Work & Energy — Set 2
Physics · कार्य और ऊर्जा · Questions 11–20 of 70
What is the dimensional formula of energy?
Correct Answer: B. [M L^2 T^-2]
Energy has the same dimensions as work. Work equals force times displacement, and force has dimension [M L T^-2]. Multiplying by L gives [M L^2 T^-2].
Efficiency of a machine is defined as?
Correct Answer: D. (Useful output energy / Input energy) x 100%
Efficiency compares useful output to total input. It is calculated as (useful output/input) multiplied by 100 percent. Losses like friction reduce efficiency.
How much work is done in lifting a 5 kg mass vertically by 2 m? (Take g = 10 m/s^2)?
Correct Answer: A. 100 J
Work against gravity equals W = mgh. Here W = 5 x 10 x 2 = 100 J. This work becomes gravitational potential energy of the mass.
Work done by friction on a moving body is generally?
Correct Answer: C. Negative
Friction usually acts opposite to the direction of motion. So the angle between friction and displacement is 180°, making work negative. Negative work reduces mechanical energy and often produces heat.
One horsepower is approximately equal to?
Correct Answer: D. 746 W
Horsepower is a non-SI unit of power. 1 horsepower is approximately 746 watts. It is often used to rate engines and motors.
In a perfectly elastic collision of an isolated system, which quantity is conserved?
Correct Answer: A. Kinetic energy
In a perfectly elastic collision, kinetic energy is conserved along with momentum. This means no kinetic energy is lost to deformation or heat overall. Real collisions are often partially inelastic.
A 1 kW device runs for 2 hours. How much energy does it consume in kWh?
Correct Answer: B. 2 kWh
Electrical energy in kWh equals power in kW times time in hours. Here energy = 1 x 2 = 2 kWh. This unit is commonly used in electricity bills.
In uniform circular motion, the work done by the centripetal force is?
Correct Answer: A. Zero
The correct answer is 'Zero'. Centripetal force is always perpendicular to the instantaneous displacement. A perpendicular force does no work because cos 90° = 0. It changes the direction of velocity but not the speed.
Gravitational potential energy of a body is taken as zero at?
Correct Answer: D. A chosen reference level
Potential energy is measured relative to a reference level. You can choose the zero level based on convenience in a problem. Only differences in potential energy have physical significance.
Which formula represents work done by a constant force?
Correct Answer: C. W = F s cosθ
Work by a constant force depends on the component of force along displacement. That is captured by W = F s cosθ. If force and displacement are in the same direction, θ = 0°.