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A body of mass 2 text kg begins to move under the action of a time dependent force given by vecF = (6thati + 6t^2hatj)textN. The power developed by the force at the time t is given by:

Solution & Explanation

### Related Formula vecF = mveca veca = fracdvecvdt implies vecv = int veca dt P = vecF cdot vecv ### Core Logic First find acceleration from force and mass. Then integrate acceleration to find the velocity vector at time t (starting from rest). Finally, compute the dot product of Force and Velocity to get instantaneous power. ### Step 1: Calculate Acceleration Given vecF = (6thati + 6t^2hatj) text N and m = 2 text kg. veca = fracvecFm = frac6thati + 6t^2hatj2 veca = (3thati + 3t^2hatj) text m/s^2 ### Step 2: Calculate Velocity Assuming the body begins to move from rest (at t=0, v=0): vecv = int_0^t veca \, dt = int_0^t (3thati + 3t^2hatj) \, dt vecv = left( frac3t^22 right)hati + left( frac3t^33 right)hatj vecv = left( frac3t^22 right)hati + t^3hatj ### Step 3: Calculate Power P = vecF cdot vecv P = (6thati + 6t^2hatj) cdot left(frac3t^22hati + t^3hatjright) P = (6t) times left(frac3t^22right) + (6t^2) times (t^3) P = 9t^3 + 6t^5 text W ### Pattern Recognition When force varies as a polynomial in time t^n, acceleration does too. Velocity jumps to t^n+1. Power (F cdot v) will result in terms behaving as t^2n+1. Here t to t^3 and t^2 to t^5. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power Class 11 Physics: Motion in a Plane

Reference Study Guides

More Work, Energy and Power Previous-Year Questions — Page 4

Q46 jee_main_2024_01_february_morning Collisions
A simple pendulum of length 1mathrm~m has a wooden bob of mass 1mathrm~kg. It is struck by a bullet of mass 10^-2mathrm~kg moving with a speed of 2 times 10^2mathrm~ms^-1. The bullet gets embedded into the bob. The height to which the bob rises before swinging back is (use g = 10mathrm~m/s^2):
  • A. 0.30 m
  • B. 0.20 m
  • C. 0.35 m
  • D. 0.40 m

Solution

### Related Formula Conservation of Linear Momentum during embedded collision: mu = (M + m)V Conservation of Mechanical Energy during vertical rise: frac12(M+m)V^2 = (M+m)gh implies h = fracV^22g ### Core Logic Given data: m = 10^-2mathrm~kg, u = 2 times 10^2mathrm~ms^-1, M = 1mathrm~kg, g = 10mathrm~m/s^2. Apply momentum balance: 10^-2 times (2 times 10^2) = (1 + 0.01)V 2 = 1.01V implies V approx 2mathrm~ms^-1\ text(since 1.01 approx 1text) ### Step 1: Compute Maximum Rise Height Using the work-energy relation for the subsequent upward swing: h = fracV^22g = frac2^22 times 10 = frac420 = 0.20mathrm~m ### Pattern Recognition Perfectly inelastic collision approximation: because m ll M, we can simplify M+m approx M during velocity matching to complete the calculation rapidly. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power Class 11 Physics: Laws of Motion
Q37 jee_main_2024_29_january_evening Conservation of Mechanical Energy
The bob of a pendulum was released from a horizontal position. The length of the pendulum is 10text m. If it dissipates 10\% of its initial energy against air resistance, the speed with which the bob arrives at the lowest point is: [Use g = 10text m s^-2]
  • A. 6sqrt5text m s^-1
  • B. 5sqrt6text m s^-1
  • C. 5sqrt5text m s^-1
  • D. 2sqrt5text m s^-1

Solution

### Related Formula Potential energy at the horizontal release point relative to the lowest point: U_i = mgell If 10\% of energy is dissipated, the remaining 90\% is converted entirely to kinetic energy at the lowest point: E_f = 0.90 times U_i = frac12mv^2 ### Core Logic Let the horizontal position be our reference for potential energy relative to the lowest point. * Initial potential energy: U_i = mgell * Energy remaining after 10\% loss: 0.90(mgell) Equating this to kinetic energy at the bottom: frac910 mgell = frac12 mv^2 ### Step 1: Calculate the Velocity We can cancel m from both sides: frac910 gell = frac12 v^2 Substitute ell = 10text m and g = 10text m s^-2: frac910 (10)(10) = frac12 v^2 90 = frac12 v^2 implies v^2 = 180 v = sqrt180 = sqrt36 times 5 = 6sqrt5text m s^-1
Pendulum trajectory diagram for Q37
Pendulum trajectory diagram for Q37
### Pattern Recognition Whenever there is a fractional energy loss x, use the formula: v = sqrt2(1-x)gell. Substituting x = 0.1 gives v = sqrt1.8gell directly. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q42 jee_main_2024_29_january_evening Vertical Circular Motion
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 frac(mathrmK.E.)_mathrmA(mathrmK.E.)_mathrmB is:
Vertical circular motion path of a pendulum bob for Q42 - JEE Main 2024 29 January Shift 2
The diagram displays a bob of mass m in vertical circular motion with velocity indicators at points A, B, and C.
  • A. 3:2
  • B. 5:1
  • C. 2:5
  • D. 1:5

Solution

### Related Formula For a body to just complete a vertical loop of radius L: * Speed at the lowest point A: v_A = sqrt5gL * Speed at the highest point B: v_B = sqrtgL ### Core Logic The kinetic energy at any point is given by: textK.E. = frac12mv^2 Thus: (textK.E.)_A = frac12m v_A^2 = frac12m(5gL) (textK.E.)_B = frac12m v_B^2 = frac12m(gL) ### Step 1: Calculate the Ratio Taking the ratio of the kinetic energies at A and B: frac(textK.E.)_A(textK.E.)_B = fracfrac12m(5gL)frac12m(gL) = frac51 = 5:1 ### Pattern Recognition Since textK.E. propto v^2, the ratio of kinetic energies is simply the ratio of the squares of the critical velocities at the bottom and top of the vertical loop: (sqrt5gL)^2 : (sqrtgL)^2 = 5:1. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q39 jee_main_2024_27_jan_morning Kinetic Energy and Momentum
Two bodies of mass 4text g and 25text g are moving with equal kinetic energies. The ratio of the magnitude of their linear momentum is:
  • A. 3:5
  • B. 5:4
  • C. 2:5
  • D. 4:5

Solution

### Related Formula K = fracP^22m implies P = sqrt2mK Where P is linear momentum, m is mass, and K is kinetic energy. ### Core Logic Given K_1 = K_2, the momentum ratio simplifies directly to the square root of their masses: fracP_1P_2 = sqrtfracm_1m_2 ### Step 1: Calculate the value Substitute m_1 = 4text g and m_2 = 25text g: fracP_1P_2 = sqrtfrac425 = frac25 ### Pattern Recognition For constant kinetic energy tracking profiles, momentum maps proportionally to sqrtm. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power
Q37 jee_main_2024_29_jan_morning Potential Energy and Force Relationship
The potential energy function (in J) of a particle in a region of space is given as U = (2x^2 + 3y^3 + 2z). Here x, y and z are in meter. The magnitude of x - component of force (in N) acting on the particle at point P (1, 2, 3) m is:
  • A. 2
  • B. 6
  • C. 4
  • D. 8

Solution

### Related Formula The force vector vecF is related to the potential energy U by the negative gradient of potential energy: vecF = -vecnabla U = -left( fracpartial Upartial xhati + fracpartial Upartial yhatj + fracpartial Upartial zhatk right) Hence, the x-component of force is: F_x = -fracpartial Upartial x ### Core Logic Given the potential energy function: U = 2x^2 + 3y^3 + 2z Taking the partial derivative with respect to x (treating y and z as constants): fracpartial Upartial x = fracpartialpartial x(2x^2) = 4x ### Step 1: Substitute Coordinates The x-component of the force is: F_x = -4x At point P(1, 2, 3) mathrm~m, we substitute x = 1: F_x = -4(1) = -4 mathrm~N Magnitude of the x-component of force is: |F_x| = 4 mathrm~N ### Pattern Recognition When asked for a specific component (like x-component), only differentiate partially with respect to that specific variable. The remaining coordinates (y, z) act purely as constants and vanish. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Physics: Work, Energy and Power

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