Effects of force
A force is something you never see directly — no one has ever photographed a "push" floating in mid-air. What gives a force away is what it does: a stationary football starts rolling, a speeding cyclist slows to a stop, a straight-driven ball curves off a bat, a lump of dough flattens under your palm. Every one of these is evidence of a force at work, read off from the change it leaves behind. This article is the second half of "Exploring Forces." Having already met force as a push or a pull, we now ask a sharper question: what, exactly, can a force change? The answer sorts into four clean categories — motion starting, speed changing, direction changing, and shape changing — and a single force very often produces more than one of them at once. Spotting which effect (or effects) is at play in a scene, and explaining why, is the core skill this topic tests.
A force can start, speed up, slow down, or redirect a moving object, and/or change its shape — often several of these at the same instant, from one single force.
The core idea
The four effects, one at a time. A force acting on an object can produce any of the following, entirely by itself:
- Starts a stationary object moving. Nothing moves until some force sets it going — a football lying still on the ground stays still until a kick pushes it.
- Changes the speed of a moving object. A push in the direction of motion speeds it up (a cyclist pedalling harder); a push against the motion slows it down (squeezing the brakes).
- Changes the direction of a moving object. A sideways force redirects motion without necessarily changing how fast the object is going — a fielder's kick sends a rolling ball off at a new angle.
- Changes the shape of an object. Squeezing, stretching, bending, or pressing deforms an object's shape, and this can happen whether or not the object moves anywhere at all — think of pressing a sponge or stretching a rubber band.
One force, several effects. These four are not mutually exclusive boxes — a real force very often ticks more than one at the same time. A cricket bat striking a moving ball changes its speed and its direction in a single blow. A ball hitting a wall changes its direction and, for an instant, its shape. Reading a scenario correctly means checking each of the four possibilities separately rather than stopping at the first one you notice.
No visible effect does not mean no force. A force can be applied without producing any obvious change at all, if the object is too heavy or too rigid to respond — pushing steadily on a brick wall applies a real force even though the wall neither moves nor visibly deforms. The absence of a visible effect is not proof that no force acted; it only means that force was not enough to overcome the object's resistance to change.
Why it works
This topic has no formula in the Class 8 syllabus — it is a classification, not a calculation — so instead of a derivation, here is the reasoning that ties the four effects together and explains why they so often overlap. Think of "effect of force" as answering one question: what property of the object changed? An object at any moment has a state of motion (moving or not, and if moving, at what speed and in what direction) and a shape. A force is simply an interaction that can alter any part of that state. Since motion and shape are separate properties, a force acting on an object can, in principle, touch either or both — which is exactly why a single force is so often seen producing two or three effects together rather than just one. There is nothing in the definition of force that restricts it to changing only one property at a time; the four effects are simply the complete list of properties a push or pull can influence for objects at this level of study. This is also why "no visible change" does not mean "no force was applied." An object resists change (through its own rigidity, weight, or friction with the ground), and a small enough force can be fully absorbed by that resistance without producing a change in either motion or shape that we can see. The force is real and acting — it simply loses the contest against the object's resistance.
Worked example
Question: A batter strikes a fast, straight-travelling cricket ball. Immediately after the bat makes contact, the ball is moving off in a new direction, at a noticeably higher speed than before, and high-speed footage shows the ball was momentarily squashed against the bat's face at the instant of contact. Before this, the ball had been thrown from rest by the bowler's earlier run-up delivery. List every effect of force you can identify across the whole sequence, and state where in the sequence each one occurs.
Step 1. Identify the first stage: before the bowler releases the ball, it is held still in the hand. The act of bowling is a push that makes this stationary ball start moving — this is the "starts a stationary object moving" effect, occurring at the moment of release.
Step 2. Identify what happens at the instant of contact with the bat: the ball's speed after the shot is described as noticeably higher than before, so the bat's force has changed the ball's speed — specifically, sped it up.
Step 3. The ball is also moving off in a new direction after contact, different from the direction it was travelling in before the bat struck it — so the same single contact also produced a change of direction.
Step 4. The footage additionally shows the ball momentarily squashed at contact — a change of shape, occurring at exactly the same instant as the speed and direction changes, all from the one force of the bat hitting the ball.
Step 5. Assemble the full list. Across the sequence there are four separate instances of the effects of force: starting motion (the bowler's release), and then, all from the single force of the bat's impact, a change of speed, a change of direction, and a change of shape occurring together. The bat's one strike alone demonstrates three of the four effects of force simultaneously.
Common traps
- Assuming a force must always start something moving — a force just as easily speeds up, slows down, redirects, or reshapes an object that is already in some particular state of motion.
- Treating the four effects as one-at-a-time boxes — a single force (a bat on a ball, a foot on a rolling ball) routinely produces two or three effects together.
- Forgetting that change of shape counts on its own, even with no motion involved at all — squeezing dough that never moves anywhere is still a genuine effect of force.
- Concluding "no force was applied" just because nothing visibly happened — a force can be fully applied and still be resisted, leaving no visible change, if it is not strong enough to overcome the object's resistance.
Ask the tutor: Want to try spotting all the effects of force hidden in a single goalkeeper's save?
If a force is applied to something and nothing happens, does that mean there was no force?
No. A force can be genuinely applied and still produce no visible change if the object resists it strongly enough — pushing steadily on a heavy locked door is a real force even though the door does not move or bend. The rule is that a force is capable of producing an effect, not that it always must. Whether an effect actually shows up depends on the size of the force compared with the object's resistance to being moved or reshaped.
Can one single force produce more than one effect at the same time?
Yes, and this is one of the most tested ideas in this topic. A cricket bat hitting a moving ball is a single force, but it changes the ball's speed, its direction, and briefly its shape, all in the same instant of contact. There is no rule limiting a force to just one effect — the four effects are simply the different properties (motion state and shape) that a force is capable of altering, and a strong enough impact can alter several of them together.
Is change of shape a "real" effect of force, even if the object never moves?
Yes. Change of shape is one of the four standard effects of force in this chapter, and it is completely independent of whether the object also moves. Squeezing a sponge, stretching a rubber band, or pressing a lump of dough all change shape while the object may stay exactly where it is on the table. Do not assume an effect of force must involve motion — shape change on its own is a complete, valid answer.