is…
Pick an answer to see why.
The book stays on the same seat, so compared with the bus it is at rest. Compared with the road outside, it is moving with the bus. Rest and motion depend on what you compare with.
Which of these is at rest compared with the ground?
Pick an answer to see why.
An electric pole stays in the same place. The river, the bird and the bicycle keep changing their positions.
Types of motion
Things don’t all move in the same way. A falling stone goes straight down. A butterfly flutters this way and that. We sort motion into two main types: linear motion and curvilinear motion.
a. Linear motion
The motion of a body along a straight line is called linear motion. (It is also called rectilinear motion: recti means “straight”.)
Examples: a vehicle moving along a straight road, a ball rolling on flat ground, a stone falling straight down, a bullet fired from a gun.
b. Curvilinear motion
When a body keeps changing its direction instead of moving in a straight line, its motion is called curvilinear motion. In other words, the motion of a body along a curved path is curvilinear motion.
Examples: a vehicle going round a bend, the tips of the hands of a clock, a swing, a fluttering butterfly, a snake sliding along. When the wind blows, dust, leaves and bits of paper fly about. They have no fixed direction: they keep changing direction as they fly. Their motion is curvilinear too.
Butterfly: Andy Wilson, via PhyloPic (CC0)
Figure 6.1 The paths of moving things, drawn as dashed lines. Linear motion (top row): a stone falling straight down, a carrom striker shot across the board, a marble rolled along the ground. Curvilinear motion (bottom row): the tip of a clock’s hand going round a circle, a swing moving along an arc, a butterfly fluttering in a wavy path.
Which of these is curvilinear motion?
Pick an answer to see why.
A swing moves along an arc, a curved path. The other three move along straight lines.
The motion of the tip of a clock’s hand is…
Pick an answer to see why.
The hand itself is straight, but its tip moves round a circle, changing direction all the time. That is curvilinear motion.
6.2 Force
We need force for every kind of work. But what exactly is a force?
The bag stays at rest until it is pushed or pulled. A push or a pull moves it. A push or a pull is a force.
Pushing or pulling can change whether a body is at rest or in motion. So we define force like this:
The external push or pull that changes, or tries to change, the state of rest or motion of a body is called force.
Why “tries to change”? Push hard on the wall of your classroom. You are using force, but the wall doesn’t move. The force only tries to move it.
We use force to walk, to run, to carry loads, to play football, to ride a bicycle and to open a door. No work can be done without force. A force is needed to set a body at rest moving, and to bring a moving body to rest.
What is a force?
Pick an answer to see why.
A force is a push or a pull. It can start, stop, speed up, slow down or turn a body, or change its shape.
Measuring force
A spring balance has a spring inside. The harder you pull its hook, the more the spring stretches, and the farther its pointer moves along the scale. The scale is marked in newtons, so it reads the force directly.
Figure 6.2 Two spring balances. The small stone stretches the spring a little: its weight is 2 N. The big stone stretches it more: 6 N. The pointer shows the force on the scale.
The SI unit of force is the newton (N). It is named after the scientist Isaac Newton.
Weight is a force too, so it is also measured in newtons. The weight of a body is the force with which the Earth pulls it towards its surface. On the Earth’s surface, a body with a mass of 1 kilogram weighs about 9.8 newtons.
The SI unit of force is…
Pick an answer to see why.
Force, including weight, is measured in newtons (N). The kilogram is the unit of mass.
Which instrument measures force?
Pick an answer to see why.
A spring balance measures force: the spring stretches more for a bigger force, and the pointer shows it in newtons.
Effects of force
A force can do many things to a body. Here are six of them.
a. A force can make a body at rest move. Kick a ball lying on the ground, and it starts moving. Press the pedals, and a bicycle moves. Push a wheelbarrow, and it starts rolling.
b. A force can bring a moving body to rest. A force is needed to stop a moving ball. When the brakes of a moving bicycle or bus are pressed, it stops. In football, the goalkeeper uses force to stop the ball.
c. A force can change the speed of a moving body. (Books also call this changing its velocity: its speed in a direction.) Push a moving swing, and it goes faster. Turn a bicycle’s pedals faster, and the bicycle speeds up. A force can also slow a body down: brakes slow a bicycle before they stop it.
d. A force can change the direction of a moving body. A ball that hits a wall or a post bounces off in a new direction. When a batsman hits a cricket ball with the bat, it flies off in a new direction.
Try it
Push the ball
Push the ball with a kick from behind, a push against it, or a push from the side, and watch what each force does. The grass rubs on the ball and slowly stops it.
The ball is
at rest
Speed
0.0 m/s
The ball is at rest. Kick it from behind.
Your kick made the ball at rest start moving.
A kick from behind made the moving ball go faster.
A push against it slowed the moving ball down.
A push against it brought the moving ball to rest.
A push from the side changed the ball’s direction.
The edge of the field pushed the ball back: its direction changed.
The grass rubbing on the ball (friction) slowly brought it to rest.
e. A force can change the shape of a body. Press a balloon full of air, and its shape changes. Dough is pressed and rolled into the round, flat shape of a roti. Squeeze a paper bag or a plastic bag between your hands, and its shape changes.
f. A force can change the size of a body. Blow into a balloon, and it gets bigger. Crumple a sheet of paper, or press clothes into a bag, and they take up less space.
Figure 6.3 Forces changing shape and size. Pressing and rolling changes the shape of a ball of dough into a flat roti. Blowing changes the size of a balloon. Crumpling changes the size (and the shape) of a sheet of paper.
So a force can change a body’s state of rest or motion, its speed and its direction, and also its shape and its size.
A batsman hits a moving cricket ball. Which effect of force is this mainly?
Pick an answer to see why.
The ball was already moving towards the batsman. The bat sends it off another way: the force changes its direction (and its speed too).
When you blow up a balloon, the force…
Pick an answer to see why.
The air you blow in pushes on the rubber from inside, and the balloon gets bigger: its size changes.
Transformation of force
When you cut wood with a sickle (hasiya), a khukuri or an axe, you hold and push the handle. But the cutting is done by the sharp edge. The force you put on the handle is passed along the tool to its edge, and the edge cuts the wood. The same happens when you dig a field with a spade or a kuto: you push the handle, and the blade cuts the soil.
Figure 6.4 The force passes along the tool. With an axe, the hands push on the handle (effort), and the sharp edge splits the log (work done). With a spade (kodalo), the hands swing the handle, and the edge of the blade cuts into the soil.
To stop a moving bicycle, motorcycle or bus, we use the brakes. You press a brake lever with your hand, or a pedal with your foot. That force is carried to the turning wheels, where the brake pads press on them. The rubbing of the pads (friction) slows the wheels down and stops them. In motorcycles, buses and airplanes too, the force on the brakes is carried to the wheels. In this way, the brakes pass a force from one place to another.
Figure 6.5 Passing a force along a rope. Left: at a well, the rope runs over a pulley. Pulling down on one end lifts the bucket of water on the other. Right: the same idea in Activity 6.11, with a pipe on a stick held by two friends in place of the pulley.
When you push the piston of one syringe, the force is passed along the water in the tube to the other syringe. The passing of a force from one place to another is called the transformation of force. (It is also called the transfer or transmission of force.)
Figure 6.6 Two syringes joined by a tube full of water. Push piston A in, and the water carries the force along the tube, round its bends: piston B is pushed out by the same distance.
In most of our daily work, we pass force along in the same way. Tools like the kuto, the spade, the hoe (chande), the sickle, the khukuri, the axe and the wedge all carry the force from where we push to where the work is done. We use different tools to transfer force.
When you cut wood with a khukuri, where is your force applied, and where is the work done?
Pick an answer to see why.
You push on the handle. The force is passed along the blade to the sharp edge, which cuts the wood. That is transformation of force.
In Activity 6.12, why does piston B move out when you push piston A in?
Pick an answer to see why.
The water fills the tube and can’t be squashed, so pushing it at one end pushes it out at the other: the force is transferred.
We do all kinds of work every day, and we use tools to do it. Tools make our work easier and faster. The tools that help us work in this way are called simple machines.
In villages, a sickle (hasiya) and a khurpa are used to cut grass and firewood. An axe splits firewood and cuts down trees. A spade (kodalo) and a kuto dig the fields. At home we use a chulesi, tongs (chimta), a knife, a nut cutter (sarauta), a shovel and a spinning wheel (charkha). A pulley draws water from a well. Heavy loads are pushed up a slanting plank into a truck. A screwdriver turns screws in and out.
These tools look simple, but with them work gets done quickly and with less force. The tools of simple structure that help us do work easily, quickly and conveniently are called simple machines.
A simple machine is a single tool that works with our own effort. It uses no fuel like diesel or coal. A complex machine is made of many simple machines joined together, working as one. A bicycle is a complex machine: its pedals and gears are wheels and axles, its brake levers are levers, and its bolts are screws.
Figure 6.7 A dhiki, the wooden rice-pounder of Nepali villages. It is a lever: a person steps on the short end, beyond the pivot, and the long, heavy arm swings up with its wooden pestle; when the foot is lifted, it drops and the pestle pounds the grain in a hole in the floor. Photo: Krish Dulal, CC BY-SA 3.0
Which of these is a simple machine?
Pick an answer to see why.
Scissors are a simple machine: two levers joined at a pivot, worked by your own effort. A motorcycle, a tractor and a bus are complex machines run by fuel.
Application of simple machine
We use simple machines every day, and they help us a lot. Their main uses are:
a.A heavy load can be lifted with less effort.
b.More work can be done in less time.
c.Effort can be applied in a convenient way (in an easier direction, or from an easier place).
We need force for every kind of work. But sometimes the force we must work against is very large. Suppose you have to roll a stone with a mass of about 1000 kg. You can’t roll it alone with your hands. But with a simple machine, you can.
Push a long iron rod or a strong wooden pole under the stone, and put a small stone under the rod as its fulcrum (the point it turns on). Now press down on the other end of the rod with a small effort. The rod pushes up on the big stone, and the stone rolls over. In this way a small effort can work against a much larger force. The rod makes the work easier, and it also changes the direction of the force: you push down, and the stone is pushed up.
Try it
Tip the stone with a bar
Move the small stone (the fulcrum) along. How hard must you push down on the end of the bar to tip a 1000 kg stone?
Push needed
1,225 N
Like lifting
125 kg
Lifting the edge by hand
4,900 N
Small stone close to the big one: a push of a few hundred newtons tips it, about what a grown-up can press down with their weight. Your end moves a long way, while the big stone’s edge rises only a little.
Move the small stone closer to the big one, and the push you need gets smaller.
With the small stone far from the big one, you must push nearly as hard as lifting the edge by hand. Halfway along the bar (100 cm), just as hard: the bar no longer helps.
In the same way, scissors cut paper and cloth with a small squeeze. A shovel throws soil and sand. A pulley draws water from a deep well.
What does a single pulley over a well do?
Pick an answer to see why.
A single fixed pulley changes the direction of your force: you pull down, the bucket goes up. Pulling down is easier and safer than lifting straight up.
A companion to your textbook, not a replacement for it: study from the book your school uses, and where the two differ, follow your teacher. Found a mistake? Please tell us.
