Physics

Forces, Energy and Machines

267 Questions

Forces, energy, and machines are fundamental physics concepts focusing on mechanics, work, power, and simple machines. These topics regularly appear in general science sections of competitive exams. Use these questions to practice calculating resultant forces, mechanical advantage, and work done in various scenarios.

Resultant force calculationsMechanical advantage of leversWork and power equationsApparent weight in elevatorsBending moments

Forces, Energy and Machines Questions

Multiple choice

Complete the formula: Pressure = Force divided by

  1. Area

  2. Volume

  3. Length

  4. Density
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pressure is defined as the force applied perpendicular to the surface of an object per unit area. Therefore, the formula is Pressure = Force / Area.

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A child stands on a bathroom scale while riding in an elevator. The child's weight when the elevator is not moving is 100 Ibs. What does the scale read when the elevator accelerates upward while travelling downward?

  1. greater than 100 Ibs

  2. less than 100 Ibs

  3. exactly 100 Ibs

  4. The same as it reads when accelerating downward while travelling upward, by symmetry.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given,

When lift is not moving, gravity force act downward, weighing machine show $\vec{F}=m\vec{g}=100\,lbs\,$force.

When lift start moving upward with acceleration$\vec{a}$, Normal reaction force on child increase by $m\vec{a}$

Therefore, net weight on weighing machine become, $\vec{F}=m\vec{g}+m\vec{a}$

Hence, scale will show weigh greater than $100\,lbs$force.

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A body of mass $32 \,kg$ is suspended by a spring balance from the roof of a vertically operating lift and going downward from rest. At the instants the lift has covered $20 \,m$ and $50 \,m$, the spring balance showed $30 \,kg$ and $36 \,kg$ respectively. Then the velocity of the lift is:

  1. decreasing at $20 \,m$ and increasing at $50 \,m$
  2. increasing at $20 \,m$ and decreasing at $50 \,m$

  3. continuously decreasing at a steady rate throughout the journey

  4. constantly increasing at constant rate throughout the journey.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

According to Fbd

We know that,

$N=m\left( g-a \right)$

So, if $N<mg$

It is a case of downward acceleration

Then, the velocity is increase

Again, if $N>mg$

It is a case of downward retardation

Then, the velocity is decreases

Hence, the velocity is increasing at $20$ m and decreasing at $50\ m$

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A boy whose mass is $50$ kg stands on spring balance inside a lift. The lift starts to ascend with an acceleration of $2{ m/s }^{ -2 }$. The reading of the machine or balance (g = 10${ m/s }^{ -2 }$) is

  1. 50 kg

  2. zero

  3. 49 kg

  4. 60 kg
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The apparent weight in an accelerating lift is W = m(g + a). Here, W = 50 * (10 + 2) = 50 * 12 = 600 N. Converting back to kg-force, the reading is 60 kg.

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A man is standing on a weighting machine with a block in his hand. The machine records $W$. When he takes the block upwards with some acceleration the machine records $W _1$. When he takes the block down with some acceleration, the machine records $W _2$. Then choose correct option 

  1. $W _1 = W = W _2$
  2. $W _1 < W < W _2$
  3. $W _2 < W < W _1$
  4. $W _2 = W _1 > W$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the block is accelerated upward, the man exerts more force on the machine (W1 > W). When accelerated downward, the man exerts less force on the machine (W2 < W). Thus, W2 < W < W1.

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

Find the reading of the spring  balance, when the elevator  is going  up with an acceleration of g /10 the pulley is smooth 

  1. 3.1 kg

  2. 4.4 kg

  3. 1.8 kg

  4. 6.2 kg
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

This involves a pulley system in an accelerating elevator. The effective acceleration is g + a. The tension in the string for a pulley system with masses m1 and m2 is T = 2 * m1 * m2 * (g+a) / (m1 + m2).

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A person of mass 50 kg stands on a weighing scale on a lift. If the lift is descending with a downwards acceleration of $9 ms^{-2}$, what would be the reading of the weighing scale ? $(g = 10 ms^-2)$

  1. 5 kg

  2. 10 kg

  3. 15 kg

  4. 20 kg
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The apparent weight is W = m(g - a). Here, W = 50 * (10 - 9) = 50 * 1 = 50 N. In terms of mass reading, this is 5 kg.

Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

An object is suspended from a spring balance in a lift. The reading is $240 N$ when the lift is at rest. If the spring balance reading now changes to $220 N$, the the lift is moving 

  1. Downward with constant speed

  2. Downward with decreasing speed

  3. Downward with increasing speed

  4. Upward with increasing speed
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Since the apparent weight of the body decreases, the lift should move downward with some acceleration.
Multiple choice physics measurements and units measuring mass measurement of mass measuring instruments

A monkey is sitting in the pan of a spring balance kept in an elevator. The reading of the spring balance is maximum when the elevator.

  1. Is in stationary

  2. Accelerate upwards

  3. Falls freely

  4. Accelerate Downwards
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The reading of a spring balance represents the normal force exerted on the object. When an elevator accelerates upwards, the apparent weight increases according to the formula W = m(g + a), making the reading maximum.

Multiple choice physics measurements and experimentation vernier calliper and screw gauge least count of vernier calliper and screw gauge measurement of length

A force $\vec { F } $ is applied on a square plate of length $L$. If the percentage error in the determination of $L$ is $3$% and in $F$ is $4$%, the permissible error in the calculation of pressure is

  1. $13$%
  2. $10$%
  3. $7$%
  4. $12$%
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Pressure  $P=\cfrac { F }{ A } =\cfrac { F }{ { L }^{ 2 } } $
Percentage error in pressure  $\cfrac { \Delta P }{ P } \times 100=\cfrac { \Delta F }{ F } \times 100+2\cfrac { \Delta L }{ L } \times 100$
$ = 4+2\times 3 = 10$  %

Multiple choice physics units and measurement: error analysis rounding off digits rounding of digits standard form

The resultant of two equal forces acting at right angles to each other is $1414$ dyne. Find the magnitude of either force.

  1. $960$
  2. $1000$
  3. $1200$
  4. $1414$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given,

$F _1=F _2=F(say)$
$\theta=90^0$
$R=1414dyne$
Resultant force,
$R=\sqrt{F _1^2+F _2^2+2F _1F _2cos\theta}$
$1414dyne=\sqrt{F^2+F^2+2F^2cos90^0}=\sqrt{2F^2}$
$1414dyne=\sqrt{2}F$
$F=\dfrac{1414}{\sqrt{2}}=999.84 \approx 1000dyne$ (by rounding off)
The correct option is B.

Multiple choice physics units and measurement: error analysis rounding off digits rounding of digits standard form

Two constant force $ \overrightarrow F _1 and \overrightarrow F _2 $ acts on a body.these forces displaces the body from point P(1, -2, 3) to Q (2, 3, 7 ) in 2s starting from rest.force $\overrightarrow F _1 $ is of magnitude 9 N and acting along vector $ ( 2 \hat i - 2 \hat j + \hat k ) $ . the positions are in meter. find work done by $ \overrightarrow F _1 $.

  1. $-12J$
  2. $+12J$
  3. $36J$
  4. $-36J$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Work done is the dot product of force and displacement. Displacement vector d = Q - P = (2-1)i + (3-(-2))j + (7-3)k = 1i + 5j + 4k. Force F1 = 9 * (2i - 2j + 1k) / sqrt(2^2 + (-2)^2 + 1^2) = 9 * (2i - 2j + 1k) / 3 = 3 * (2i - 2j + 1k) = 6i - 6j + 3k. Work = (6i - 6j + 3k) dot (1i + 5j + 4k) = 6 - 30 + 12 = -12J.