Physics

Forces, Energy and Machines

281 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 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 maths congruence introduction to shapes similarity of triangles introduction to similar triangles

A man of height 1.8 metre is moving away from a lamp post at the  rate of 1.2 m/sec . If the height of the lamp post be 4.5 metre , then the rate at which the shadow of the  man is lengthening is 

  1. $0.4 m/sec$
  2. $0.8m/sec$
  3. $1.2m/sec$

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let x be the distance of the man from the post and y be the length of the shadow. By similar triangles, y/1.8 = (x+y)/4.5. Simplifying gives 4.5y = 1.8x + 1.8y, so 2.7y = 1.8x, or y = (2/3)x. The rate of change dy/dt = (2/3) * dx/dt = (2/3) * 1.2 = 0.8 m/sec.

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.

Multiple choice physics static electricity properties of charges charge properties of charge

Two charges of equal magnitude and at a distance'r' exert a force $F$ on each other. If the charges are halved and distance between them is doubled, then the new force acting on each charge is

  1. $F / 8$
  2. $F / 4$
  3. $4 F$
  4. $F / 16$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Original force$,$

$F = k\dfrac{{q\,q}}{{{r^2}}}$
New force$,$
$F = k\dfrac{{\dfrac{q}{2}\,\dfrac{q}{2}}}{{{{\left( {2r} \right)}^2}}} = \dfrac{1}{{16}}F = \dfrac{F}{{16}}$
Hence,
option $(D)$ is correct answer. 

Multiple choice physics behaviour of perfect gas and kinetic theory of gases degree of freedom: law of equipartition of energy law of equipartition of energy law of equipartition of energy and mean free path

An ant is moving on a plane horizontal surface. The number of degrees of freedom of the ant will be

  1. 1

  2. 2

  3. 3

  4. 6
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Degrees of freedom represent the number of independent coordinates required to specify the position of an object. An ant on a 2D plane requires two coordinates (x, y).

Multiple choice physics behaviour of perfect gas and kinetic theory of gases degree of freedom: law of equipartition of energy law of equipartition of energy law of equipartition of energy and mean free path

A man is climbing up a spiral type staircase. His degrees of freedom are :

  1. 1

  2. 2

  3. 3

  4. more than 3
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

There will be three degrees of freedom. Two are along x-direction and y-directions due to translation and the last degree of freedom due to angular rotation as the  man climbs up.

Hence, Option C is correct.

Multiple choice physics behaviour of perfect gas and kinetic theory of gases degree of freedom: law of equipartition of energy law of equipartition of energy law of equipartition of energy and mean free path

An ant is walking on the horizontal surface. The number of degree of freedom of ant will be

  1. $1$
  2. $2$
  3. $3$
  4. $6$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

An ant walking on a horizontal surface is constrained to move in a 2D plane (x and y coordinates). However, if the ant is considered a rigid body capable of rotation about its center of mass, it possesses 3 degrees of freedom (2 translational and 1 rotational).