Work, Energy and Power
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See all →- The kinetic energies of two similar cars A and B are 100 J and 225 J respectively. On applying breaks, car A stops after 1000 m and car B stops after 1500 m. If F_A and F_B are the forces applied by the breaks on cars A and B, respectively, then the… Answer: 2. 2/3 Key idea: by the work-energy theorem, the brake force does work equal to the kinetic energy lost: F d = K . F_A=100/1000=0.1\... 1
- A bob of heavy mass m is suspended by a light string of length l. The bob is given a horizontal velocity v_0 as shown in figure. If the string gets slack at some point P making an angle θ from the horizontal, the ratio of the speed v of the bob at po… Answer: 4. (sinθ/2+3sinθ)^(1/2) Key idea: the string goes slack when the tension becomes zero, and energy is conserved along the way. At P (... 1
- Two bodies A and B of same mass undergo completely inelastic one dimensional collision. The body A moves with velocity v_1 while body B is at rest before collision. The velocity of the system after collision is v_2. The ratio v_1 : v_2 is : Answer: 3. 2:1 Key idea: In a completely inelastic collision the bodies stick together and momentum is conserved. mv₁+0=(m+m)v₂ , so v₂=v₁/2... 1
- At any instant of time t, the displacement of any particle is given by 2t-1 (SI unit) under the influence of force of 5N. The value of instantaneous power is (in SI unit): Answer: 2. 10 Key idea: Instantaneous power P=vec F·vec v . Velocity v=d/dt(2t-1)=2 m/s (constant). Then P=Fv=5×2=10 W. Option 2. 1
- The potential energy of a long spring when stretched by 2 cm is U. If the spring is stretched by 8 cm, potential energy stored in it will be : Answer: 1. 16U Spring energy U=1/2kx²∝ x² . Extension goes from 2 cm to 8 cm, a factor of 4, so energy goes up by 4²=16 . The new energy is... 1
- An electric lift with a maximum load of 2000 kg (lift + passengers) is moving up with a constant speed of 1.5 ms^-1. The frictional force opposing the motion is 3000 N. The minimum power delivered by the motor to the lift in watts is : (g = 10 ms^-2) Answer: 4. 34500 At constant speed the motor force balances weight plus friction. F=mg+f=2000×10+3000=23000\ N P=Fv=23000×1.5=34500\ W Optio... 1