CBSE Class 9 Science Chapter 06 How Forces Affect Motion MCQs Set 02

Practice MCQs for Class 9 Science Chapter 06 How Forces Affect Motion

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Question: The unit of force is named after Isaac Newton. Which of the following correctly follows the convention explained in the chapter for writing this unit and its symbol?
A. The unit is written as 'newton' and its symbol as 'N'
B. The unit is written as 'Newton' and its symbol as 'N'
C. The unit is written as 'newton' and its symbol as 'n'
D. The unit is written as 'Newton' and its symbol as 'n'
Show Answer & Explanation

Answer: (A) The unit is written as 'newton' and its symbol as 'N'

Explanation:
Even though the unit honours a person, the full name stays lowercase (newton) while only the symbol is capitalised (N) - this is a general rule for units named after scientists.

Question: When several forces act on an object at the same time, the single combined force that actually decides how the object moves is referred to as the
A. net force
B. normal force
C. tension force
D. buoyant force
Show Answer & Explanation

Answer: (A) net force

Explanation:
Multiple forces may act on a body, but only their combined effect - the net force - determines whether it stays put, speeds up, slows down or changes direction.

Question: A fireperson often finds it hard to keep a water hose steady while it sprays water forcefully. Which idea from the chapter best accounts for this?
A. The water rushing out pushes back on the hose with an equal and opposite force
B. The hose becomes heavier as water passes through it
C. Friction between the fireperson's hands and the hose increases suddenly
D. The gravitational force on the hose increases while spraying
Show Answer & Explanation

Answer: (A) The water rushing out pushes back on the hose with an equal and opposite force

Explanation:
This is the same reasoning used for balloons and rockets in the chapter - the hose exerts a forward force on the ejected water, so by Newton's third law the water exerts an equal backward force on the hose, making it hard to hold steady.

Question: A spacecraft drifting in deep space, far from any planet, experiences almost no gravitational pull. Using the reasoning applied to rockets in the chapter, how could it still change its velocity?
A. By expelling some mass, such as gas, in one direction so it gets pushed in the opposite direction
B. By waiting for a nearby star's gravity to act on it
C. By increasing its own total mass
D. It cannot change velocity without nearby gravity
Show Answer & Explanation

Answer: (A) By expelling some mass, such as gas, in one direction so it gets pushed in the opposite direction

Explanation:
Just as a rocket's engine expels gas downward to receive an upward push, a spacecraft in nearly gravity-free space can change its velocity by ejecting mass in one direction, since the ejected matter pushes back on it in the opposite direction as per Newton's third law.

Question: In Activity 6.1, once the stack of coins is placed near the middle of points A and B on the rubber band, what must be done just before releasing it to observe its motion?
A. Push the coins back until the rubber band is stretched to the mark C
B. Measure the distance travelled immediately
C. Switch to a different surface first
D. Wrap adhesive tape around the coins again
Show Answer & Explanation

Answer: (A) Push the coins back until the rubber band is stretched to the mark C

Explanation:
The described sequence is to first pull the stack back to mark C using a finger, stretching the rubber band, and only then let go of the coins so their motion can be timed and measured.

Question: The chapter's note on measuring force compares forces we can feel with those scientists can detect in the lab. What does this comparison reveal?
A. Human touch can sense forces around millinewtons, while specialised experiments can detect forces as tiny as yoctonewtons
B. Humans can feel forces smaller than scientists can measure
C. Both humans and scientific instruments are limited to forces above one newton
D. Forces smaller than one newton cannot be measured at all
Show Answer & Explanation

Answer: (A) Human touch can sense forces around millinewtons, while specialised experiments can detect forces as tiny as yoctonewtons

Explanation:
• A light touch, something we can directly feel, is on the order of 10^-3 N (millinewtons)
• Scientists using specialised setups can measure forces as small as 10^-24 N (yoctonewtons)
• This shows a huge gap between everyday sensation and laboratory precision

Question: Before Newton stated his laws of motion, Galileo used thought experiments involving a horizontal plane. Which long-held but mistaken belief did his reasoning challenge?
A. That a force must be continuously applied to keep an object moving at constant velocity
B. That objects fall at different rates depending on their weight
C. That force has no particular direction
D. That friction always speeds up a moving object
Show Answer & Explanation

Answer: (A) That a force must be continuously applied to keep an object moving at constant velocity

Explanation:
For a long time people assumed motion at constant velocity needed a continuous push; Galileo argued that if all obstacles to motion were removed, a body moving on a horizontal plane would keep moving indefinitely without any force being applied.

Question: A person pushes a moving box forward with a force exactly equal in magnitude to the friction opposing it. What happens to the box as a result?
A. It keeps moving at a constant velocity
B. It gradually speeds up
C. It slows down and eventually stops
D. It reverses its direction of motion
Show Answer & Explanation

Answer: (A) It keeps moving at a constant velocity

Explanation:
The applied force and friction are equal and opposite, so they balance each other out completely. With zero net force acting on the box, Newton's first law tells us its velocity stays unchanged.

Question: Two children of different masses sit on identical swings. To give both children exactly the same initial acceleration, on which child must a larger force be applied?
A. The heavier child
B. The lighter child
C. Both require the same force
D. Neither child needs a force to accelerate
Show Answer & Explanation

Answer: (A) The heavier child

Explanation:
Since acceleration equals force divided by mass, achieving an identical acceleration for a larger mass demands a proportionally larger applied force.

Question: Fragile glass items are usually cushioned with bubble wrap or hay before being transported. What is the underlying reason this protects them from breaking?
A. It stretches out the time over which any impact force acts, which lowers the acceleration and hence the force felt by the glass
B. It makes the glass items permanently heavier and sturdier
C. It cancels out the gravitational force acting on the items
D. It locks the items in place so friction alone stops all motion
Show Answer & Explanation

Answer: (A) It stretches out the time over which any impact force acts, which lowers the acceleration and hence the force felt by the glass

Explanation:
This works on the same principle as airbags and a fielder softening a catch - spreading the change in velocity over a longer time reduces the acceleration involved, and a smaller acceleration means a smaller force acts on the fragile item during a jolt.

Question: During snake boat practice, all 100 oarsmen apply a horizontal force of 200 N each. If 95 row correctly to push the boat forward while 5 mistakenly row in the opposite direction, what is the resulting net force on the boat (ignoring drag)?
A. 18000 N in the forward direction
B. 20000 N in the forward direction
C. 1000 N in the forward direction
D. 19000 N in the forward direction
Show Answer & Explanation

Answer: (A) 18000 N in the forward direction

Explanation:
The forward-pulling oarsmen outnumber the mistaken ones by 90, so the net force equals 90 times 200 N, which works out to 18000 N acting in the forward direction.

Question: A sailor standing in a small stationary boat leaps forward onto the shore. What happens to the boat immediately as the sailor jumps?
A. The boat moves backward, away from the shore
B. The boat stays completely still
C. The boat also moves forward, following the sailor
D. The boat's mass suddenly increases
Show Answer & Explanation

Answer: (A) The boat moves backward, away from the shore

Explanation:
As the sailor pushes off the boat to jump forward, the boat simultaneously experiences an equal and opposite backward push from the sailor's feet, so it drifts away from the shore.

Question: High jump events place a thick mat or sand bed for athletes to land on instead of hard ground. What is the physics reason behind this?
A. The mat increases the time taken for the athlete's velocity to fall to zero, reducing the force experienced on landing
B. The mat temporarily increases the athlete's body mass
C. The mat removes the effect of gravity during the fall
D. The mat provides a reaction force that cancels out the athlete's weight entirely
Show Answer & Explanation

Answer: (A) The mat increases the time taken for the athlete's velocity to fall to zero, reducing the force experienced on landing

Explanation:
A softer surface stretches out the duration of the impact compared to hard ground; a longer stopping time means a smaller acceleration, and by Newton's second law, a smaller force acts on the athlete's body.

Question: A hand cart loaded with vegetables collides head-on with an identical but empty hand cart. As per Newton's third law, how do the forces the two carts exert on each other during the collision compare?
A. They are equal in magnitude and opposite in direction, regardless of the difference in load
B. The loaded cart exerts a larger force because it carries more mass
C. The empty cart exerts a larger force since it can move more freely
D. The forces are unequal because the carts have different masses
Show Answer & Explanation

Answer: (A) They are equal in magnitude and opposite in direction, regardless of the difference in load

Explanation:
Newton's third law always produces equal and opposite forces on the two interacting objects, no matter how different their masses or loads are; only the resulting accelerations differ because of the mass difference.

Question: Holding a bar magnet close to a magnetic compass makes the needle swing around while the much heavier magnet in your hand stays put, even though the two exert equal and opposite magnetic forces on each other. Why does only the needle visibly move?
A. The needle's mass is far smaller, so the same force produces a much larger acceleration in it than in the bulky magnet
B. The forces on the needle and the magnet are actually unequal
C. Only the needle experiences a genuine force; the magnet experiences none
D. The bar magnet is somehow held fixed by a separate opposing force
Show Answer & Explanation

Answer: (A) The needle's mass is far smaller, so the same force produces a much larger acceleration in it than in the bulky magnet

Explanation:
This mirrors the Earth-and-fruit example in the chapter: equal forces act on both objects, but acceleration equals force divided by mass, so the light compass needle accelerates noticeably while the far more massive magnet barely moves at all.

Question: In a tug of war, what actually decides whether the rope stays still or begins to slide toward one team?
A. Whether the two pulling forces are equal or unequal in magnitude
B. The total length of the rope being used
C. How many players stand at the very back of each team
D. The direction each team happens to be facing
Show Answer & Explanation

Answer: (A) Whether the two pulling forces are equal or unequal in magnitude

Explanation:
Equal and opposite pulls are balanced forces, so the rope stays put; the moment one side pulls harder, the forces become unbalanced and the rope slides toward the stronger pull.

Question: Two forces of 10 N and 6 N act on a block in the same direction, as shown in one of the chapter's worked examples. What is the net force on the block?
A. 16 N in the direction of both forces
B. 4 N in the direction of the larger force
C. 4 N opposite to both forces
D. 16 N opposite to both forces
Show Answer & Explanation

Answer: (A) 16 N in the direction of both forces

Explanation:
When forces point the same way, their magnitudes simply add up, giving 10 N + 6 N = 16 N acting in that same direction.

Question: When a spring balance is used to find the weight of an object, what physical quantity is it really measuring?
A. The gravitational force with which Earth pulls the object
B. The mass of the object in kilograms
C. The normal force from the table beneath it
D. The frictional force acting on the object
Show Answer & Explanation

Answer: (A) The gravitational force with which Earth pulls the object

Explanation:
Weight is just the name given to the pull of Earth's gravity on an object, and the spring balance measures the strength of this pulling force directly.

Question: An object rests on a flat table. The table exerts a normal force on it that balances its weight. In which direction does this normal force act?
A. Perpendicular to the surface, pushing away from it
B. Parallel to the surface, opposing any sliding
C. Straight downward, same as gravity
D. Diagonally, at an angle to the surface
Show Answer & Explanation

Answer: (A) Perpendicular to the surface, pushing away from it

Explanation:
The normal force always acts perpendicular to the contact surface and pushes outward from it, which is why it can cancel the downward pull of gravity on a resting object.

Question: Newton's first law refers to an object moving with 'constant velocity'. What does this phrase precisely mean?
A. Neither the speed nor the direction of motion changes
B. Only the speed stays the same while direction can vary
C. Only the direction stays fixed while speed can vary
D. The object has stopped moving altogether
Show Answer & Explanation

Answer: (A) Neither the speed nor the direction of motion changes

Explanation:
Velocity has both magnitude and direction, so calling it constant means both must stay fixed - a straight-line motion at unchanging speed, not just unchanging speed alone.

Question: The chapter mentions a more complete version of Newton's second law written in terms of momentum instead of just mass and acceleration. Why is this version considered more general?
A. It links net force to the rate of change of momentum, so it still works even when an object's mass is not fixed
B. It eliminates the need to consider force altogether
C. It only applies to objects that are perfectly at rest
D. It replaces acceleration with distance travelled
Show Answer & Explanation

Answer: (A) It links net force to the rate of change of momentum, so it still works even when an object's mass is not fixed

Explanation:
F = ma assumes constant mass, but real situations like rockets losing fuel involve changing mass. Expressing the law as force being proportional to the rate of change of momentum covers such cases too.

Question: A heavy stone and a light pebble are dropped together near Earth's surface, with air resistance ignored. Based on the chapter's note on gravitational acceleration, what happens?
A. Both fall with the same acceleration, since g does not depend on mass
B. The heavier stone falls faster because it has more weight
C. The lighter pebble falls faster because it has less mass to move
D. Their accelerations depend on their shapes, not their masses
Show Answer & Explanation

Answer: (A) Both fall with the same acceleration, since g does not depend on mass

Explanation:
The text explicitly notes that the acceleration due to gravity does not depend on an object's mass, so both objects speed up at the same rate as they fall.

Question: In the balloon-and-straw activity described in the chapter, what makes the balloon shoot forward once its tied neck is released?
A. The escaping air pushes on the balloon material, and the balloon pushes back on the air with an equal force in the opposite direction
B. The straw physically drags the balloon along the taut thread
C. The balloon becomes lighter than air and simply floats away
D. Outside air pressure builds up behind the balloon and shoves it forward
Show Answer & Explanation

Answer: (A) The escaping air pushes on the balloon material, and the balloon pushes back on the air with an equal force in the opposite direction

Explanation:
As the stretched balloon expels air out through the straw, the air exerts an equal and opposite reaction force on the balloon, sending it moving in the direction opposite to the escaping air - the same idea used to explain rocket propulsion.

Question: During its descent, the Vikram lander of Chandrayaan-3 briefly fired its engine in the same direction as its motion. What was the purpose of doing this?
A. To slow the lander down so it could reach the right speed for a safe touchdown
B. To speed up the lander for a quicker descent to the surface
C. To change the lander's horizontal direction of travel
D. To keep the lander's velocity completely unchanged
Show Answer & Explanation

Answer: (A) To slow the lander down so it could reach the right speed for a safe touchdown

Explanation:
Firing exhaust gases in the direction of motion pushes back on the spacecraft, decelerating it - exactly what was needed to bring Vikram's speed down for a gentle landing near the Moon's south pole.

Question: Two boxes joined by a string are pulled together by an outside force on a frictionless floor. The chapter treats both boxes and the string as one single system rather than analysing each box separately. Why is this useful? • It removes the need to work out the internal string tension acting between the boxes • Only the outside force needs to be considered against the combined mass • The resulting acceleration formula becomes as simple as for a single object
A. Because it simplifies finding the acceleration without tracking internal forces
B. Because it makes the string tension larger than the applied force
C. Because it eliminates the need for any external force in the calculation
D. Because it reduces the combined mass of the system to zero
Show Answer & Explanation

Answer: (A) Because it simplifies finding the acceleration without tracking internal forces

Explanation:
Internal forces like tension act between parts of the same system and cancel out when the whole system is considered together, leaving only the external force to divide by the total mass - a much simpler calculation than handling each box on its own.

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