Work, Energy, and Simple Machines MCQs for UPSC Prelims
130 practice questions covering Work, Energy, and Simple Machines, organised into 1 topics.
130 questions include a written explanation.
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A body of mass $2\ \text{kg}$ starts from rest on a smooth level floor, and a net force does $16\ \text{J}$ of work on it. Which equation should be solved to find the body's final speed $v$?
A$2 \times v^{2} = 16$
B$\frac{1}{2}\times 2 \times v^{2} = 16$
C$2 \times v = 16$
D$\frac{1}{2}\times 2 \times v = 16$
Show answer and explanation
Correct answer: B - $\frac{1}{2}\times 2 \times v^{2} = 16$
Starting from rest, the net work equals the final kinetic energy, and kinetic energy is $\frac{1}{2}mv^{2}$. Leaving out the factor $\frac{1}{2}$ overstates the kinetic energy. Using $v$ in place of $v^{2}$ drops the square that the kinetic energy formula demands, and doing both at once is wrong twice over.
Q2
medium
Assertion (A): When a car on a straight level road speeds up, the work done by the net force on the car is positive. Reason (R): The work done by the net force on a body equals the final kinetic energy of that body.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: C - A is true, but R is false
(A) is true, because the kinetic energy rises as the car speeds up and the net work equals that rise. (R) is false: the net work equals the change in kinetic energy, not the final value, and the two agree only when the body starts from rest.
Q3
medium
A lift raises a load of mass $400\ \text{kg}$ through a height of $12\ \text{m}$. Taking $g = 10\ \text{m/s}^{2}$, how much gravitational potential energy does the load gain?
A$48000\ \text{J}$
B$480000\ \text{J}$
C$4000\ \text{J}$
D$24000\ \text{J}$
Show answer and explanation
Correct answer: A - $48000\ \text{J}$
The gain is $mgh = 400 \times 10 \times 12 = 48000\ \text{J}$. Leaving out the height gives $4000\ \text{J}$. Bringing in a factor of $\frac{1}{2}$ from the kinetic energy formula gives $24000\ \text{J}$. Writing one zero too many gives $480000\ \text{J}$.
Q4
medium
Assertion (A): A man who pushes hard on a stationary wall does a large amount of work on the wall. Reason (R): The work done by a force is zero when the object on which it acts has no displacement.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: D - A is false, but R is true
(A) is false: however hard he pushes, the wall does not move, so the work done on it is zero. (R) is a correct statement of when work vanishes, and it is exactly the rule that shows (A) to be wrong. So a true reason sits beside a false assertion.
Q5
easy
The mechanical energy of an object is defined as which of these?
AIts kinetic energy on its own
BThe difference between its kinetic energy and its potential energy
CIts potential energy on its own
DThe sum of its kinetic energy and its potential energy
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Correct answer: D - The sum of its kinetic energy and its potential energy
Mechanical energy is the total of the energy due to motion and the energy due to position, so the two are added. Subtracting them would let a raised, still object come out with a negative total, which is not what mechanical energy means. Taking only one of the two leaves out the other whenever both are present.
Q6
easy
A pendulum bob is pulled to one side and released, and friction is ignored. At the lowest point of its swing, which statement is correct?
AIts kinetic energy is greatest and its potential energy is least
BBoth its kinetic energy and its potential energy are greatest
CBoth its kinetic energy and its potential energy are zero
DIts potential energy is greatest and its kinetic energy is least
Show answer and explanation
Correct answer: A - Its kinetic energy is greatest and its potential energy is least
The bob is lowest at that point, so its potential energy is at its smallest, and since the total stays fixed the kinetic energy must be at its largest, which is why the bob is moving fastest there. The reverse pattern belongs to the ends of the swing. The two cannot both be greatest, or both zero, while the total is fixed and the bob is moving.
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