**1.** Two spherical balls of mass $10$ kg each are placed $10$ cm
apart. Find the gravitational force of attraction between
them.

Gravitational force of attraction $\\$ F = $\frac{GMm}{r^2}$ $\\$ $\frac{6.67\times 10^{-11} \times 10 \times 10}{(0.1)^2}$ = 6.67 $\times 10^{-7}$ N

**2.** Four particles having masses m, $2$ m, $3$ m and $4$ m are
placed at the four corners of a square of edge a. Find the gravitational force acting on a particle of mass m
placed at the centre.

2 None

Solutions**3.** 15. A pole of length $1'00 m$ stands half dipped in a
swimming pool with water level $50'0 cm$ higher than the
bed. The refractive index of water is $1'33$ and sunlight
is coming at an angle of $45°$ with the vertical. Find the
length of the shadow of the pole on the bed.

Shadow length =$ BA^1$ = $BD + A^1D $ = 0.5 + 0.5 tan r$\\$ Now. 1.33 = $\frac{ sin 45^0}{ sin r }$ $\Rightarrow$ sin r = 0.53.$\\$ $\Rightarrow$ cos r = $\sqrt{1-sin^2r}$ = $\sqrt{1-(0.53)^2}$ = 0.85 $\\ $ So, tan r = 0.6235 $\\$ So, shadow length = (0.5) (1 + 0.6235) = 81.2 cm