Physics · Work, Energy And Power · NEET
To lift a body of mass m slowly through a height h, you apply an upward force equal to its weight mg. Work done against gravity = force x displacement = mg x h = mgh. This work does not vanish; it gets stored in the body as gravitational potential energy. So PE = mgh. NCERT derives it by noting that when the body is released, this stored mgh reappears exactly as kinetic energy 1/2 mv^2 on reaching the ground.
Gravity acts only vertically (downward). Work by gravity depends only on the vertical drop or rise, never on the horizontal path. If a body slides down a curved ramp or a straight incline of the same height h, the change in gravitational PE is mgh in both cases. This is why gravity is a conservative force. For NEET, always take h as the vertical height, not the length of the slope.
The value of mgh is not absolute; it depends on where you set the reference level (h = 0). You can put h = 0 at the ground, at a table top, or anywhere convenient. Only the CHANGE in potential energy, mg(h2 - h1), has physical meaning and is the same no matter which reference you pick. In NEET problems, choose the reference that makes the numbers simple, usually the lowest point of the motion.
PE = mgh is stored energy due to position (height). KE = 1/2 mv^2 is energy of motion (speed). During free fall PE keeps converting into KE while their sum (mechanical energy) stays constant. At the top: all PE, zero KE. At the bottom: all KE, zero PE. Both are measured in joules.
With the simple mgh formula, PE is negative when the body is below your chosen h = 0 level (h is negative). This is normal and just reflects your reference choice. Separately, in gravitation using the exact formula -GMm/r, PE is always negative because the zero is fixed at infinity. For the Work-Energy-Power chapter, stick to mgh and let the sign of h follow your reference.
A particle is released from height S from the surface of the Earth. At a certain height its kinetic energy is three times its potential energy. The height from the surface of the Earth and the speed of the particle at that instant are respectively
The sum of the kinetic energy and potential energy of a simple pendulum bob is 0.02 J. The speed of the bob at the equilibrium position is approximately (mass of the bob = 20 g)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
PE = mgh, where m is mass in kg, g is acceleration due to gravity (9.8 m/s^2, often taken as 10 in NEET), and h is the vertical height above the reference level. The unit is the joule (J).
The SI unit is the joule (J). Its dimensional formula is [M L^2 T^-2], the same as all forms of energy and work.
It is a scalar. It has only magnitude (and a sign that depends on the reference level), no direction.
Read the question. If g is given as 10 m/s^2, use 10; otherwise use 9.8 m/s^2. Most NEET problems state g = 10 m/s^2 to keep arithmetic clean.
Work done in lifting a body against gravity is not lost; it is stored by virtue of the body's raised position. When released, this stored energy converts fully into kinetic energy, showing it was truly stored.
