A Book on a Table Has Forces on It and Still Does Nothing
Learn what a force does, why forces come in contact and non-contact kinds, how to add forces along a line to find the net force, and why a rigid body can only be moved while a non-rigid one can be reshaped.
If forces act on a book lying on a table, why does it stay put?
There are two forces on it. Gravity pulls it down, and the table pushes it up.
They are equal in size and opposite in direction, so they cancel. The book therefore behaves exactly as if no force were acting — it does not move.
This is the first thing to get straight about force: a force does not guarantee motion, and motionlessness does not mean no forces. What matters is whether the forces cancel.
This page covers the first part of the ICSE Class 8 Physics chapter on force and pressure: what a force does, the kinds of force, adding forces along a line, and why the same push reshapes clay but only moves a brick.
They are equal in size and opposite in direction, so they cancel. The book therefore behaves exactly as if no force were acting — it does not move.
This is the first thing to get straight about force: a force does not guarantee motion, and motionlessness does not mean no forces. What matters is whether the forces cancel.
This page covers the first part of the ICSE Class 8 Physics chapter on force and pressure: what a force does, the kinds of force, adding forces along a line, and why the same push reshapes clay but only moves a brick.
What can a force actually do?
A force is a push or a pull on a body. It cannot be seen; it is recognised only by its effects, of which there are five.
- Start motion. A stationary football moves when kicked.
- Stop motion. A moving ball is caught and brought to rest.
- Change speed. Pedalling harder speeds a bicycle up; braking slows it down.
- Change direction. A batted ball travels back the way it came; a magnet bends the path of a rolling steel ball.
- Change shape or size. Squeezing a sponge, stretching a rubber band, denting a car.
The first four are all changes in the state of motion. The fifth is a change in shape, and it is the one students forget to list.
Its SI unit is the newton, symbol . From the earlier chapter on units, force is a derived quantity:
Force has direction. Stating is not a complete description — pushing east and pushing west have opposite results. A quantity needing both a size and a direction is a vector, and force is one. This is why forces are drawn as arrows, with the length showing the size and the arrowhead showing the direction.
Measuring it. A force is measured with a spring balance, which works because the extension of a spring grows in step with the force stretching it. A spring balance reading for a hanging object is measuring the weight of that object — the force of gravity on it:
for a mass of , taking .
- Start motion. A stationary football moves when kicked.
- Stop motion. A moving ball is caught and brought to rest.
- Change speed. Pedalling harder speeds a bicycle up; braking slows it down.
- Change direction. A batted ball travels back the way it came; a magnet bends the path of a rolling steel ball.
- Change shape or size. Squeezing a sponge, stretching a rubber band, denting a car.
The first four are all changes in the state of motion. The fifth is a change in shape, and it is the one students forget to list.
Its SI unit is the newton, symbol . From the earlier chapter on units, force is a derived quantity:
Force has direction. Stating is not a complete description — pushing east and pushing west have opposite results. A quantity needing both a size and a direction is a vector, and force is one. This is why forces are drawn as arrows, with the length showing the size and the arrowhead showing the direction.
Measuring it. A force is measured with a spring balance, which works because the extension of a spring grows in step with the force stretching it. A spring balance reading for a hanging object is measuring the weight of that object — the force of gravity on it:
for a mass of , taking .
Which forces need contact and which do not?
Forces split into two families by whether the two bodies must touch.
Contact forces act only on touching.
- Muscular force — pushing a door, lifting a bag, a bullock pulling a cart. Breathing and digestion also use muscular force inside the body.
- Frictional force — always opposes the relative motion between two surfaces in contact. It is what slows a rolling ball and lets you walk without slipping.
- Normal reaction — the push a surface gives perpendicular to itself. This is the upward push of the table on the book.
- Tension — the pull along a stretched string or rope.
Non-contact forces act across a distance, with nothing between.
- Gravitational force — the pull of the Earth on every object, which we call its weight. It acts on a falling stone with no contact at all.
- Magnetic force — a magnet attracts iron from a distance, and two like poles repel without touching.
- Electrostatic force — a plastic comb rubbed on dry hair picks up small pieces of paper from slightly above them.
The test to apply. Ask whether the force would still act if a gap were opened between the two bodies. Friction would vanish; gravity would not. That distinction, and not the strength of the force, is what puts it in one family or the other.
Non-contact forces are also called field forces, because each acts through a region of influence surrounding the source.
Contact forces act only on touching.
- Muscular force — pushing a door, lifting a bag, a bullock pulling a cart. Breathing and digestion also use muscular force inside the body.
- Frictional force — always opposes the relative motion between two surfaces in contact. It is what slows a rolling ball and lets you walk without slipping.
- Normal reaction — the push a surface gives perpendicular to itself. This is the upward push of the table on the book.
- Tension — the pull along a stretched string or rope.
Non-contact forces act across a distance, with nothing between.
- Gravitational force — the pull of the Earth on every object, which we call its weight. It acts on a falling stone with no contact at all.
- Magnetic force — a magnet attracts iron from a distance, and two like poles repel without touching.
- Electrostatic force — a plastic comb rubbed on dry hair picks up small pieces of paper from slightly above them.
The test to apply. Ask whether the force would still act if a gap were opened between the two bodies. Friction would vanish; gravity would not. That distinction, and not the strength of the force, is what puts it in one family or the other.
Non-contact forces are also called field forces, because each acts through a region of influence surrounding the source.
Formula
How do you add forces to find the net force?
For forces along the same straight line, add those pointing the same way and subtract those pointing opposite ways. The single force that results is the net or resultant force.
Same direction — add. Two people push a crate in the same direction with and :
in the direction of the pushes.
Opposite directions — subtract. In a tug-of-war one team pulls with and the other with :
in the direction of the stronger team. The rope moves that way — slowly, because is all that is left of a effort.
Equal and opposite — balanced. If both teams pull with :
The forces are balanced. The rope does not move, though it may stretch.
What each case produces.
- Balanced forces () produce no change in the state of motion. A body at rest stays at rest; a body already moving keeps moving as it was. They can still change the body's shape — the tug-of-war rope is under great tension and stretches.
- Unbalanced forces () do change the state of motion — starting, stopping, speeding up, slowing down or turning.
Back to the book. Gravity pulls down with for the book, and the table pushes up with :
Balanced, so no motion. Pull the table away and only the downward force remains — now unbalanced, and the book falls.
Same direction — add. Two people push a crate in the same direction with and :
in the direction of the pushes.
Opposite directions — subtract. In a tug-of-war one team pulls with and the other with :
in the direction of the stronger team. The rope moves that way — slowly, because is all that is left of a effort.
Equal and opposite — balanced. If both teams pull with :
The forces are balanced. The rope does not move, though it may stretch.
What each case produces.
- Balanced forces () produce no change in the state of motion. A body at rest stays at rest; a body already moving keeps moving as it was. They can still change the body's shape — the tug-of-war rope is under great tension and stretches.
- Unbalanced forces () do change the state of motion — starting, stopping, speeding up, slowing down or turning.
Back to the book. Gravity pulls down with for the book, and the table pushes up with :
Balanced, so no motion. Pull the table away and only the downward force remains — now unbalanced, and the book falls.
Why does the same push reshape clay but only move a brick?
Because a brick is rigid and clay is non-rigid.
A rigid body does not change its shape or size when a force is applied. The distance between any two points in it stays fixed. A force on a rigid body can therefore only change its state of motion — it can be slid, lifted or spun, but not squashed. A brick, a stone, a steel rod and a glass marble are rigid.
A non-rigid body changes shape or size under a force. Its particles are able to shift relative to one another, so a force can deform it as well as move it. Clay, dough, a sponge, a rubber band, a spring and a plastic bottle are non-rigid.
The consequence for the five effects. All five are available on a non-rigid body, but a rigid body only offers the first four. That is a favourite one-line question: which effect of force cannot be produced on a rigid body? — a change of shape or size.
Deformation can be temporary or lasting. A rubber band or a spring springs back when released; a rubber ball recovers its roundness after bouncing. These are elastic. Clay, dough and a dented car panel keep the new shape; these are plastic in behaviour.
No body is perfectly rigid. A steel girder in a bridge does flex a little when a lorry crosses, and the tug-of-war rope stretches under balanced forces. Rigid means the deformation is far too small to matter for the question at hand — a simplification, not a claim about the material being unsqueezable.
Which is why a brick dropped on clay leaves a clear dent in the clay and none in itself: both bodies experience the same force, and only the softer one has particles free enough to rearrange.
A rigid body does not change its shape or size when a force is applied. The distance between any two points in it stays fixed. A force on a rigid body can therefore only change its state of motion — it can be slid, lifted or spun, but not squashed. A brick, a stone, a steel rod and a glass marble are rigid.
A non-rigid body changes shape or size under a force. Its particles are able to shift relative to one another, so a force can deform it as well as move it. Clay, dough, a sponge, a rubber band, a spring and a plastic bottle are non-rigid.
The consequence for the five effects. All five are available on a non-rigid body, but a rigid body only offers the first four. That is a favourite one-line question: which effect of force cannot be produced on a rigid body? — a change of shape or size.
Deformation can be temporary or lasting. A rubber band or a spring springs back when released; a rubber ball recovers its roundness after bouncing. These are elastic. Clay, dough and a dented car panel keep the new shape; these are plastic in behaviour.
No body is perfectly rigid. A steel girder in a bridge does flex a little when a lorry crosses, and the tug-of-war rope stretches under balanced forces. Rigid means the deformation is far too small to matter for the question at hand — a simplification, not a claim about the material being unsqueezable.
Which is why a brick dropped on clay leaves a clear dent in the clay and none in itself: both bodies experience the same force, and only the softer one has particles free enough to rearrange.
Exam tip
Exam tip: never say no force because nothing moved
The book does not move, so no force acts on it is wrong. The correct statement is that the forces on it are balanced, so the net force is zero.
List all five effects of force when asked, and remember the fifth — change of shape or size — is the one that fails on a rigid body.
When adding forces, state a direction with the answer: towards the stronger team, not just .
Keep weight as a force in newtons, distinct from mass in kilograms: an body has weight . Writing a weight in kilograms loses marks.
For the contact-or-not question, apply the gap test rather than trying to recall a list — and remember friction and normal reaction are contact, while gravitational, magnetic and electrostatic are not.
Name friction carefully: it opposes relative motion, so it acts opposite to the direction the surfaces slide.
And draw forces as arrows whose length suggests the size, with the body shown as a simple box. A clear diagram often carries the mark by itself.
List all five effects of force when asked, and remember the fifth — change of shape or size — is the one that fails on a rigid body.
When adding forces, state a direction with the answer: towards the stronger team, not just .
Keep weight as a force in newtons, distinct from mass in kilograms: an body has weight . Writing a weight in kilograms loses marks.
For the contact-or-not question, apply the gap test rather than trying to recall a list — and remember friction and normal reaction are contact, while gravitational, magnetic and electrostatic are not.
Name friction carefully: it opposes relative motion, so it acts opposite to the direction the surfaces slide.
And draw forces as arrows whose length suggests the size, with the body shown as a simple box. A clear diagram often carries the mark by itself.
Did you know
Why does a balanced rope still snap?
In a tug-of-war with two evenly matched teams, the net force on the rope is zero and nothing moves. Yet ropes do break in exactly that situation.
The reason is that net force zero is a statement about the rope as a whole, not about what is happening inside it. Each team is pulling the rope's fibres apart, and the fibres feel the full pull from each side — not the difference between them.
So the balanced case is the worst case for the rope: doubling both pulls to each keeps the net force at zero while doubling the strain on every fibre.
This is why balanced forces do nothing is only true of the state of motion. Balanced forces are perfectly capable of changing a body's shape, and if the body is non-rigid enough, of destroying it.
The reason is that net force zero is a statement about the rope as a whole, not about what is happening inside it. Each team is pulling the rope's fibres apart, and the fibres feel the full pull from each side — not the difference between them.
So the balanced case is the worst case for the rope: doubling both pulls to each keeps the net force at zero while doubling the strain on every fibre.
This is why balanced forces do nothing is only true of the state of motion. Balanced forces are perfectly capable of changing a body's shape, and if the body is non-rigid enough, of destroying it.
Key takeaways
Force, balance and rigidity: quick revision
- A force is a push or pull, recognised only by its effects, measured in newtons with a spring balance, and it is a vector — size plus direction.
- Its five effects: start motion, stop motion, change speed, change direction, change shape or size. The first four are changes in the state of motion.
- , and weight , so weighs with .
- Contact forces: muscular, frictional, normal reaction, tension. Non-contact (field) forces: gravitational, magnetic, electrostatic. Test by opening a gap.
- Net force along a line: add if same direction (), subtract if opposite ().
- Balanced forces give and no change in motion — but they can still change shape. Unbalanced forces change the state of motion.
- The book on the table: , so it rests. Remove the table and downward is unbalanced.
- A rigid body cannot be reshaped, so only the first four effects apply. A non-rigid body can also be deformed — elastically like a spring, or permanently like clay.
- No real body is perfectly rigid; rigidity is a useful approximation.
Work through some net-force and force-type questions now — most marks in this chapter come from stating the direction and naming the right family.
- Its five effects: start motion, stop motion, change speed, change direction, change shape or size. The first four are changes in the state of motion.
- , and weight , so weighs with .
- Contact forces: muscular, frictional, normal reaction, tension. Non-contact (field) forces: gravitational, magnetic, electrostatic. Test by opening a gap.
- Net force along a line: add if same direction (), subtract if opposite ().
- Balanced forces give and no change in motion — but they can still change shape. Unbalanced forces change the state of motion.
- The book on the table: , so it rests. Remove the table and downward is unbalanced.
- A rigid body cannot be reshaped, so only the first four effects apply. A non-rigid body can also be deformed — elastically like a spring, or permanently like clay.
- No real body is perfectly rigid; rigidity is a useful approximation.
Work through some net-force and force-type questions now — most marks in this chapter come from stating the direction and naming the right family.