Class 8 Science, Chapter 5 (NCERT). 15 questions by concept, 1 with pictures. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
A hand presses a table, then lifts clear of it. What happens to the contact forces between the hand and table when they separate?
(d) End as soon as contact ends
The contact forces between the two objects end as soon as contact ends. Other forces, such as gravity, can still act on them.
A box stands on the floor. It remains still. How do the forces on the box compare?
(b) Forces balancing one another
A still object can have forces acting on it. The chapter says those forces balance one another.
A hand starts a ball rolling. Which is a force: the hand’s push or the ball’s movement?
(c) The hand’s push only
A force is a push or pull from an interaction. Movement is not force stored in an object.
A person stays away from the ball. Which setup lets the person push it using muscular force?
(b) A held stick touching the ball
Muscular force needs a path of contact. A held rope or stick can provide it.
A boat moves through still water. What force does the water exert against its motion?
(b) Friction
Air and water exert friction on objects moving through them.
A block rests on a table. Someone tries to slide the upper object. How do tiny bumps in the touching surfaces produce friction?
(d) Tiny bumps on the two surfaces catch against one another and oppose relative sliding.
Even smooth-looking surfaces have tiny bumps. These bumps catch when the surfaces touch and oppose one surface sliding over the other.
Two ring magnets fit loosely on a smooth upright wooden guide. The upper ring slides freely; like poles face across a gap. The upper magnet springs back up after being pressed down gently and released. What keeps the upper magnet in the air?
(b) The upward magnetic force from the lower magnet
Like poles repel, so the lower magnet pushes the upper magnet upwards across the gap. This magnetic force keeps the upper magnet in the air. The loose, smooth guide keeps the rings aligned but does not hold the upper ring up.
Which poles must face each other for two magnets to push each other away?
(c) Like poles
Like poles repel, which means they push each other away. The facing poles must be north and north, or south and south.
A magnet attracts an iron pin across a gap. Which statement describes how magnetic force depends on contact?
(d) It can act without contact and does not automatically stop when the objects touch.
A non-contact force does not require the objects to touch. Gravity and magnetic force can also act when their objects are touching.
A charged balloon attracts the charged woollen cloth used to rub it. It repels another balloon rubbed in the same way. How many kinds of electrical charge are there?
(b) Two
There are two kinds of electrical charge: positive and negative. The balloons repel because they carry like charges. Each balloon attracts its charged cloth because they carry unlike charges.
At the same place, object A weighs 2 N, B weighs 5 N, and C weighs 5 N. Which objects does Earth pull with equal force?
(a) B and C only
Weight measures Earth's gravitational pull on an object. The two objects with equal weights experience equal pulls. The third object's different weight means it experiences a different pull.
An object weighs one sixth as much on the Moon as on Earth. Its mass on Earth is 30 kg. What is its mass on the Moon?
(d) 30 kg
Mass is the amount of matter. Moving the object to the Moon changes its weight, not its mass.
Spring balances A and B each have a range of 0 N to 10 N. Between adjacent 1 N marks, A has 6 equal gaps and B has 5 equal gaps. How do their smallest divisions compare?
(d) Smaller on A than on B
A divides each 1 N interval into more equal gaps than B. Each gap therefore represents less weight on A, so A has the smaller smallest division. Equal ranges do not mean equal smallest divisions.
A large pumice rock floats while a smaller coin sinks. Which feature of pumice can help explain why it floats?
(b) Its air spaces can make its weight low enough for water’s upthrust to support it.
Pumice can float because its air spaces affect how its weight compares with upthrust. Size alone is insufficient. At rest, the floating pumice has equal weight and upthrust.
Three objects are hung from the same 10 N spring balance, one at a time, in the order listed. Each hangs still at the same place. A small metal block and a pencil box each weigh 4 N. A large empty carton weighs 2 N. How does the spring's stretch compare for the three objects?
(d) Equal for the metal block and pencil box; less for the carton
The same spring stretches equally for objects of equal weight. A greater weight produces a greater stretch. An object's size alone does not determine the stretch.
Wrong
There is no force on a box that stays still while I push it.
Why it feels right. I use motion as proof that a force exists.
Right
Forces can balance and leave an object at rest.
Worked example
