One joule of work is said to be done when:

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.

$A$ box of mass $10 \, kg$ is placed at point $A$ on a horizontal surface. It is moved to point $B$,which is at a distance of $2 \, m$ from $A$. If the line joining $A$ and $B$ is horizontal,find the work done by the gravitational force on the box. Justify your answer. $(g = 10 \, m s^{-2})$ (in $, J$)

$(a)$ Two bodies have equal masses and are moving with uniform velocities of $2\,v$ and $3\,v$. What is the ratio of their kinetic energies?
$(b)$ $A$ mass is dropped from height $h$. At halfway to the ground,what is the total energy?

State the law of conservation of energy.

Define watt. Express kilowatt in terms of joule per second. $A$ $150 \, kg$ car engine develops $500 \, W$ for each $kg$. What force does it exert in moving the car at a speed of $20 \, m \, s^{-1}$ (in $N$)?