Force: push, pull and its effects
A vendor pushes a loaded handcart to start it, pulls the rope to swing it round a corner, and squeezes the brake handle to slow it down. Each of these actions is a push or a pull.
A push or a pull on an object is called a force. A force comes from the interaction of two or more objects. Its SI unit is the newton (N).
Key idea 1: Effects of a force — A force can make a resting object move or a moving object stop, change its speed, change its direction of motion, or change its shape. One force can show more than one effect at a time.
- Easy: A kick makes a football at rest start to move.
- Medium: Pressing a sponge or stretching a rubber band changes the shape of the object.
- Exam-type: A batsman hits a ball so that it goes back faster and in a new direction; speed and direction both change.
In the figure: Each column shows one effect of force with an everyday example.
Key idea 2: Force needs interaction — When you push a wall, the wall also pushes back on your hand. Two objects must interact for a force to appear; when the interaction ends, the force ends. A book resting on a table still has forces on it, but they balance each other.
- Easy: At least two objects are needed for a force.
- Medium: If a push and a pull of equal size act on a box from opposite sides, the box does not move.
- Exam-type: "No motion" does not mean "no force"; it can mean that the forces are balanced.
Force = push or pull; unit = newton; effects = motion, speed, direction, shape.
Thinking that a force can change the colour of an object. A force changes motion or shape, not colour.
To list the effects, think of MDS. Memory clue: MDS = Motion, Direction, Shape (speed belongs to motion).
Spot the difference
| Action | Push or pull | Effect |
|---|---|---|
| Kicking a football | push | starts motion |
| Drawing water from a well | pull | moves the bucket up |
Practice questions
- 1What is a force?
- 2What is the SI unit of force?
- 3Which effect of force is seen when you press a sponge?
- 4A goalkeeper dives and stops a rolling ball. Which effect of force is this?
- 5Name the effects a force can have on an object.
- 6Why does a wall seem to push back when you push it?
- 7A tennis player hits a ball so that it moves faster and in a new direction. Which effects are seen?
- 8A book lies at rest on a table. Does this mean that no force acts on it?
Show answers (try them yourself first)
- 1A push or a pull.
- 2Newton (N).
- 3Change in shape.
- 4A decrease in speed (to zero).
- 5Start or stop motion, change speed, change direction, change shape.
- 6The two objects interact, so each exerts a force on the other.
- 7Change in speed and change in direction.
- 8No; forces such as weight and the push of the table act on it, but they balance each other.
Contact forces: muscular force and friction
A farmer's bullock pulls a cart along a mud road. When the animal stops pulling, the cart rolls a little and then stops. The pull came from muscles; the stopping came from friction.
A contact force acts only when objects touch. Muscular force is the force exerted by the muscles of people and animals. Friction is the force between two surfaces in contact that opposes motion.
Key idea 1: Friction — The surfaces look smooth, but tiny bumps on them lock into each other. Friction acts opposite to the direction of motion. A rough surface gives more friction and a smooth surface gives less. Liquids and gases also apply friction on moving objects.
- Easy: Friction slows down a moving object.
- Medium: A marble rolls farther on a smooth tile than on a rough carpet because friction is less on the tile.
- Exam-type: If a box is pushed to the right, friction on it acts to the left.
In the figure: On the smooth surface the ball rolls far; on the rough surface more friction stops it sooner.
Contact forces: muscular force and friction
In the app you can move this figure with your finger.
Open in the appKey idea 2: Muscular force, and friction at work — Lifting a bucket, pulling a cart and kicking a ball use muscular force; so do the heart and the chest inside the body. Friction helps us walk, grip and brake, but it also wears out surfaces and slows machines. Aeroplanes, ships and fast trains get streamlined shapes to reduce friction from air or water.
- Easy: A bullock pulling a cart uses muscular force.
- Medium: Pulling a bucket up with a rope is still muscular force, acting through the rope.
- Exam-type: On a wet smooth floor there is very little friction between shoe and floor, so we slip.
Muscular force and friction are both contact forces.
Thinking friction is always bad. We could not walk, write or stop a cycle without it.
Link surface and friction. Memory clue: Rough = Resist (more friction); Smooth = Slip (less friction).
Spot the difference
| Surface | Friction | Result |
|---|---|---|
| Smooth | less | object slides far |
| Rough | more | object stops soon |
Practice questions
- 1Which force arises from the action of muscles?
- 2Is friction a contact force or a non-contact force?
- 3In which direction does friction act on a moving object?
- 4Why does a rolling ball stop on level ground?
- 5Give one use and one harm of friction.
- 6Why do ships and fast trains have streamlined shapes?
- 7A marble slides 40 cm on tile, 12 cm on carpet and 3 cm on gravel. Arrange the surfaces from least to most friction.
- 8Why is walking on a wet smooth floor risky, and how does a rough tile pattern help?
Show answers (try them yourself first)
- 1Muscular force.
- 2A contact force.
- 3Opposite to its motion.
- 4Friction between the ball and the ground opposes its motion.
- 5Use: grip for walking or braking. Harm: it wears surfaces and slows machines.
- 6The shape reduces friction from air or water.
- 7Tile, carpet, gravel (the marble stops soonest on gravel, so gravel has the most friction).
- 8There is very little friction, so shoes slip; a rough pattern increases friction and grip.
Non-contact forces: magnetic and electrostatic
A comb rubbed on dry hair picks up tiny bits of paper without touching them, and a fridge magnet pulls itself onto the steel door from a small distance. Some forces work across a gap.
A non-contact force acts even when objects are not touching. Magnetic force acts between magnets, or between a magnet and a magnetic material such as iron. Electrostatic force is exerted by a charged object on another object.
Key idea 1: Magnetic force — Every magnet has two poles, north and south. Like poles repel each other and unlike poles attract each other. The force acts across a gap, even through paper or a thin sheet of plastic.
- Easy: Bringing two like poles together makes the magnets push apart.
- Medium: A magnet picks up an iron nail but not a plastic pen.
- Exam-type: Two ring magnets on a pencil with like poles facing: the top one floats. Turn it over and it drops and sticks.
In the figure: Magnetic and electrostatic forces can attract or repel; gravity only attracts.
Key idea 2: Electrostatic force — Rubbing two different materials together, such as a plastic straw and polythene, makes them charged. There are two kinds of charge, positive and negative. Like charges repel and unlike charges attract. A charged object can also attract light uncharged things like paper bits.
- Easy: A rubbed plastic straw attracts tiny bits of paper.
- Medium: Two balloons rubbed with the same wool hang apart because they carry the same kind of charge.
- Exam-type: A rubbed balloon and the wool used to rub it attract each other, because they carry opposite charges.
Magnetic, electrostatic and gravitational forces are non-contact forces.
Thinking that all non-contact forces only pull. Magnetic and electrostatic forces can also push apart.
For two poles or two charges, ask "same or different?". Memory clue: Like = Leave (repel); Unlike = Unite (attract).
Spot the difference
| Force | Acts between | Can repel? |
|---|---|---|
| Magnetic | magnets and iron | yes |
| Electrostatic | charged objects | yes |
Practice questions
- 1What is a non-contact force?
- 2Name the two kinds of electric charge.
- 3What happens when two like poles of magnets are brought close?
- 4A rubbed comb picks up tiny paper bits. Which force acts?
- 5What do two unlike charges do to each other?
- 6Name one non-contact force other than magnetic and electrostatic force.
- 7Two ring magnets on a pencil have like poles facing and the top one floats. What happens if you turn the top magnet upside down, and why?
- 8Two balloons are rubbed with the same woollen cloth and hung side by side from threads. What will they do, and why?
Show answers (try them yourself first)
- 1A force that acts without the objects touching.
- 2Positive and negative.
- 3They repel each other.
- 4Electrostatic force.
- 5They attract each other.
- 6Gravitational force (gravity).
- 7It comes down and sticks, because unlike poles now face each other and attract.
- 8They move apart, because both carry the same kind of charge and like charges repel.
Gravity, weight and mass
A mango falls from the tree, and a ball thrown up always comes back down. The Earth pulls every object towards itself, and a spring balance in a shop shows how strong that pull is.
The force with which the Earth attracts an object is gravitational force (gravity). The weight of an object is this pull on it, measured in newtons. The mass of an object is the amount of matter in it, measured in kg or g.
Key idea 1: Weight and mass are different — Weight is a force and changes with place; the Moon pulls with about one-sixth of the Earth's pull. Mass does not change anywhere. On Earth, a 5 kg object weighs about 50 N.
- Easy: Gravity always pulls; it never pushes.
- Medium: A 5 kg object weighs about 50 N on Earth and about 8 N on the Moon.
- Exam-type: "The wheat bag weighs 10 kg" is not scientific; 10 kg is its mass, and its weight is about 100 N.
In the figure: The mass stays 5 kg everywhere, but the weight is greatest on Earth and least on the Moon.
Key idea 2: Measuring them — Weight is measured with a spring balance: a heavier object stretches the spring more. Never hang a load heavier than the maximum mark. Mass is found with a beam balance, which compares the object with known masses. The smallest reading is the gap between two big marks divided by the number of small divisions.
- Easy: A spring balance measures weight.
- Medium: A balance marked 0 to 10 N cannot be used to weigh a 15 N object.
- Exam-type: Big marks at 0, 2, 4 N with 4 small divisions between them give 2 divided by 4, which is 0.5 N per division.
Weight is a force (newton); mass is the amount of matter (kg).
Saying "my weight is 40 kg". In science, 40 kg is the mass.
Mass stays, weight moves. Memory clue: Mass = Matter, same everywhere; Weight = Where you are, it changes.
Spot the difference
| Property | Mass | Weight |
|---|---|---|
| What it is | amount of matter | pull of gravity |
| Unit | kg or g | newton |
| Changes with place | no | yes |
Practice questions
- 1What is the SI unit of weight?
- 2Which instrument measures weight?
- 3What is mass?
- 4Does the mass of an object change on the Moon?
- 5A 6 kg bag weighs about 60 N on Earth. About how much would it weigh on the Moon, where weight is one-sixth?
- 6A spring balance has big marks at 0, 5 and 10 N with 5 equal small divisions between big marks. What is its smallest reading?
- 7A shopkeeper says, "The weight of this sack is 25 kg." Correct the statement scientifically.
- 8Can a 0 to 10 N spring balance be used to weigh a stone of weight 14 N? Why?
Show answers (try them yourself first)
- 1Newton (N).
- 2A spring balance.
- 3The amount of matter in an object.
- 4No; mass is the same everywhere.
- 5About 10 N.
- 61 N.
- 7The mass of the sack is 25 kg; its weight is about 250 N.
- 8No; the weight is more than the maximum mark and the balance may get damaged.
Upthrust: floating and sinking
In a river, a child easily lifts a big stone while it is under water, but the same stone feels much heavier once it is out. The water pushes the stone upwards and helps the child.
The upward force exerted by a liquid on an object placed in it is called upthrust or buoyant force. The Greek scientist Archimedes found that the upthrust equals the weight of the liquid pushed aside by the object.
Key idea 1: Weight against upthrust — Two forces act on an object in water: its weight pulls it down and the upthrust pushes it up. If the weight equals the upthrust, the object floats. If the weight is more than the upthrust, it sinks.
- Easy: Upthrust always acts upward.
- Medium: An object weighing 8 N that pushes aside 5 N of water sinks.
- Exam-type: An object weighing 6 N that pushes aside 6 N of water floats.
In the figure: Upthrust (blue, upward) and weight (red, downward) act on both objects. The left one floats and the right one sinks.
Key idea 2: What decides floating — Not only heaviness but also size, shape and the liquid matter. Pumice floats because it is full of tiny air pockets; a small stone sinks. A sealed empty bottle floats, but filled with sand it sinks. All liquids apply upthrust, not only water.
- Easy: Pumice floats on water.
- Medium: A coin sinks but a big wooden block floats, because the coin's weight is more than the upthrust on it.
- Exam-type: Adding sand to a floating bottle increases its weight beyond the upthrust, so it sinks.
Two forces act in water: weight downward and upthrust upward.
Thinking that heavy objects always sink. A huge ship floats, while a tiny coin sinks.
Compare the two forces. Memory clue: Equal = Float; Weight wins = Drown.
Spot the difference
| Condition | Result |
|---|---|
| weight = upthrust | floats |
| weight > upthrust | sinks |
Practice questions
- 1What is upthrust?
- 2In which direction does upthrust act?
- 3Which scientist linked upthrust to the weight of the liquid pushed aside?
- 4When does an object float in a liquid?
- 5Why does a stone feel lighter when lifted inside water?
- 6Why does pumice float on water?
- 7An object weighs 9 N and pushes aside 7 N of water. Will it float or sink? Why?
- 8A sealed empty bottle floats, but it sinks when filled with sand. Explain using weight and upthrust.
Show answers (try them yourself first)
- 1The upward force exerted by a liquid on an object placed in it.
- 2Upward.
- 3Archimedes.
- 4When its weight is equal to the upthrust.
- 5Water pushes it upward, so less force is needed to lift it.
- 6Tiny air pockets make it light for its size.
- 7It sinks, because the upthrust (7 N) is less than its weight (9 N).
- 8Sand raises the weight above the upthrust, so it sinks; the empty bottle's weight equals the upthrust.