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 physics turning effects of forces centre of gravity forces - vectors and moments acceleration due to gravity

The apparent weight of a person of mass m in an elevator is 2mg. The elevator is moving

  1. up with an acceleration of $\frac{g}{2}$

  2. up with an acceleration of g

  3. up with an acceleration of 2g

  4. down with an acceleration of g
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The apparent weight is given by N = m(g + a) for upward acceleration. If N = 2mg, then m(g + a) = 2mg, which simplifies to g + a = 2g, so a = g. The elevator is moving upward with an acceleration of g.

Multiple choice physics oscillations a few applications of linear shm simple pendulum example of simple harmonic motion

A person weighing $60\ kg$ stands on a platform which oscillates up and down at a frequency of $2\ Hz$ and amplitude $5\ cm$. The maximum and minimum apparent weights are nearly: ($g$ = 10$\ m/s^2$)

  1. $108$ kg-wt, $12$ kg-wt
  2. $108$ kg-wt, $24$ kg-wt
  3. $54$ kg-wt, $12$ kg-wt
  4. $54$ kg-wt, $24$ kg-wt
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$a=\omega^{2}x$
So, $a _{max}=$ $\omega^{2}A$
We know that $\omega=2\pi  f$
So, $a _{max}=\dfrac{(2\pi\times 2)^{2}\times 5}{100}$
Case I:
$N-mg=ma _{max}$
$N=m(a _{max}+g)$
$=60(10+\dfrac{16\times \pi^{2}\times 5}{100})$
$=1080 $ 

$ N=108$ kg-wt

Case II:
$mg-N=ma _{max}$
or, $N=mg-ma _{max}$
$=60(10-8)$
$=120\ N=12$ kg-wt

Multiple choice physics simple harmonic motion a few applications of linear shm simple pendulum example of simple harmonic motion

A person normally weighing 60kg stands on a platform which oscillates up and down simple harmonically with a frequency $2Hz$ and an amplitude $5cm$.if a machine on the platform gives the person's weight,then consider the following statements :

  1. The maximum reading of machine will be $108$kg
  2. The maximum reading of machine will be $90kg$
  3. The minimum reading of machine will be $12kg$

  4. The minimum reading of the machine will be zero correct statements are:
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

Maximum a=$w^2A$
$=(4\pi)^2\times 0.05$
$=16 \pi^2\times 0.05$
$=8$
$mg=60 \Rightarrow m=\dfrac{60}{10}=6$
$W _{max}=m(g+a)$
$=6\times (18)$
$=108kg$
$W _{min}=6(g-a)
$=12kg

Multiple choice physics types of energy renewable and non-renewable resources renewable and non-renewable sources of energy substances, objects and energy

A body of mass $15\ kg$ is raised from certain depth. If the work done in raising it by $10m$ is $1620J$, its velocity at this position is$(g=10 { ms }^{ -2 })$:

  1. ${ 2\ ms }^{ -1 }$
  2. ${ 4\ ms }^{ -1 }$
  3. ${ 1\ ms }^{ -1 }$
  4. ${ 8\ ms }^{ -1 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

work done=PE+KE

$1620=mgh+\dfrac{mv^2}{2}$
$1620=15\times 10\times 10+\dfrac{15v^2}{2}=1500+\dfrac{15v^2}{2}$
$v=4m/s$

Multiple choice luminous intensity measurements physics

Light with energy flux 36$\mathrm { w } / \mathrm { cm } ^ { 2 }$ is incident on a well polished metal slate of side 2$\mathrm { cm }$ . Theforce experienced by it is

  1. 0.96$\mu N$
  2. 0.24$\mu N$
  3. 0.12$\mu N$
  4. 0.36$\mu N$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The force exerted by light on a surface is F = P/c, where P is the power (energy flux * area). For a perfectly reflecting surface, F = 2IA/c. For an absorbing surface, F = IA/c. Assuming absorption, F = (36 W/cm^2 * 4 cm^2) / (3 * 10^8 m/s) = 144 / 3 * 10^-8 N = 4.8 * 10^-7 N = 0.48 uN. If reflected, it is 0.96 uN. Given the answer 0.96 uN, the metal plate is considered a perfect reflector.

Multiple choice physics energy transformations and energy transfers forms of energy and energy conservation energy for everything forms of energy

 person trying to lose weight by burning fat lifts a mass of $10kg$ upto a height of $0.5\, m\, with\, in\, 1000$ times. Assume that the potential energy lost each time he lowers the mass is dissipated. How much fat will he use up considering the work done only when the weight is lifted up? Fat supplies $3.8 \times 10^7 J$ of energy per kg which is converted to mechanical energy with a $20 \%$ efficiency rate. 

  1. $6.45 \times {10^{ - 3}}kg$
  2. $9.89 \times {10^{ - 3}}kg$
  3. $12.89 \times {10^{ - 3}}kg$
  4. $2.45 \times {10^{ - 3}}kg$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Work done per lift = mgh = 10 * 9.8 * 0.5 = 49 J. Total work for 1000 lifts = 49,000 J. With 20% efficiency, energy required from fat = 49,000 / 0.20 = 245,000 J. Fat energy density = 3.8 * 10^7 J/kg. Mass of fat = 245,000 / (3.8 * 10^7) = 6.447 * 10^-3 kg.

Multiple choice physics along with motion what forces can do? force and it's unit force and its effects

Study the impact force when a golfer drives a golf ball $230$ meters down the fairway.

  1. The impact force on the golf ball is greatest

  2. The impact force on the club head is greatest

  3. The impact force is the same for both

  4. The impact force has no effect on the club

  5. The impact force has no effect on the ball
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The impact force or impulse is given by $I=F\times t$  

 by Newton's third law both will exert an equal and opposite force on each other and time of impact is also same therefore  impact force (impulse) is same for both.

Multiple choice physics along with motion what forces can do? force and it's unit force and its effects

 A force of $15 N$ increases the length ofa by $1 \mathrm { mm }$. The additional force require increase the length by $2.5 \mathrm { mm }$ in N is 

  1. $2.25$
  2. $22.5$
  3. $37.5$
  4. $3.75$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

We know,

$Y=\dfrac{FL}{A\Delta L}$

For a wire, A,L,Y are constant.

$\dfrac{F}{\Delta L}=$constant

Let extra Force needed be $ F$,

$\dfrac{15}{1}=\dfrac{F+15}{2.5}$

$F+15=2.5\times 15$

$F=1.5\times 15=22.5$

Option $\textbf B$ is the correct answer
Multiple choice physics along with motion what forces can do? force and it's unit force and its effects

A lift coming down is just about to reach the ground floor. Taking the ground floor as origin and positive direction upwards for all quantities, which of the following is correct ($x$=displacement, $v$= velocity, $a$=acceleration):

  1. $x < o, v<0, a>0$
  2. $x > 0, v < 0, a < 0$
  3. $x > 0, v < 0, a> 0$
  4. $x > 0, v >0,a >0$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If the lift is below the origin (ground floor), x < 0. If it is moving down, v < 0. If it is slowing down to reach the ground, it must have an upward acceleration, so a > 0.

Multiple choice physics along with motion what forces can do? force and it's unit force and its effects

Eqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force ?

  1. Eqbal

  2. Seema

  3. Both apply same force

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

Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surface of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus to push the heavier box, Seema has to apply a greater force than Eqbal.

Multiple choice physics forces - vectors and moments concept of force and its unit force - push or pull forces

Which of the following pairs of forces cannot be added to give a resultant force of $4N$?

  1. $2 N$ and $8 N$
  2. $2 N$ and $2 N$
  3. $2 N$ and $6 N$
  4. $2 N$ and $4 N$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Resultant force, 

$F=\sqrt{F _1^2+F _2^2+2F _1F _2cos\theta}$. . . . . . . . .(1)
Lets consider the option A,

$F _1=2N$
$F _2=8N$

For maximum resultant force, $cos\theta=1$
From equation (1),
$F _{max}=\sqrt{F _1^2+F _2^2+2F _1F _2}=\sqrt{(F _1+F _2)^2}$
$F _{max}=F _1+F _2$
$F _{max}=8+2=10N$
For minimum resultant force, $cos\theta=-1$
$F _{min}=\sqrt{F _1^2+F _2^2-2F1F _2}=\sqrt{(F _1-F _2)^2}$
$F _{min}=\sqrt{(2-8)^2}=6N$
So, the $4N$ does not lie within this range. Thus it is not possible to have it as resultant force.
The correct option is A.

Multiple choice physics forces - vectors and moments concept of force and its unit force - push or pull forces

Two forces 12 N and 5 N are acting perpendicular to each other. Then the net force acting is.

  1. 17 N

  2. 18 N

  3. 7 N

  4. zero

  5. 13 N
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

$ \vec F _{net} = \vec F _1 +\vec F _2$

$| \vec F _{net}| = \sqrt{F _1^2 +F _2^2 +2F _1F _2\cos\theta}$
$| \vec F _{net}| = \sqrt{12^2 +5^2 +2\times 12 \times 5\cos 90^0}$
$| \vec F _{net}| = \sqrt{12^2 +5^2 }$
$| \vec F _{net}| = 13N$
Therefore, E is correct option.

Multiple choice physics forces - vectors and moments concept of force and its unit force - push or pull forces

Two forces of $12 \mathrm { N } \text { and } 8 \mathrm { N }$ acts upon a body. The resultant force on the body has maximum value of

  1. $4 \mathrm { N }$
  2. $0 \mathrm { N }$
  3. $20 \mathrm { N }$
  4. $8 \mathrm { N }$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given,

$|\vec F _1|=12N$
$|\vec F _2|=8N$

The resultant force on the body is 
$|\vec R|=\sqrt{F _1^2+F _2^2+2F _1F _2cos\theta}$

For maximum value, $cos\theta=1$
$|\vec R|=\sqrt{(12)^2+(8)^2+2\times 12\times 8\times 1}$

$|\vec R _{max}|=\sqrt{144+64+24\times 8}$

$|\vec R _{max}|=\sqrt{400}=20N$
The correct option is C.

Multiple choice physics force concept of force and its unit force - push or pull forces

Which of the following pairs of forces will never give resultant force of $2N$

  1. $2N$ and $2N$
  2. $1N$ and $1N$
  3. $1N$ and $3N$

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Because if these two forces Act then it cancelled out because they have the same value. Either it acts in same or opposite direction