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 force concept of force and its unit force - push or pull forces

In a game of tug of war, three girls of team $A$ pull the rope with forces of $100 N$, $120 N$ and $170 N$. In team $B$, the three members pull the rope with forces of $130 N$, $150 N$ and $155 N$. Who will win the tug of war? What is the resultant force?

  1. Team $A$, $55 N$
  2. Team $B$, $55 N$
  3. Team $A$, $45 N$
  4. Team $B$, $45 N$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Resultant force is nothing but the total force acting on a body including their directions. If the object is at rest or it moves with a same velocity, the resultant force should be zero. All forces are acting on same direction, resultant force should be the sum of all the forces.

In this case, the forces applied by team A are 100 N, 120 N and 170 N. The total force is 100 N+120 N+170 N = 390 N. The forces applied by tema B are 130 N, 150 N and 155 N. The total force is 130 N+150 N+155 N=435 N.
Now, both the total forces are in the opposite direction as the team are in the opposite direction. So, the resultant force is 435 - 390 = 45 N. 
Therefore, Team B will win in the game as they apply more force of 45 N.

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

A force F is given by $F=at+bt^2$, where t is time. The dimensions of a and b are:

  1. $[MLT^{-3}]$and$ [MLT^{-4}]$
  2. $[MLT^{-4}]$and$ [MLT^{-3}]$
  3. $[MLT^{-1}]$and$ [MLT^{-2}]$
  4. $[MLT^{-2}]$and$ [MLT^{0}]$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given that $F=at+bt^2$ where $t$ is time.


For this equation to be dimensionally correct, the each term in the right hand side should have the dimensions of $F$

Dimensional formula of $F=[ MLT^{-2}]$

$\implies aT=[ MLT^{-2}]$
$\implies$ Dimension of $a$ is $[ MLT^{-3}]$

and
$\implies bT^2=[ MLT^{-2}]$
$\implies$ Dimension of $b$ is $[ MLT^{-4}]$

Hence, option $(A)$ is correct.

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

What happens when two forces are applied in the opposite direction on an object?

  1. Net force is the addition of forces.

  2. Net force is the subtraction of forces.

  3. Net force is the division of forces.

  4. Net force is the multiplication of forces.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When two forces are applied in the opposition direction on an object, the net force will be the difference between the two forces.

Multiple choice maths concept of directed numbers and number line subtraction of directed numbers subtraction of integers subtraction of integers on number line

A driver is $20$ m below sea level. If he goes further down by $10$m, then find his new position.

  1. $10$m
  2. $-10$m
  3. $30$m
  4. $-30$m
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Sea Level $\rightarrow$ $0$m

Original Position $\rightarrow$ $-20$m (Since he is below sea level)
New Position $\rightarrow$ $-$ ($20+10$) $\rightarrow$ $-30$m


Multiple choice physics motion and measurement physical quantities units - definitions and systems physical quantities like mass and weight

Read the following statements carefully and pick out the correct choice of answer.
A: Electromotive force is expressed in Newtons.
B: Electric intensity is expressed in ${ VC }^{ -1 }$.

  1. Both A and B are correct.

  2. Both A and B are wrong.

  3. A is correct but B is wrong.

  4. A is wrong but B is correct.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Electromotive force is the voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined as the electrical potential for a source in a circuit.

It is not a force but voltage and its unit is volt.

Electric intensity at a point is defined as the force experienced per unit positive charge at a point placed in the electric field. 
Its unit is $Newton/Coloumb=NC^{-1}$

Both statements are wrong.

Multiple choice physics upthrust in fluids, archimedes' principle and floatation applications of floatation principle of floatation and its applications when do objects float on water?

A person of mass $m$ is standing on one end of a plank of mass $M$ and length $L$ and floating in water. The person moves from one end to another and stops. The displacement of the plank is :

  1. $\dfrac {Lm}{(m+M)}$
  2. $Lm(M+m)$
  3. $\dfrac {(m+M)}{Lm}$
  4. $\dfrac {LM}{(m+M)}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Since there is no external horizontal force, the center of mass of the system remains stationary. m*x + M*X = 0, where x and X are displacements. The person moves distance L relative to the plank, so x_person = L - X_plank. Solving m(L - X) + MX = 0 gives X = -mL / (m+M). The magnitude is mL / (m+M).

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.