11. A satellite of mass m revolves around the earth of radius R at a height x from its surface. If g is the acceleration due to gravity on the surface of the earth, calculate the orbital speed of the satellite.

12. An object is placed at a distance of 40 cm from a convex spherical mirror of radius of curvature 20 cm. At what distance from the object should a plane mirror be placed so that the image in the spherical mirror and plane mirror may be in one plane?13. What is displacement reaction? Explain with example. Also write down the reactivity series of the metals.

The value of orbital speed does not depend on the mass of satellite.Let mass of earth=M,mass of satellite=m,Radius of earth=R

orbital speed=v,r=distance of satellite from the center=R+x,
where x=height of satellite above the surface of earth

Now the centripetal force required to keep the satellite in it's orbit
$\mathrm{F}=\frac{{\mathrm{mv}}^2}{\mathrm{r}}$
Gravitational force acting on the satellite

$$\begin{gathered} \frac{\mathrm{GMm}}{{\mathrm{r}}^2} \\ \mathrm{In} \mathrm{equilibrium}, \\ \frac{{\mathrm{mv}}^2}{\mathrm{r}}=\frac{\mathrm{GMm}}{{\mathrm{r}}^2} \\ \mathrm{v}=\sqrt{\frac{\mathrm{GM}}{\mathrm{r}}} \\ =\sqrt{\frac{{\mathrm{gR}}^2}{\mathrm{r}}} \\ =\mathrm{R}\sqrt{\frac{\mathrm{g}}{\mathrm{R}+\mathrm{x}}} \end{gathered}$$ 

13-

Displacement reaction is a type of oxidation-reduction chemical reaction when an element or ion moves out of one compound and into another - that is, one element is replaced by another in a compound. This is represented by the general reaction scheme:

A + BC → AC + B
Example- Reaction between magnesium, Mg, and hydrochloric acid, HCl, forms magnesium chloride, MgCl2, and hydrogen, H2.

Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
For the reactivity series you can refer to the link:
https://www.meritnation.com/ask-answer/question/what-do-u-mean-by-reactivity-series-i-just-want-a-reactivit/chemical-reactions-and-equations/4774348