$A$ girl of mass $40\, kg$ climbs a rope $6\, m$ long at a constant speed in $15$ seconds. What power does she expend during the climb (in $W$)?

Name at least three commonly used units of energy.

Calculate the kinetic energy of a car of mass $750 \,kg$ moving with a velocity of $54 \,km \cdot h^{-1}$. Find the new kinetic energy of the car if a passenger of mass $50 \,kg$ sits in the car.

$(a)$ An arrow moves forward when released from a stretched bow. Explain the transformation of energy in the process.
$(b)$ $A$ boy of mass $50 \, kg$ climbs up a vertical height of $100 \, m$. Calculate the amount of potential energy he gains.

State whether the following statement is true or false:
When an arrow is released from a bow,potential energy changes into kinetic energy.

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.