Class 9 Science, Chapter 6 (NCERT). 12 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
A cart is pulled with 5 N eastward. What does that pull describe?
(a) An interaction of magnitude 5 N, direction east
The pull is an interaction between the cart and whatever pulls it. Force is a vector, so both magnitude and direction are needed: 5 N towards the east. The newton measures force, not mass.
A 12 N force acts right and a 7 N force acts left. What is the net force?
(d) 5 N to the right
Opposite forces subtract: 12 N − 7 N = 5 N. The larger force acts to the right, so the net force is 5 N to the right.
A trolley moves in a straight line at constant velocity while opposing forces act on it. What is the net force on the trolley?
(c) Zero
The velocity does not change, so the acceleration is zero. By , the net force is zero. The opposing forces still act; they are balanced and their sum is zero.
A sideways unbalanced force acts on a moving ball. What can it do?
(d) Change the direction of the ball's velocity
Acceleration is any change in velocity, including direction.
Why can a person walk forward without slipping?
(d) Static friction from the ground acts forward on the foot.
The foot pushes backward on the ground, and static friction from the ground provides forward traction.
Why are wheels used to move heavy loads?
(a) Rolling friction is generally smaller than sliding friction.
Replacing sliding with rolling commonly reduces the resisting force.
Why does a standing passenger fall forward when a bus stops suddenly?
(a) The passenger's body tends to continue its forward motion because of inertia.
When the bus slows, the body tends to retain its previous velocity; inertia is not an extra force.
Which object has greater inertia: a loaded truck or an empty trolley?
(d) The loaded truck, because it has greater mass
Mass measures inertia, so the more massive object resists velocity change more strongly.
A 4 kg object accelerates at 3 m/s² under a net force of 12 N. What net force gives the same object an acceleration of 6 m/s²?
(a) 24 N
For the same mass, doubling acceleration doubles net force: 12 × 2 = 24 N. The 6 N option incorrectly halves the force. The 4 N option treats mass in kilograms as force in newtons. Adding the initial force and new acceleration gives 18, but these quantities cannot be added to calculate force.
Why does an airbag reduce injury during a collision?
(c) It increases stopping time, reducing average force for the same momentum change.
Impulse is fixed by momentum change; spreading it over more time lowers average force.
A gun pushes a bullet forward. What is the third-law partner of that push?
(c) The bullet pushing the gun backward with equal force
The partner of the gun's push on the bullet is the bullet's push on the gun. It is backward, equal in size and acts at the same moment. The two forces act on two different bodies, so they never cancel.
Two persons of unequal mass stand on smooth ice and push each other. What are the forces on them, and which one has the greater acceleration?
(d) Equal opposite forces; the lighter person has the greater acceleration
The two pushes are an action-reaction pair, so they act at the same time, are equal in magnitude and opposite in direction. Each force acts on a different person, so nothing cancels. From , the same force size gives a larger acceleration to the smaller mass, so the lighter person accelerates more.
Wrong
A trolley moving steadily must have a forward net force.
Why it feels right. I associate pushing with keeping it moving.
Right
A net force changes velocity.
Worked example
