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 power work and power work, energy and power physics energy and its forms

The average power required to lift a $100kg$ mass through a height of $50$ metres in approximately $50$ seconds would be ( in J/s)

  1. $50$
  2. $5000$
  3. $100$
  4. $980$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$Power= Work/Time= F.s/t=mgs/t=(100\times 9.8\times 50)/50= 980W $


Hence correct answer is option $D $ 

Multiple choice power work and power work, energy and power physics energy and its forms

A force $\vec {F}=(3\hat {i}+4\hat {j})N$ acts on $2kg$ movable object that moves from an initial position $\vec {r} _{1}=(-3\hat {i}-2\hat {j})m$ to a final position $\vec {r} _{1}=(5\hat {i}+4\hat {j})m$ in $6s$. The average power delivered by the force during the interval of $6s$ is equal to :

  1. $8\ watt$
  2. $\dfrac{50}{6}\ watt$
  3. $15\ watt$
  4. $\dfrac{50}{3}\ watt$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

 

Given,

Force, $\vec {F}=(3\hat {i}+4\hat {j})N$

Displacement, $\vec{d}={{\vec{r}} _{2}}-{{\vec{r}} _{1}}=(5\hat{i}+4\hat{j})-(-3\hat{i}-2\hat{j})=\left( 8\hat{i}+6\hat{j} \right)\,m$

Work, $W=\vec{F}.\vec{d}=\left( 3\hat{i}+4\hat{j} \right)\left( 8\hat{i}+6\hat{j} \right)=48\,J$

Average power $P=\dfrac{W}{t}=\dfrac{48}{6}=8\,W$

Average power is $8\,W$ 

Multiple choice power work and power work, energy and power physics energy and its forms

A force F acting on a body depends on its displacement $S$ as $F \propto S^{1/3}$. The power delivered by $F$ will depend on displacement as:

  1. $S^{2/3}$
  2. $S^{-5/3}$
  3. $S^{1/2}$
  4. $S^0$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We know Power=$\dfrac{work \ done}{time}$

Also work done = Force $\times$displacement

As given $F \ \alpha \ {S}^{1/3}$

$F=KS^{ 1/3 }$------(1) (where $K$= constant of proportionality)

Now Power= $\dfrac{F \times S}{time}$----(2)
Putting value of 1 in 2 we get 

$P= {\dfrac{KS^{\dfrac{2}{3}}}{t}}$

Multiple choice power work and power work, energy and power physics energy and its forms

A $0.25 kg$ book falls from a height of $1.00 m$, initially at rest. What is the AVERAGE power output of gravity on the book in this time period?

  1. $2.45 W$
  2. $5.43 W$
  3. $10.9 W$
  4. $24.1 W$

  5. None of the above
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given :   $m = 0.25$ kg               $S =1.00$ m                    $u =0$ m/s                $a = g = 9.8m/s^2$

Using   $S = ut+\dfrac{1}{2}at^2$
$\therefore$    $1.00 = 0 + \dfrac{1}{2} \times 9.8t^2$                                     $\implies t = 0.45$ s
 Work done by gravity         $W = mgh  =0.25 \times 9.8 \times 1.00 = 2.45$  J
$\therefore$Output power  of gravity        $P = \dfrac{W}{t} = \dfrac{2.45}{0.45} = 5.43$  $W$

Multiple choice power work and power work, energy and power physics energy and its forms

A girl of mass $40 kg$ climbs $50$ stairs each of average height $20 cm$ in $50 s$. Find the power of the girl $\left( g=10m{ s }^{ -2 } \right) $.

  1. $50 W$
  2. $50\times 20W$
  3. $80 W$
  4. $50\times 20\times 2W$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Power $=\dfrac{work \quad done}{time}$

$=\dfrac { change\quad in\quad PE }{ time } \ =\dfrac { 40\times 10\times 50\times 20\times { 10 }^{ -2 } }{ 50 } \ =80W$

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

A hydraulic machine exerts a force of 900 N on a piston of diameter 1.80 cm. The output force is exerted on a piston of diameter 36 cm. What will be the output force?

  1. $12\times 10^4 N$
  2. $16\times 10^4 N$
  3. $36\times 10^4 N$
  4. $38\times 10^4 N$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$f=900 N$
$a=\pi \times (\frac {1.8}{2})^2=\pi \times \frac {(1.8)^2}{4}$
$A=\pi \times (\frac {36}{2})^2=\pi \times \frac {(36)^2}{4}$
Now $\frac {f}{a}=\frac {F}{A}$
$\Rightarrow \frac {900\times 4}{\pi (1.8)^2}=\frac {F\times 4}{\pi \times (36)^2}$
$\therefore F=\frac {900\times 4}{\pi \times 1.8\times 1.8}\times \frac {\pi \times 36\times 36}{4}$
$=36\times 10^4N$.

Multiple choice physics pressure in fluids and atmospheric pressure hydraulic equipment applications of pascals law examples of hydraulic press

In a typical hydraulic press, a force of 20 N is exerted on small piston of area 0.050 m2. What is force exerted by large piston on load if it has an area of 0.50 m2?

  1. 200N

  2. 100N

  3. 50N

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

Using   $\dfrac{F _1}{A _1} = \dfrac{F _2}{A _2}$

$\therefore$   $\dfrac{20}{0.05} = \dfrac{F _2}{0.5}$            $\implies F _2 = 200$  N

Multiple choice physics machines levers and pulleys pulley pulleys

In a single movable pulley, if the effort moves by a distance $x$ upwards, by what height is the load raised?

  1. $x/2$
  2. $x$
  3. $4x$
  4. $2x$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Load $L$ is attached to the movable pulley. A rope is fi xed at a point in ceiling that passes through the moveable pulley. 
In order to lift the effort $E$ by $ x$ meters, the load $L$ needs to 
Travel the distance of $ x/2$ meters. (The entire rope needs to be pulled by $E$ for a distance of $ x$) 
Now, Velocity Ratio equals (Distance traveled by $E$) divided by (Distance traveled by $L$) 
So it becomes : $ x/( x/2)$. This equals to 2
Since it is a ratio. it has no units of measurement. 
Multiple choice physics a little effort, lot of work levers and pulleys pulley pulleys

Calculate the effort in newtons required to raise 1000 kg anchor on boat with a wheel and axle of radii 50 cm and 10 cm respectively.

  1. 1000

  2. 1500

  3. 2000

  4. 3000
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The mechanical advantage of a wheel and axle $= \displaystyle \frac{Radius  of  wheel}{Radius  of  the  axle} $      ..........  (1)
But $MA = \displaystyle \frac{load}{effort}$      .............    (2)
From (1) and (2)
$\displaystyle \frac{Radius  of  wheel}{Radius  of  the  axle} = \frac{load}{effort}$
$\displaystyle \frac{50  cm}{10  cm} = \frac{1000 \times  10  N}{effort}$
$Effort = \displaystyle \frac{1000  \times  10  \times 10}{50}$
$ Effort = 2000 N$

Multiple choice physics a little effort, lot of work levers and pulleys pulley pulleys

Two crowbars of lengths 5 m and 7 m are used to lift a boulder, using a small stone, as a fulcrum. Does it affect the effort required to lift the load?

  1. Yes

  2. No

  3. Maybe

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

Yes it effects the effort required to lift the load as a crow bar of 7 m length provides greater effort arm than a crow bar of 5 m length. Thus crow bar of 7 m length requires lesser effort.

Multiple choice physics machines levers and pulleys pulley pulleys

In a single fixed pulley, if the effort moves by a distance $x$ downwards, by what height is the load raised upwards?

  1. $2x$
  2. $x$
  3. $x/2$
  4. $x/g$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

We know that for a single fixed pulley, Distance moved by effort is equal to the distance moved by load.
$ Distance\; moved \;by\; effort = x$
$\therefore Distance \;moved \;by \;load = x$

Multiple choice physics machines levers and pulleys pulley pulleys

A block and tackle system has the velocity ratio $3$. A man can exert a pull of $200kgf$. What is the maximum load he can raise with this pulley system if its efficiency is $60\%$

  1. $250 kgf$
  2. $260 kgf$
  3. $350 kgf$
  4. $360 kgf$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$Mechanical advantage = Velocity Ratio \times Efficiency $
$M.A = 3 \times \dfrac{60}{100} = 1.8$
$M.A = \dfrac{load}{effort} $
$\therefore load = M.A \times effort$
$\Rightarrow 1.8 \times 200 = 360kgf$

Multiple choice physics machines levers and pulleys pulley pulleys

A force of $250 N$ is required to lift a $75 kg$ mass through a pulley system. In order to lift the mass through $3 m$, the rope has to be pulled through $12m$. The percent efficiency of system is

  1. $50$
  2. $75$
  3. $33$
  4. $90$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Height lift was $h=3m$
Work done by the force is $W=Fd=250\times 12=3000Joule$

Work against the gravity i.e. $increase $ in potential energy of the  mass is $E=mgh=75\times 10\times 3=2250Joule $

so efficiency will be $\eta \times 100=\dfrac{E}{W}\times 100$% = $75$%