Work & Energy — Set 7
Physics · कार्य और ऊर्जा · Questions 61–70 of 70
If force and displacement are in opposite directions, the work done by the force is?
Correct Answer: B. Negative
Opposite directions mean the angle θ between force and displacement is 180°. Since cos 180° = -1, work becomes negative. Negative work reduces the kinetic energy of the body.
A block is pulled by 30 N over 5 m while friction of 10 N opposes motion. What is the net work done?
Correct Answer: A. 100 J
Net force along motion is 30 - 10 = 20 N. Net work W = F_net x s = 20 x 5 = 100 J. Net work equals the change in kinetic energy by the work-energy theorem.
The CGS unit of energy is?
Correct Answer: D. erg
In the CGS system, energy is measured in erg. 1 erg equals 1 dyne-centimeter in CGS units. 1 joule equals 10^7 erg.
What is the change in gravitational potential energy of a 1 kg mass moved from 2 m to 5 m height? (Take g = 10 m/s^2)?
Correct Answer: A. 30 J
Change in potential energy is ΔPE = mg(h2 - h1). Here ΔPE = 1 x 10 x (5 - 2) = 30 J. Only the height difference matters for ΔPE in a uniform field.
Work done in stretching a spring from extension x1 to x2 is?
Correct Answer: C. (1/2)k(x2^2 - x1^2)
Spring energy at extension x is (1/2)kx^2. So work from x1 to x2 equals the change in this energy, which is (1/2)k(x2^2 - x1^2). This follows because spring force varies with x.
Power in terms of work W done in time t is?
Correct Answer: B. P = W/t
Power measures how fast work is done. It is defined as work divided by time, so P = W/t. The SI unit watt equals joule per second.
A 2 kW heater runs for 30 minutes. What energy is used in kWh?
Correct Answer: D. 1 kWh
Convert time: 30 minutes equals 0.5 hour. Energy = power x time = 2 x 0.5 = 1 kWh. This is a standard calculation for electrical energy consumption.
When friction acts on a moving system, mechanical energy usually decreases because it is converted into?
Correct Answer: A. Thermal energy
Friction is a non-conservative force that does negative work on motion. The lost mechanical energy appears mainly as heat due to microscopic interactions. This is why surfaces warm up when rubbed.
The SI unit of work is?
Correct Answer: A. Joule
The SI unit of work is Joule (J), equal to the work done when a force of 1 Newton moves an object through 1 meter (1 J = 1 N·m). Work is a scalar quantity.
Kinetic energy of an object depends on its?
Correct Answer: C. Both mass and velocity
Kinetic energy = ½mv², so it depends on both mass (m) and velocity (v). Doubling the velocity quadruples the kinetic energy, while doubling the mass only doubles it. This is why high-speed collisions are more dangerous.