Motion in a Straight Line Class 11 NEET MCQs

Explanation:
When a moving body encounters a force opposite to its motion, the effect on its velocity depends on the force’s magnitude and direction. According to Newton’s laws, a body resists changes in its state of motion due to inertia. An opposing force produces acceleration in the opposite direction, gradually reducing the body’s speed. The body continues along its straight path initially because a reversal of direction requires sufficient force to overcome its momentum. The rate of slowing depends on both the applied force and the object’s Mass, illustrating the relationship between force, acceleration, and velocity.

An analogy is pushing a sliding book backwards; it slows before it can move in the opposite direction. motion remains along the same line until other forces act.

Explanation:
Retardation, a form of negative acceleration, reduces the body’s speed over time. While acceleration remains constant in magnitude, the forward velocity diminishes steadily. Displacement still increases as the object continues moving, but at a decreasing rate. Observing velocity and displacement graphs under uniform retardation shows a linearly decreasing velocity curve and a gently increasing displacement curve. The interplay of velocity, speed, and acceleration under constant opposing force determines which quantity reduces consistently.

A car slowing steadily on a straight road demonstrates this behavior: its speed decreases, displacement continues but at a slower rate, and acceleration remains constant.

Explanation:
Straight-line motion occurs when an object moves along a path where all points lie on a single straight trajectory. motion along curved paths, circular or otherwise, deviates from this definition because the direction continuously changes. Even if speed is constant, a curved path implies changing velocity due to the continuous change in direction, which contrasts with motion purely along a straight line.

For example, a wheel rolling in a circle or a body moving in a circular orbit exhibits constant speed but changes direction, highlighting that straight-line motion requires both consistent path and unchanging direction.

Explanation:
Constant velocity implies that both speed and direction remain unchanged. While the particle moves, its position Vector changes continuously because it moves away from its initial location. Acceleration is zero in magnitude since there is no change in velocity, ensuring the particle’s motion is uniform.

Visualizing this, a car moving at constant velocity along a straight road covers equal distances in equal intervals of time. The position Vector keeps changing as it advances, although the velocity and acceleration stay consistent.

Explanation:
Understanding velocity, speed, and acceleration is key to evaluating statements about motion. A body can have zero velocity but still accelerate, such as at the highest point of a vertical throw. A body moving in a curved path can maintain constant speed while velocity changes due to changing direction. Acceleration can remain constant even as the velocity direction changes. These principles clarify which motion scenarios defy common assumptions about speed and velocity.

For example, a ball thrown upwards stops momentarily at the peak but experiences gravitational acceleration.

Explanation:
The trajectory of a particle depends on both its velocity and acceleration. Velocity dictates the direction of motion at each instant, while acceleration alters either speed or direction. The combination of these Vectors determines how the particle moves through space, whether along a straight line, curve, or circle.

For instance, a projectile follows a parabolic path because initial velocity gives forward motion while gravitational acceleration pulls it downward, shaping the trajectory. The interplay of these Vectors defines the overall path.

Explanation:
At the highest point of vertical motion, the body’s upward velocity becomes zero as it transitions from ascending to descending. However, acceleration remains active due to gravity, continuously influencing the motion. This ensures the body starts descending immediately after reaching the peak. Acceleration is uniform and points downward throughout the motion, illustrating that velocity can be zero while acceleration persists.

A ball tossed vertically momentarily stops at the top, yet gravity immediately changes its motion direction.

Explanation:
Relative motion defines how moving objects are observed from different frames of reference. The train passenger perceives the car’s velocity relative to the train’s own motion, combining the Vectors of both velocities. The resulting direction depends on the Vector addition of the train’s and car’s velocities, giving an apparent diagonal motion in the passenger’s frame.

For example, crossing rivers with a boat: the water current and boat speed combine to give the actual direction seen by a passenger inside the boat.

Explanation:
Even when speed is constant, circular motion involves continuous change in velocity direction. Acceleration, known as centripetal acceleration, always points towards the center of the circle, maintaining the circular path. Both velocity and acceleration Vectors are constantly changing direction while the speed magnitude remains unchanged.

A stone tied to a string and swung in a circle demonstrates this: its speed is constant, but velocity direction keeps rotating, requiring inward acceleration.

Explanation:
Applying brakes produces a force opposite to the car’s direction of motion, creating acceleration toward the rear. This deceleration reduces the car’s speed gradually until it stops. The direction of acceleration is determined by the applied braking force relative to the motion.

For instance, when braking on a straight road, the car slows in the same line of travel due to the opposing frictional force, illustrating the direct link between applied force and change in motion.

Explanation:
The coin’s motion relative to the train depends on the train’s acceleration. If the train decelerates, objects inside tend to move forward relative to the train due to inertia. Conversely, if the train accelerates forward, the coin appears to fall backward as it resists the train’s change in velocity. This demonstrates Newton’s first law, where objects resist changes to their state of motion.

For example, standing in a bus that brakes suddenly causes a feeling of lurching forward, illustrating relative motion and inertia.

Explanation:
Newton’s first law states that an object maintains its velocity if no NET external force acts upon it. Balanced forces produce no acceleration, allowing the object to continue moving uniformly in the same direction. Any unbalanced force would change the object’s speed or direction, interrupting constant velocity.

A puck sliding on a frictionless ice surface continues at a steady speed because no NET force opposes its motion.

Explanation:
Friction is a resistive force arising between surfaces in contact. It always opposes the relative motion or tendency of motion between surfaces. The magnitude depends on the nature of the surfaces and the normal force pressing them together. Friction can prevent motion entirely or slow an already moving object.

For instance, pushing a heavy box on a floor encounters friction opposing the direction of push, reducing acceleration.

Explanation:
Acceleration occurs whenever velocity changes in magnitude or direction. Motion at constant speed along a straight line produces no acceleration, whereas curved motion or speed changes introduce acceleration. Observing velocity Vectors and speed magnitudes over time indicates whether acceleration is present.

For example, a car cruising at constant speed on a straight highway exhibits zero acceleration despite ongoing motion.

Explanation:
When an object accelerates uniformly, its velocity changes over time according to the equation v = u + a × t, where u is the initial velocity, a is the acceleration, and t is the time elapsed. The velocity increases linearly if the acceleration is constant.

Here, the body starts at a given initial speed and experiences constant acceleration. Over the given duration, the acceleration adds incrementally to the initial velocity, showing how motion evolves in a straight line under uniform acceleration. Understanding this relationship helps predict the final speed without solving the problem directly.

For example, a car moving at 15 m/s that accelerates steadily at 10 m/s² increases its speed gradually. After each second, the velocity rises by the amount equal to the acceleration, demonstrating a uniform increase in motion. Graphing velocity versus time would show a straight line with slope equal to acceleration, clearly visualizing this change.

This principle illustrates that an object’s speed grows steadily with time when acted upon by constant acceleration, while displacement during this period can also be calculated using standard kinematic relations.

Explanation:
Distance, displacement, and speed describe motion differently. Distance measures total path length traveled, always non-negative. Displacement measures NET change in position and can be zero if the particle returns to its starting point. Speed, being the magnitude of velocity, is always positive for motion.

For example, a person walking around a circular track and returning to the starting point covers a finite distance, yet their displacement is zero. This highlights that displacement can vanish even though motion occurs, while distance and speed remain non-zero.

Understanding these distinctions helps differentiate scalar and Vector measures of motion in a straight line or curved path.

Explanation:
Average velocity is the NET displacement divided by total time, while average speed is total distance divided by time. When motion occurs in a straight line without reversing direction, both are equal. However, if the motion involves curves or back-and-forth movement, displacement is less than total distance, reducing the ratio.

For instance, jogging around a circular track: total distance traveled is greater than the NET displacement from start to finish, making the ratio of average velocity to average speed less than one. This shows the difference between scalar and Vector measurements of motion.

Explanation:
Maximum height in projectile motion depends on the vertical component of initial velocity and gravitational acceleration. As the projectile rises, vertical velocity decreases until it becomes zero at the peak. Acceleration due to gravity remains constant downward, shaping the trajectory.

For example, throwing a ball upward faster results in a higher peak. The motion is controlled by how the upward velocity counteracts gravity, with the vertical speed becoming zero at the highest point while downward acceleration persists.

Explanation:
Objects under free fall experience uniform acceleration due to gravity. When one object starts with an initial downward speed, the separation between it and a free-falling object increases over time. Using kinematic relations such as s = ut + ½ a t² allows calculation of the growing distance without needing exact values here.

For instance, dropping two balls simultaneously with one having an initial velocity shows that the faster-moving object moves ahead, increasing the gap between them predictably with time.

Explanation:
When braking uniformly, each car decelerates at the same rate until stopping. The distance each car travels while stopping can be calculated using kinematic formulas like v² = u² + 2 a s. Subtracting the sum of their stopping distances from the initial separation gives the final gap.

An example is two cars slowing simultaneously: their initial speeds and the uniform deceleration determine how far apart they remain when motion ceases. This illustrates the connection between relative motion, braking, and stopping distances.

Explanation:
The coefficient of restitution (e) measures how elastic a collision is, defined as the ratio of relative speed after collision to before collision. In a perfectly elastic collision, no kinetic energy is lost, and bodies rebound completely. This coefficient helps quantify elasticity in collisions and determine energy transfer during impact.

For example, two ideal billiard balls colliding without deformation exhibit perfect elasticity. The value of e indicates the efficiency of kinetic energy conservation in the collision process.

Explanation:
Equilibrium occurs when the NET force and NET torque on an object are zero. For multiple forces acting on a point object, the Vector sum must close geometrically, forming a closed polygon. This ensures no unbalanced force acts, maintaining the object’s state of rest or uniform motion.

An analogy is connecting Vectors tip-to-tail in a loop; if the polygon closes, forces balance perfectly, producing equilibrium.

Explanation:
Rocket motion relies on Newton’s third law: every action has an equal and opposite reaction. As fuel is expelled backward, the rocket gains forward momentum. This is a direct consequence of the conservation of linear momentum, which ensures total momentum remains constant in a system when no external forces act.

For example, a toy rocket propelled by compressed air moves forward as air is pushed backward, illustrating momentum conservation in action.

Explanation:
Projectile motion combines initial velocity and gravitational acceleration. The vertical displacement after time t can be found using s = ut – ½ g t² for upward motion. The bullet rises initially, while gravity reduces its vertical speed each second.

For example, a ball thrown vertically upward slows due to gravity; its displacement after one second is slightly less than initial speed times time, reflecting the decelerating effect of gravity.

Explanation:
The dust separates due to inertia. Objects at rest resist changes to their state of motion. When the carpet moves suddenly, the dust tends to remain in its original position and falls off, illustrating Newton’s first law.

An analogy is shaking a tray of small balls; the tray moves, but the balls resist motion and may scatter due to inertia.

Explanation:
A coin in free fall inside a lift experiences acceleration due to gravity alone. Constant lift speed does not affect the coin’s acceleration because velocity is constant, not changing the gravitational force acting on the coin.

For example, dropping a coin in a smoothly moving elevator, whether ascending or descending at constant speed, shows it accelerates downward at g.

Explanation:
Tension in strings depends on the Mass distribution and acceleration. With frictionless pulleys and equal masses, the forces balance partially, but the acceleration of connected masses affects the tension between segments. Using Newton’s second law and F = ma for each Mass segment, the tension can be calculated.

For instance, identical weights on a pulley produce predictable tension differences, illustrating force transmission through strings.

Explanation:
A gun recoils due to conservation of momentum. Initially, the total momentum is zero. When the bullet moves forward rapidly, the gun must move backward so that the total momentum remains unchanged. This is a practical example of Newton’s third law: action on the bullet produces equal and opposite reaction on the gun.

For example, firing a toy gun or slingshot demonstrates backward motion as a reaction to forward-propelled object.

Explanation:
Stopping a moving object requires an unbalanced force opposite to its motion. Balanced forces maintain current velocity, while a single unbalanced force creates acceleration or deceleration. The NET force determines the rate and effectiveness of stopping.

For instance, pressing brakes on a car generates unbalanced frictional force to reduce speed and eventually stop the vehicle.

Explanation:
Inertia is a measure of an object’s resistance to changes in motion. It depends directly on Mass: heavier objects resist acceleration more than lighter ones. Size, shape, or material does not determine inertia.

For example, pushing a heavy boulder is harder than pushing a small stone, even if both are the same shape, because Mass determines resistance to motion.

Explanation:
When an insect collides with a moving car, Newton’s third law and momentum conservation govern the interaction. Both the insect and car experience equal and opposite forces, resulting in equal changes in momentum relative to their masses. The perceived effect differs for passengers due to frame of reference, but the physical forces remain symmetric.

For example, a fly hitting a moving bus experiences the same magnitude of force as the bus applies to it, even though the bus hardly slows.

Explanation:
Inertia is directly proportional to Mass. If two objects have masses in the ratio 1:4, their inertia follows the same ratio, as greater Mass resists motion changes more strongly. This explains why heavier objects are harder to accelerate.

For example, pushing a small and a large cart illustrates that inertia grows linearly with mass.

Explanation:
Momentum is mass × velocity. At the highest point, the vertical velocity of the ball becomes zero, so momentum in the vertical direction is zero. Acceleration due to gravity still acts downward, but the instantaneous momentum at that point vanishes.

An example is a basketball tossed upward: at the peak, the ball momentarily stops before descending, showing zero vertical momentum.

Explanation:
The stone follows Newton’s first law: it continues in a straight line tangential to the circle at the instant the string breaks. The circular path ceases because the centripetal force disappears, but motion along the tangential direction continues due to inertia.

For example, swinging a ball on a rope and releasing it shows it moves straight along the tangent of the circle.

Explanation:
Newton’s third law states that for every action, there is an equal and opposite reaction. These forces always act on different bodies, never on the same object. The action force on one body produces a reaction on the other, which ensures momentum is conserved.

For example, when you push against a wall, your hand experiences a force backward while the wall experiences a forward force from your push. Both forces are equal in magnitude but act on different bodies, demonstrating the law of action and reaction.

Explanation:
Momentum is given by p = m × v. At the same speed, the heavier object has greater momentum and requires more force to stop. The tennis ball has less mass and therefore less momentum, making it easier to decelerate and stop.

For instance, a tennis ball rolls on the ground and comes to rest quickly under friction, whereas a cricket ball at the same speed takes longer because it has higher momentum.

Explanation:
This demonstrates inertia of rest. Objects at rest resist changes to their state of motion. When the carpet moves suddenly, the dust tends to remain stationary and separates from the carpet.

An analogy is shaking a tray of small balls: the tray moves, but the balls resist motion and scatter, illustrating inertia of rest.

Explanation:
Luggage experiences inertia. Sudden acceleration, braking, or turns can cause it to slide or fall if not secured. Tying luggage prevents accidents by keeping it stationary relative to the bus during motion changes.

For example, books on a moving cart slide when the cart accelerates or stops. Tying them prevents motion caused by inertia.

Explanation:
External unbalanced forces like friction and air resistance oppose motion and gradually bring the ball to rest. Even though the ball has inertia, these forces reduce its velocity to zero over time.

For instance, sliding a puck on a table eventually stops because friction continuously reduces its speed until it halts.

Explanation:
This is Newton’s first law of motion. It explains that objects maintain their state of motion unless acted upon by unbalanced external forces. Inertia is the inherent tendency of objects to resist changes in motion.

For example, a puck sliding on ice continues moving straight unless friction or another force acts, illustrating the law of inertia.

Explanation:
If the NET external force is zero, the object experiences no acceleration. It either remains at rest or moves with constant velocity, meaning it is in equilibrium. Newton’s first law explains this situation mathematically and conceptually.

For example, a book lying on a table remains stationary because gravitational and normal forces are balanced.

Explanation:
Inertia depends on mass: the greater the mass, the more an object resists changes in motion. Other properties like size, volume, or surface area do not affect inertia. Mass is the quantitative measure of resistance to acceleration.

For example, pushing a heavy boulder is harder than pushing a small rock because the boulder’s higher mass gives it greater inertia.

Explanation:
Newton defined linear momentum as “mass in motion,” calculated by p = m × v. Momentum is a Vector quantity, representing both magnitude and direction. It explains how mass and velocity together determine resistance to motion changes and force interactions.

For example, a moving car has momentum proportional to its mass and speed, determining how difficult it is to stop or change its motion.

Explanation:
When the net external force is zero, an object experiences no acceleration. It can remain at rest or move at a constant velocity. Inertia ensures motion continues unchanged unless unbalanced forces act.

For instance, a puck sliding on a frictionless ice surface keeps moving uniformly because no external force changes its velocity.