The $CGS$ unit of work is:

When is the work done by a force said to be negative? Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work.

Can any object have momentum even if its mechanical energy is zero? Explain.

$A$ spring which has been kept compressed by tying it is allowed to be dissolved in an acid. What happens to the potential energy of the spring?

$(a)$ Define kinetic energy.
$(b)$ The masses of a scooter and a bike are in the ratio of $2:3$,but they are moving with the same speed of $108 \, km/h$. Compute the ratio of their kinetic energy.

$(i)$ An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the total work done against the force of gravity and by the force of gravity on the object? Explain with proper mathematical expression.
$(ii)$ $A$ certain force acting on a $20 \, kg$ mass changes its velocity from $5 \, m s^{-1}$ to $2 \, m s^{-1}$. Calculate the work done by the force.