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6.(JEE Main 2024 (Online) 4th April Evening Shift )

A body of m   kg slides from rest along the curve of vertical circle from point A to B in friction less path. The velocity of the body at B is:

JEE Main 2024 (Online) 4th April Evening Shift Physics - Work Power & Energy Question 7 English

(given, R = 14   m , g = 10   m / s 2 and 2 = 1.4 )

A.10.6 m/s

B.19.8 m/s

C.16.7 m/s

D.21.9 m/s

Correct option is (D)

From energy conservation

m g ( R + R sin 45 ) = 1 2 m v 2 10 ( 1 + 1 2 ) × 14 = 1 2 v 2 10 ( 1 + 2 2 ) × 28 = v 2 10 ( 1 + 0.7 ) × 28 = v 2 v = 21.81

7.(JEE Main 2024 (Online) 4th April Morning Shift )

If a rubber ball falls from a height h and rebounds upto the height of h / 2 . The percentage loss of total energy of the initial system as well as velocity ball before it strikes the ground, respectively, are :

A. 50 % , 2 gh

B. 50 % , gh

C. 50 % ,  gh  2

D. 40 % , 2 gh

Correct option is (A)

To solve this problem, we need to analyze both the energy loss and the initial velocity of the rubber ball before it strikes the ground.

First, let's consider the energy loss. The energy involved here is gravitational potential energy. The initial potential energy of the ball when it is about to fall is given by U i = m g h , where U i is the initial potential energy, m is the mass of the ball, g is the acceleration due to gravity, and h is the initial height from which the ball falls. After the ball rebounds, it reaches a height of h / 2 . The potential energy at this new height is U f = m g h 2 .

The energy loss can be calculated as the difference between the initial and final potential energies, and to find the percentage energy loss, we divide this difference by the initial energy and multiply by 100:

Energy loss percentage = ( U i U f ) U i × 100

Substituting the values of U i and U f gives:

Energy loss percentage = ( m g h m g h 2 ) m g h × 100

By simplifying, we find:

Energy loss percentage = m g h 1 2 m g h m g h × 100 = 1 2 × 100 = 50 %

This tells us that the energy loss percentage is indeed 50 % .

Next, we'll find the velocity of the ball just before it strikes the ground. The velocity can be determined using the formula for the velocity of an object in free fall:

v = 2 g h

Here, v is the velocity of the ball just before impact, g is the acceleration due to gravity, and h is the height from which the ball falls. This formula shows that the initial velocity of the ball before it strikes the ground is 2 g h , not taking into account air resistance and assuming it starts from rest.

Therefore, the correct answer is Option A: 50 % , 2 g h .

8.(JEE Main 2024 (Online) 30th January Morning Shift )

A particle is placed at the point A of a frictionless track A B C as shown in figure. It is gently pushed towards right. The speed of the particle when it reaches the point B is :

(Take g = 10   m / s 2 ).

JEE Main 2024 (Online) 30th January Morning Shift Physics - Work Power & Energy Question 11 English

A. 2 10   m / s

B. 10   m / s

C. 10   m / s

D. 20   m / s

Correct option is (C)

By COME

KE A + U A = KE B + U B 0 + mg ( 1 ) = 1 2 mv 2 + mg × 0.5 v = g = 10   m / s

9.(JEE Main 2024 (Online) 29th January Evening Shift )

A bob of mass ' m ' is suspended by a light string of length ' L '. It is imparted a minimum horizontal velocity at the lowest point A such that it just completes half circle reaching the top most position B. The ratio of kinetic energies ( K . E ) A ( K . E ) B is :

JEE Main 2024 (Online) 29th January Evening Shift Physics - Work Power & Energy Question 13 English

A.5 : 1

B.3 : 2

C.1 : 5

D.2 : 5

Correct option is (A)

Apply energy conservation between A & B

1 2 mV L 2 = 1 2 mV H 2 + mg ( 2   L ) V L = 5 gL

So, V H = gL

( K . E ) A ( K . E ) B = 1 2   m ( 5 gL ) 2 1 2   m ( gL ) 2 = 5 1

10.(JEE Main 2024 (Online) 27th January Evening Shift )

A bullet is fired into a fixed target looses one third of its velocity after travelling 4   cm . It penetrates further D × 10 3   m before coming to rest. The value of D is :

A.23

B.32

C.42

D.52

Correct option is (B)

v 2 u 2 = 2 a S ( 2 u 3 ) 2 = u 2 + 2 ( a ) ( 4 × 10 2 ) 4 u 2 9 = u 2 2 a ( 4 × 10 2 ) 5 u 2 9 = 2 a ( 4 × 10 2 ) ( 1 ) 0 = ( 2 u 3 ) 2 + 2 ( a ) ( x ) 4 u 2 9 = 2 a x ( 2 )

( 1 ) / ( 2 )

5 4 = 4 × 10 2 x x = 16 5 × 10 2 x = 3 2 × 10 2   m x = 32 × 10 3   m