Energy In Simple Harmonic Motion

At the maximum height, all the kinetic energy is converted into potential energy. The gravitational potential energy at the maximum height is given by:
where:
- (mass),
- (acceleration due to gravity),
- is the maximum height.
The initial kinetic energy is 20 J.
By conservation of energy:

Solving for :
Hence, the correct answer is (D).

When an object is at rest and there is an attempt to move it, the frictional force that opposes the initiation of motion is called static friction.
Static friction prevents the object from moving until a certain threshold force is applied. Once the object starts moving, static friction is replaced by kinetic (or dynamic) friction, which is generally smaller than static friction.
Explanation of other options:
- (B) Fluid friction: This type of friction occurs when an object moves through a fluid (liquid or gas), not when an object is stationary.
- (C) Sliding friction: This is the friction between two objects in relative motion to each other.
- (D) Kinetic friction: This type of friction opposes the motion of objects that are already in motion.
- (E) Rolling friction: This friction occurs when an object rolls over a surface, typically smaller than sliding friction.
Thus, the correct answer is option (A), static friction.