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 rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

A man, sitting firmly over a rotating stool has his arms stretched. If he folds his arms, the work done by the man is :

  1. zero

  2. positive

  3. negative

  4. may be positive or negative
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When the arms are stretched the rotational inertia is higher. When the man folds his arm he is basically decreasing his rotational inertia thereby gaining more angular velocity using the principle of conservation of angular momentum. This implies more angular distance is covered in the same time giving us positive work.

Multiple choice physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

A force $\vec F = \alpha \hat i + 3\hat j + 6\hat k$ is acting at a point $\vec r = 2\hat i - 6\hat j - 12\hat k$. The value of $\alpha$ for which angular momentum about the origin is conserved is:-

  1. $1$
  2. $-1$
  3. $2$

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

Given,
$\vec r=2\hat i-6\hat j-12\hat k$
$\vec F=\alpha\hat i +3\hat j+6\hat k$
For the conservation of angular momentum about origin, the torgue acting on the particle will be zero.
$\tau=\vec r\times \vec F$
$\vec r\times \vec F=0$
$(\alpha \hat i+3\hat j+6\hat k)\times (2\hat i-6\hat j-12\hat k)=0\hat i+0\hat j+0\hat k$
$\hat i(-36+36)-\hat j (12+12\alpha)+\hat k(6+6\hat k)=0\hat i+0\hat j+\hat k$
By equating it's coefficient, we get
$12+12\alpha =0$,   $6+6\alpha =0$
$12=-12\alpha$
$\alpha =-1$
The correct option is B.

Multiple choice physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

What is the magnitude of vertical force required to produced a moment of 20 Nm at point A (1 m, 1 m) if the force is acting at point B (2m,2m)

  1. 40 N

  2. 30 N

  3. 20 N

  4. 10 N
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Moment = F * d_perpendicular. The force is vertical, so the perpendicular distance is the difference in x-coordinates: |2 - 1| = 1 m. 20 Nm = F * 1 m, so F = 20 N.

Multiple choice physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

A man standing on a platform holds weight in his outstreached arms. The system rotates freely about a central vertical axis. If he now draws the weights inward close to his body 

  1. The angular velocity of the system will increase

  2. The angular momentum of the system will increase

  3. The kinetic energy of the system will increase

  4. He will have to expend some energy to draw the weights in
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
$\tau=0=\cfrac{dl}{dt}$
$\Rightarrow L=constant$, Angular momentum remains constant.
By drawing arms in, $I$ is decreased.
$L=I\omega$
For $L$ to remain constant, $\omega$ increases.
$KE=\cfrac{L^{2}}{2I}$, $I$ decreases.
$\Rightarrow KE \,\,increases$
The energy spent in drawing weight is converted into KE of rotation

Multiple choice physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

A force $\vec { F } =\alpha \hat { i } +3\hat { j } +6\hat { k }$ is acting at a point $\vec { r } =2\hat { i } -6\hat { j } -12\hat { k }$. The value of $\alpha$ for which angular momentum about origine is conserved is

  1. $1$
  2. $-1$
  3. $2$

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

For  conservation of angular momentum about origin, external torque acting about origin should be zero.

$\therefore$ Torque $=r \times F$
$\Rightarrow (2\hat i-6\hat j- 12\hat k ) \times (\alpha \hat i + 3 \hat j + 6 \hat k)= (-36 +36) \hat i + (-12 \alpha -12 ) \hat j + ( 6+6\alpha ) \hat k=0$   $\Rightarrow \alpha =-1$

Multiple choice physics fluid pressure pressure in air introduction to atmospheric pressure pressure exerted by air devices to measure pressure

If one newton force is applied to unit crossesctional area (in $m^2$)  of any subject then the amount of pressure applied is

  1. one pascal

  2. one atmospheric pressure

  3. one gauge pressure

  4. none of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pressure is defined as force per unit area applied on a surface.

$P=\dfrac{F}{A}$ 
In SI units, pressure has unit $N/m^2$ which is equivalent to $Pa$(Pascal).

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

When the height of the inclined plane decreases :

  1. the mechanical advantage decreases

  2. the mechanical advantage increases

  3. effort increases

  4. it will not have any effect on the mechanical advantage
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The mechanical advantage of an inclined plane is given by length/height. As the height decreases, the ratio length/height increases, thus increasing the mechanical advantage.

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

It is difficult to climb the steps having greater inclination. Why?

  1. Less erect steps require less energy

  2. Greater inclination requires less effort because of the high mechanical advantage

  3. Greater inclination requires more effort because of the low mechanical advantage

  4. None of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A steeper incline (greater inclination) effectively acts as a shorter inclined plane for a given horizontal distance, which reduces the mechanical advantage. Consequently, more effort is required to climb it.

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

A pulley system of VR of 2 and efficiency of 50%. If the effort moves through a distance of 5m. Calculate the distance moved by the load.

  1. 2m

  2. 5.6m

  3. 10m

  4. 2.5m
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Given:  $VR=2$


From the relation $VR=\dfrac{\text{Distance moved by effort}}{\text{distance moved by load}}$


Using formula we get distance moved by the load is $2.5m$

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

Akhil has to lift a load of $800N$ onto a platform of $2m$ height. Instead, he pushes the load up a ramp $4m$ long. Find the force required to lift the load vertically up.

  1. $200N$
  2. $600N$
  3. $400N$
  4. $800N$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The vertical force is the gravitational force means weight of the load. 

According to question, the weight of load is $mg=800 N$, this will be the required force to lift the load vertically.  

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

Which of the following statements is NOT correct?

  1. In a first order lever, the fulcrum is placed between the effort and the load
  2. The mechanical advantage of a second class levers is greater than $1$ as load is situated in between to effort and fulcrum.

  3. An inclined plane in a hospital, helps pushing a wheel chair easily.

  4. A crow bar is an example of second class lever
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A crowbar is a classic example of a first-class lever, where the fulcrum is between the effort and the load. Therefore, stating it is a second-class lever is incorrect.

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

In a lifting machine, an effort of $500$ N is to be moved by a distance of $20$ m to raise a load of $10000$ N by a distance of $0.8$ m. Determine the velocity ratio and mechanical advantage?

  1. 10 and 35

  2. 20 and 35

  3. 10 and 25

  4. 25 and 20
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Distance moved by effort is 20m,that of load is 0.8m

$VR=\dfrac{{E}{} _{d}}{{L}{} _{d}}$

$VR=\dfrac{20}{0.8}$

$VR=25$

the load is 10,000N and effort=500N

$MA=\dfrac{{load}{} _{d}}{{effort}{} _{d}}$

$MA=\dfrac{10000}{500}$

$MA=20$