$\Lambda$ trolley of mass $100 \mathrm{~kg}$, starting from rest, describes $100 \mathrm{~m}$ in 10 second. \Lambdat that instant, i.e., at the commencement of the $11^{\text {th }}$ second, two packets, each of mass $12.5 \mathrm{~kg}$, are gently placed in the trolley. I low far does it move in the next 10 seconds assuming that the forces on the trolley remain the same throughout? Solution Since $s-u t+\frac{1}{2} a t^{2}$
$100-0 \div \frac{1}{2} a 100 \quad \therefore \quad a-2 \mathrm{~m} / \mathrm{sec}^{2}$
The velocity at the cnd of 10 seconds $=v=u+a t=0+2 \times 10=20 \mathrm{~m} / \mathrm{s}$ $F=m a=100 \times 2=200 \mathrm{~N}$
During the next 10 seconds, acceleration $-\frac{200}{125}-1.6 \mathrm{~m} / \mathrm{sec}^{2}$ The velocity al the commenecment of the 11 th socond $v^{\prime}$ is given by $m u=m^{\prime} v^{\prime}$ (Taw of conscrvation of momentum) $100 \times 20=125 \times v^{\prime}$
$\therefore \quad v^{\prime}-\frac{100 \times 20}{125}-16 \mathrm{~m} / \mathrm{scc}$
$\therefore$ The distance travelled in the next 10 seconds $=s^{\prime}-u t+\frac{1}{2} a t^{2}$ $-16 \times 10+\frac{1}{2} \times 1.6 \times 100-240 \mathrm{~m}$