On the table nothing pushes the disc — until it meets a wall. At that instant the wall shoves back, perpendicular to its surface, and the disc springs away. That touch-only push is a contact force, and the wall feels the disc push back just as hard — Newton's third law.
Watch where the disc's motion changes: never in the open, only at a wall. That is the signature of a contact force — it acts solely while the surfaces touch. Each push is perpendicular to the wall and lasts only an instant, so the speed readout never moves; only the direction flips. By Newton's third law the disc shoves the wall exactly as hard as the wall shoves the disc.
A contact force is a push or a pull that one object exerts on another only while their surfaces are touching. Break the contact and the force vanishes at once. The wall's push on the disc, your hand on a door, a foot on a pedal, the pull of a rope — all are contact forces. They are the opposite of non-contact forces like gravity or magnetism, which reach across empty space. On Kinetica's table the single contact force is the wall.
When two surfaces meet, each pushes on the other at right angles to the surface — this perpendicular contact push is the normal force. By Newton's third law the two pushes are equal in size and opposite in direction: the wall pushes the disc, and the disc pushes the wall just as hard. The force lasts only for the brief moment of contact, delivering an impulse that changes the disc's motion. No touch, no contact force — which is why the disc coasts freely between walls.
Between the walls nothing touches the disc, so there is no contact force and it glides straight. The instant it meets a wall, the wall delivers a normal force — a shove perpendicular to the surface — that reverses the disc's approach and sends it away. On the square and rectangle the push is perpendicular to a flat wall; on the circle it points along the radius toward the centre. The push is brief and perpendicular, so the disc's speed is unchanged — only its direction turns.
Contact forces are easiest to feel when you press a surface and watch the push grow. Load a block on a table, decide which forces need touching, and turn a brief bounce into a force.
At rest the table's normal force exactly matches the block's weight, so the net force is zero and the block stays put. Pile on more mass and the table simply pushes back harder.
The floor pushes up on your feet with a normal force exactly equal to your weight, which is why you neither sink nor float. Stand on bathroom scales and the number you read is really that contact force.
For the brief instant a ball touches the ground, the ground pushes up on it with a large contact force that reverses its motion. The harder and faster the bounce, the bigger that short-lived push.
Push hard on a wall and it pushes back on you just as hard — Newton's third law. You feel that equal contact force in your arms, even though the heavy wall does not move at all.
A contact force is the push that appears the instant two surfaces meet. This FAQ travels from the disc and the wall to books on tables, bouncing balls, rockets in space, airbags, and the equal-and-opposite pairs of Newton's third law.
A contact force is a push or a pull that one object exerts on another only while their surfaces are touching. The moment the contact ends, the force disappears. Most of the forces you meet every day are contact forces: the normal push of a floor, friction between surfaces, the tension in a rope, and the applied push of your hand. They stand in contrast to non-contact forces like gravity and magnetism, which act across empty space. On Kinetica's table, the only contact force is the wall, felt at each bounce.
The normal force is the contact push a surface exerts at right angles to itself — "normal" is the physics word for perpendicular. It always points away from the surface: straight up from a level floor, sideways from a wall, and along the radius on the circular table. The normal force is what holds a resting object up against gravity and what reverses the disc when it strikes a wall. It is one half of a contact force; the other half, acting along the surface, is friction.
| Property | Contact force | Non-contact force |
|---|---|---|
| Touching needed? | Yes | No |
| Acts across a gap? | No | Yes |
| Carried by | Surfaces or connections | A field |
| Examples | Normal force, friction, tension | Gravity, magnetism |
Newton's third law says that whenever one object pushes another, the second pushes back with an equal force in the opposite direction. So a contact force is never one-sided: when the wall pushes the disc, the disc pushes the wall just as hard the other way. These two forces are an action-reaction pair, and they act on two different objects — one on the disc, one on the wall — which is exactly why they do not cancel. The wall barely moves only because it is far more massive and fixed in place.
A solid table pushes up on the book with a normal force exactly equal to the book's weight, so the two forces cancel and the book rests in balance. The table can do this because its material is slightly springy at a tiny scale: it compresses just enough to push back as hard as it is pushed. Add more mass and the normal force grows to match — until the load is so great that the material can no longer supply enough force, and the table finally breaks.
Between the walls nothing touches the disc, so it feels no contact force and coasts in a straight line. The instant it reaches a wall, the two surfaces touch and the wall delivers a normal force — a perpendicular shove — that reverses the disc and sends it away. The push lasts only for that brief contact and acts at right angles to the wall, so the disc's speed is unchanged and only its direction turns. It is a clean, isolated example of a contact force at work.
Impulse measures the total effect of a force that acts for a certain time. It is force multiplied by the time the force acts, and it equals the change in the object's momentum: impulse = force × time = change in momentum. A contact force is usually brief but can be large, and the impulse it delivers is what flips the disc's momentum at a bounce. The key insight is that the same impulse can come from a big force over a short time or a small force over a long time.
Stopping a moving object always needs the same impulse — enough to remove all its momentum. But impulse is force times time, so if you stretch the stop over a longer time, the force needed drops. Drawing your hands back as you catch a fast ball, landing with bent knees, or falling onto a soft mat all lengthen the stopping time and so reduce the force on you. The momentum change is fixed; only the way you spread it over time is in your control.
Yes. Friction appears only where two surfaces are in contact and slide, or try to slide, against each other, so it is firmly a contact force. It acts along the surface, resisting the sliding, which makes it the sideways partner of the perpendicular normal force. Together the normal force and friction make up the full contact force between two surfaces. Friction cannot act across a gap, which is what separates it from non-contact forces like gravity.
The wall's push is perpendicular to the wall and lasts only an instant. Because it acts across the disc's motion rather than along it, it reverses the disc's direction without doing work on it, so no speed is gained or lost. A force only changes speed when part of it points along the direction of travel; a purely sideways push just turns the path. This is why the speed readout never flickers at a frictionless bounce — the contact force changes direction alone.
A rocket does not push against the ground or the air — it pushes against its own exhaust. The engine hurls hot gas backward, and by Newton's third law the gas pushes the rocket forward with an equal force. Because this action-reaction pair is entirely between the rocket and its fuel, it works perfectly in the vacuum of space, where there is nothing outside to push on. The same idea lets a swimmer move by pushing water back and a squid jet forward by pushing water out.
Tension is the pulling force carried along a rope, string, cable, or chain when it is stretched. It passes a pull from one end of the connection to the other, like a tug-of-war rope transmitting each team's pull. Tension is a contact force, because it needs the physical connection of the rope to act. A defining feature is that tension can only pull, never push — which is why a rope can drag a sled forward but cannot shove it backward.
The two forces really are equal, but they act on objects of wildly different mass. The same force produces a large change in motion for the light disc and an utterly tiny one for the massive, anchored wall. Newton's second law makes this precise: acceleration equals force divided by mass, so the heavier object barely budges. The wall does move, in principle, by an immeasurably small amount — but for all practical purposes only the disc rebounds.
In a crash, a passenger's momentum must be brought to zero, and that needs a fixed impulse. Airbags and crumple zones work by stretching out the time over which the stop happens. The crumple zone folds, and the airbag squashes, so instead of hitting a hard surface in a few thousandths of a second, the body slows over a longer interval. Because impulse is force times time, the longer time means a much smaller force on the person — turning a deadly stop into a survivable one.
Almost all of them are. When you open a door, kick a ball, lean on a desk, or pull a cart, you are applying a contact force through touching surfaces or a connection. The important exceptions are the non-contact forces: gravity, which pulls everything downward without touching; magnetism; and static electricity. A good habit when analysing a situation is to list every force and label each as contact or non-contact — it makes the whole picture, and any free-body diagram, far clearer.
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