Under what condition is a force considered constant when it causes displacement?

An adult weighing $600 \, N$ raises the centre of gravity of his body by $0.25 \, m$ while taking each step of $1 \, m$ length in jogging. If he jogs for $6 \, km$,calculate the energy utilised by him in jogging assuming that there is no energy loss due to friction of ground and air. Assuming that the body of the adult is capable of converting $10 \, \%$ of energy intake in the form of food,calculate the energy equivalents of food that would be required to compensate for the energy utilised for jogging.

Consider a particle on which constant forces $F_{1}=\hat{i}+2 \hat{j}+3 \hat{k} \text{ N}$ and $F_{2}=4 \hat{i}-5 \hat{j}-2 \hat{k} \text{ N}$ act together, resulting in a displacement from position $r_{1}=20 \hat{i}+15 \hat{j} \text{ cm}$ to $r_{2}=7 \hat{k} \text{ cm}$. The total work done on the particle is:

$A$ body moves a distance of $10 \, m$ along a straight line under the action of a force of $5 \, N$. If the work done is $25 \, J$,the angle which the force makes with the direction of motion of the body is.....$^o$

$A$ man carries a load on his head through a distance of $5 \,m$. The maximum amount of work is done when he ..........

$A$ force $\vec{F} = -5 \hat{i} - 7 \hat{j} + 3 \hat{k}$ acting on a particle causes a displacement $\vec{s} = 3 \hat{i} - 2 \hat{j} + a \hat{k}$. If the work done is $14 \ J$,then the value of '$a$' is: