Track quiz
Show on a big screenHere are the ten rules first. Use them if a question feels hard.
| Idea | The rule |
|---|---|
| No force, no change | An object at rest stays at rest unless a net force acts. A moving object keeps a constant velocity unless a net force acts. |
| Inertia and mass | Inertia is an object's tendency to resist a change in its motion. A larger mass has more inertia. |
| Force and acceleration | Net force equals mass times acceleration, F = ma. One newton gives 1 kg an acceleration of 1 m/s². |
| Friction with numbers | Friction acts against the motion, or against the push when nothing moves. Net force is the push minus the friction. Equal forces leave the motion unchanged. |
| Action and reaction | If A pushes B, B pushes A equally the other way. The two forces act on different objects. |
| Equal forces, unequal motion | The two forces of a pair are equal. They act on different objects, so they do not cancel. If nothing else pushes, the lighter object accelerates more. |
| Softening a stop | The velocity drops by the same amount. A longer stopping time means a smaller acceleration. So the average force needed is smaller. |
| Which law answers | First law: no net force, no change in velocity. Second law: F = ma. Third law: forces come in equal, opposite pairs. |
| Force from a graph | The slope of a velocity–time graph is the acceleration. Then net force = mass × acceleration. |
| Two boxes, one system | Boxes joined by a light, tight string move as one system. On a smooth floor, acceleration = outside pull ÷ total mass. |
A skater glides at 4 m/s. The net force on her is zero. What happens next?
Compare a 4 kg bag with a 9 kg suitcase. Which resists a change in motion more?
A net force of 45 N acts on a 9 kg trolley. What is its acceleration?
A 30 kg cart is pulled with 50 N. Friction on it is 35 N. What is the net force?
A hand pushes a door with 20 N. What does the door do to the hand?
A spring pushes two carts apart with 12 N each. One is 2 kg, one is 6 kg. Nothing else pushes them. Which speeds up more?
Why do high jumpers land on a thick mat?
A skater pushes a wall and rolls backwards. Which law explains it?
A 2 kg trolley gains 6 m/s in 3 s, from rest. What is the net force?
A light string ties two boxes, 3 kg and 7 kg. The floor is smooth. A 30 N pull acts on one. What is a?
This is Chapter 6 of your Class 9 book, Exploration. It is called How Forces Affect Motion. Read it this week.
Next is Work, Energy, and Simple Machines. It asks what a force can do when it moves something.
Reveal every answer to finish this lesson.