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As shown below, bob A of a pendulum having massless string of length 'R' is released from 60^circ to the vertical. It hits another bob B of half the mass that is at rest on a friction less table in the centre. Assuming elastic collision, the magnitude of the velocity of bob A after the collision will be (take g as acceleration due to gravity) [cite: 1, 2]
Collisions diagram for Q10 - JEE Main 2025 Morning
The diagram displays a pendulum bob A suspended at an angle of 60 degrees ready to strike bob B at the lowest equilibrium center point.

Solution & Explanation

### Related Formula u = sqrt2gh = sqrt2gR(1 - costheta) v_1 = left(fracm_1 - m_2m_1 + m_2right)u + left(frac2m_2m_1 + m_2right)v_2i ### Core Logic
Collisions explanation diagram for Q10
The diagram displays a pendulum bob A suspended at an angle of 60 degrees ready to strike bob B at the lowest equilibrium center point.
Velocity of bob A just prior to collision : u = sqrt2gleft(R - Rcos 60^circright) = sqrt2gfracR2 = sqrtgR Using conservation of momentum and coefficient of restitution e=1 for elastic interaction [cite: 660, 662]: m_A u = m_A v_1 + m_B v_2 implies m u = m v_1 + fracm2 v_2 implies 2v_1 + v_2 = 2u ### Step 1: Apply Restitution Velocity Difference v_2 - v_1 = u Subtracting equations yields : 3v_1 = u implies v_1 = fracu3 = frac13sqrtgR ### Pattern Recognition In an elastic head-on collision where one body hits half its mass at rest, it retains exactly one-third of its initial hitting speed[cite: 661, 663]. ### Chapter Mix Class 11 Physics: Work, Energy and Power

Reference Study Guides

More Work, Energy and Power Previous-Year Questions — Page 3

Q19 2025 Variable Force
An object with mass 500 g moves along x-axis with speed v=4sqrtx~textm/s. The force acting on the object is: [cite: 164]
  • A. 8 N [cite: 165]
  • B. 5 N [cite: 167]
  • C. 6 N [cite: 166]
  • D. 4 N [cite: 168]

Solution

### Related Formula a = vfracdvdx F = m cdot a [cite: 778] ### Core Logic Given velocity as a function of position x: [cite: 164, 779] v = 4sqrtx implies v^2 = 16x [cite: 164, 779] Differentiating both sides with respect to position coordinate x: [cite: 780] 2vfracdvdx = 16 implies vfracdvdx = 8 [cite: 780, 781] Thus, the acceleration of the object is a constant value a = 8\ textm/s^2[cite: 781]. Converting mass to kilograms (m = 500\ textg = 0.5\ textkg) [cite: 164, 788]: F = 0.5 times 8 = 4\ textN [cite: 788] ### Pattern Recognition When velocity depends on position coordinate x like v = ksqrtx, squaring instantly reveals that acceleration is constant, since v^2 = k^2 x matches the third kinematic profile v^2 = 2ax directly[cite: 779]. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q19 2025 Work Done by a Variable Force
A force F=alpha+beta x^2 acts on an object in the x-direction. The work done by the force is 5J when the object is displaced by 1 m. If the constant alpha=1N then beta will be
  • A. 15 N/m^2
  • B. 10 N/m^2
  • C. 12 N/m^2
  • D. 8 N/m^2

Solution

### Related Formula The work done W by a variable force component F(x) over a displacement step is given by: W = int_x_1^x_2 F(x) dx ### Core Logic Assuming the object moves from the origin x=0 to x=1text m : W = int_0^1 (alpha + beta x^2) dx = 5text J ### Step 1: Integration and Variable Isolation Perform the integration step : W = left[ alpha x + fracbeta x^33 ight]_0^1 = alpha + fracbeta3 = 5 Given alpha = 1text N , substitute this into the equation to find beta : 1 + fracbeta3 = 5 implies fracbeta3 = 4 implies beta = 12text N/m^2 ### Pattern Recognition For polynomial forces, the integration steps always yield fractional coefficients matching their power index (1 for constant, frac13 for squared terms). ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q14 2025 Average and Instantaneous Power
A body of mass 4mathrm\;kg is placed on a plane at a point mathrmP having coordinate left( 3,4right) mathrmm. Under the action of force overrightarrowmathrmF = left( 2widehatmathrmi + 3widehatmathrmjright) mathrmN ,it moves to a new point Q having coordinates (6,10) m in 4 \sec. The average power and instantaneous power at the \end of 4 \sec are in the ratio of :
  • A. 13:6
  • B. 6:13
  • C. 1:2
  • D. 4 : 3

Solution

### Related Formula * **Average Power** (P_textavg) = fractextTotal Work DonetextTotal Time = fracvecF cdot vecst * **Instantaneous Power** (P_textinst) = vecF cdot vecv(t) ### Core Logic Given parameters: * Force vector: vecF = 2hati + 3hatj * Displacement coordinates: P(3,4) rightarrow Q(6,10) implies vecs = (6-3)hati + (10-4)hatj = 3hati + 6hatj * Time window, t = 4text s Calculate Average Power : W = vecF cdot vecs = (2hati + 3hatj) cdot (3hati + 6hatj) = (2 times 3) + (3 times 6) = 6 + 18 = 24 text J P_textavg = fracWt = frac244 = 6 text W Now, analyze the instantaneous dynamics to extract final velocity vecv at t=4text s: Acceleration vector : veca = fracvecFm = frac2hati + 3hatj4 = 0.5hati + 0.75hatj Assuming the body starts from rest, velocity at t=4text s is : vecv = veca cdot t = (0.5hati + 0.75hatj) times 4 = 2hati + 3hatj Calculate Instantaneous Power at t=4text s : P_textinst = vecF cdot vecv = (2hati + 3hatj) cdot (2hati + 3hatj) = 2^2 + 3^2 = 4 + 9 = 13 text W Taking the final ratio : fracP_textavgP_textinst = frac613 *(Note: There is a minor kinematic inconsistency in the question data layout regarding matching coordinate parameters independently, but the calculations follow the standard intended framework directly).* ### Pattern Recognition For constant force acceleration from rest, average power equals frac12 F a t while instantaneous power scales linearly as F a t, meaning the structural ratio simplifies exactly to 1:2. The custom displacement vector here alters that baseline baseline ratio as tracked. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q16 2025 Vertical Circular Motion
A body of mass 'm' connected to a massless and unstretchable string goes in a vertical circle of radius 'R' under gravity g. The other end of the string is fixed at the center of a circle. If velocity at top of circular path is n sqrtgR , where, n geq 1 , then ratio of kinetic energy of the body at bottom to that at top of the circle is [cite: 1, 2]
  • A. fracmathrmnmathrmn + 4
  • B. fracmathrmn + 4mathrmn
  • C. fracmathrmn^2mathrmn^2 + 4
  • D. fracmathrmn^2 + 4mathrmn^2

Solution

### Related Formula V_textBottom = sqrtV_textTop^2 + 4gR ### Core Logic Given velocity at the top position : V_textTop = sqrtn^2 gR By work-energy theorem, mechanical energy conservation between the top and bottom positions gives : V_textBottom = sqrtn^2 gR + 4gR Since KE = frac12m v^2, the ratio of kinetic energy at the bottom to that at the top is [cite: 2, 112]: textRatio = fracV_textBottom^2V_textTop^2 = fracn^2 gR + 4gRn^2 gR = fracn^2 + 4n^2 ### Pattern Recognition Kinetic energy change in vertical circles always gains a fixed additive value of 2mg(2R) = 4mgR due to gravity work[cite: 112, 687]. ### Chapter Mix Class 11 Physics: Work, Energy and Power

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