Kinematics - NEET Physics Questions
Question 31: easy

If the figure below represents a parabola, identify the physical quantities representing Y and X for constant acceleration

1. X = time, Y = velocity
2. X = velocity, Y = time
3. X = time, Y = displacement
4. X = time, Y = acceleration
View Answer

\[ S= \frac{1}{2}at^{2}\]

As, S is proportional to t graph is a parabola

Question 32: moderate

The v-t graph for a particle is as shown. The distance travelled in the first four second is

1. 12 m
2. 16 m
3. 20 m
4. 24 m
View Answer

Area bounded by v-t graph represents displacement. Here.

Area = (1/2)×4×8= 16

So, Distance travelled is 16 m.

Question 33: moderate

A particle starts form rest. Its acceleration (a) versus time (t) is as shown in the figure. The maximum speed of the particle will be

1. 110 ms–¹
2. 55 ms–¹
3. 550 ms–¹
4. 660 ms–¹
View Answer

Area of acceleration time graph represents change in velocity. As acceleration is always positive speed is always increasing.

Area = (1/2)× 10× 11 = 55

So, max. speed of 55 m/s

Question 34: difficult

A bus begins to move with an acceleration of 1 ms–². A man who is 48 m behind the bus starts running at 10 ms–¹ to catch the bus. The man will be able to catch the bus after

1. 6 s
2. 5 s
3. 8 s
4. 7 s
View Answer

Using relative speed, let’s re-calculate with the correct setup:

1. Acceleration of the bus: \( a = 1 \, \text{m/s}^2 \)

2. Initial distance behind the bus: \( d = 48 \, \text{m} \)

3. Speed of the man: \( v_m = 10 \, \text{m/s} \)

4. The equation of motion for the bus after \( t \) seconds:
\[
d_b = \frac{1}{2} a t^2 = \frac{1}{2} \cdot 1 \cdot t^2 = \frac{1}{2} t^2
\]

5. Distance traveled by the man:
\[
d_m = v_m \cdot t = 10t
\]

6. Setting the distances equal considering the man starts 48 m behind:
\[
10t = \frac{1}{2} t^2 + 48
\]
\[
\frac{1}{2} t^2 - 10t + 48 = 0
\]

7. Multiplying by 2:
\[
t^2 - 20t + 96 = 0
\]

8. Using the quadratic formula:
\[
t = \frac{20 \pm \sqrt{20^2 - 4 \cdot 96}}{2}
\]
\[
t = \frac{20 \pm \sqrt{400 - 384}}{2}
\]
\[
t = \frac{20 \pm 4}{2}
\]
\[
t = 12 \, \text{s} \quad \text{or} \quad t = 8 \, \text{s}
\]

 the man will catch the bus is 8 seconds. 

Question 35: difficult

A car is moving along a straight road with a uniform acceleration. It passes through two points P and Q separated by a distance with velocity 30 km/hr and 40 km/hr respectively. The velocity of the car midway between P and Q is :

1. 33.3 km/hr
2. 20√3 km/hr
3. 25√2 km/hr
4. 35 km/hr
View Answer

Speed at mid point is given by

\[ V_{mid}=\sqrt{\frac{V_{1}^{2}+V_{2}^{2}}{2}} \]

Question 36: difficult

Between two stations, a train accelerates from rest uniformly at first, then moves with constant velocity, and finally retards uniformly to come to rest. If the ratio of the time taken is 1 : 8 : 1 and the maximum speed attained be 60 km h–¹, then what is the average speed over the whole journey?

1. \[48 km h^{-1}\]
2. \[52 km h^{-1}\]
3. \[54 km h^{-1}\]
4. \[56 km h^{-1}\]
View Answer
  1. Time Ratios: The time ratios are given as
    1:8:11:8:1
     

    . So, if the total time is tt 

    , the train spends t10\frac{t}{10} 

    accelerating, 8t10\frac{8t}{10} 

    at constant velocity, and t10\frac{t}{10} 

    decelerating.

  2. Velocity-Time Graph:
    • The graph forms a trapezium.
    • The train accelerates linearly from 0 to 60 km/h, holds constant at 60 km/h, and then decelerates back to 0.

    Key points:

    • The area under this graph represents the total distance traveled.
    • The height (maximum velocity) = 60 km/h.
    • The time intervals are in the ratio
      1:8:11:8:1
       

      .

  3. Average Speed:
    The area of the trapezium is given by: 

    Area=12×(ttotal)×(initial velocity+final velocity)\text{Area} = \frac{1}{2} \times (t_{\text{total}}) \times (\text{initial velocity} + \text{final velocity})For the constant velocity portion:

     

    Average speed=60×(1+8+1)10=54km/h\text{Average speed} = \frac{60 \times (1 + 8 + 1)}{10} = 54 \, \text{km/h}

Thus, the average speed is 54 km/h.

Question 37: moderate

A particle moving with a velocity equal to 0.4 m/s is subjected to an uniform acceleration of 0.15 m/s² for 2 sec in a direction at right angles to its initial direction of motion. The resultant

1. 0.7 m/s
2. 0.5 m/s
3. 0.1 m/s
4. None of these
View Answer

To find the resultant velocity using unit vectors:

- Initial velocity: \( \vec{u} = 0.4 \, \hat{i} \, \text{m/s} \)
- Acceleration: \( \vec{a} = 0.15 \, \hat{j} \, \text{m/s}^2 \)
- Time: \( t = 2 \, \text{seconds} \)

Using the equation of motion \( \vec{v} = \vec{u} + \vec{a}t \):

- In the \( x \)-direction: \( v_x = 0.4 \, \text{m/s} \)
- In the \( y \)-direction: \( v_y = 0 + 0.15 \times 2 = 0.3 \, \text{m/s} \)

The resultant velocity is:

\[
|\vec{v}| = \sqrt{v_x^2 + v_y^2} = \sqrt{(0.4)^2 + (0.3)^2} = \sqrt{0.25} = 0.5 \, \text{m/s}
\]

Thus, the resultant velocity is 0.5 m/s

Question 38: easy

Two bodies of different masses \[m_{a} and m_{b}\] are dropped from two different heights, viz, a and b. The ratio of time taken by the two to drop through these distances is :

1. a : b
2. \[\frac{m_{a}}{m_{b}}:\frac{b}{a}\]
3. √a : √b
4. a² : b²
View Answer

The time taken by an object to fall from a height is given by the equation:

\[
t = \sqrt{\frac{2h}{g}}
\]

For mass \( m_a \) dropped from height \( a \), the time taken is:

\[
t_a = \sqrt{\frac{2a}{g}}
\]

For mass \( m_b \) dropped from height \( b \), the time taken is:

\[
t_b = \sqrt{\frac{2b}{g}}
\]

The ratio of time taken by the two bodies is:

\[
\frac{t_a}{t_b} = \frac{\sqrt{\frac{2a}{g}}}{\sqrt{\frac{2b}{g}}} = \sqrt{\frac{a}{b}}
\]

So, the ratio of the time taken is:

\[
\frac{t_a}{t_b} = \sqrt{\frac{a}{b}}
\]

Question 39: moderate

From the top of a tower a ball is thrown vertically upwards. When the ball reaches h below the top of tower, it’s speed is double of what it was at height h above the tower. Find maximum height attained by ball from top of tower?

1. 4h/3
2. 3h/4
3. 5h/3
4. 5h/4
View Answer

Let the initial velocity of the ball be \( u \), and let \( v \) be the velocity of the ball at a distance \( h \) above the tower. Using the equation of motion:

\[
v^2 = u^2 - 2gh
\]

At a distance \( h \) below the tower, the velocity is doubled, so:

\[
(2v)^2 = u^2 + 2gh
\]

Simplifying these:

\[
4v^2 = u^2 + 2gh
\]

Substitute \( v^2 = u^2 - 2gh \) from the first equation:

\[
4(u^2 - 2gh) = u^2 + 2gh
\]

Expanding and solving:

\[
4u^2 - 8gh = u^2 + 2gh
\]

\[
3u^2 = 10gh
\]

\[
u^2 = \frac{10gh}{3}
\]

The maximum height \( H \) from the top of the tower is given by:

\[
H = \frac{u^2}{2g} = \frac{10gh}{6g} = \frac{5h}{3}
\]

Thus, the maximum height attained by the ball from the top of the tower is \( \frac{5h}{3} \).

Question 40: moderate

A stone dropped from the top of a tower travels 5/9th of the height of tower during the last second of fall. Height of the tower is : (Take g = 10 m/s²)

1. 52 m
2. 36 m
3. 45 m
4. 78 m
View Answer

From Galileo's ratio of odd number distances travelled in consecutive seconds are in the order of 1:3:5:7... Here distance travelled in 3rd second is 5/9th of total distance total time of flight is 3 second.

\[ s= \frac{1}{2}gt^{2}= \frac{1}{2}\times 10\times 3^{2}= 45 m \]