What do mast cells secrete?

In a potentiometer experiment,the balancing length for a cell is $240 \ cm$. When a resistance of $2 \ \Omega$ is connected in parallel with the cell,the balancing length becomes $120 \ cm$. The internal resistance of the cell is ................. $\Omega$.

The velocities of three particles of masses $20 \ g$,$30 \ g$ and $50 \ g$ are $10 \hat{i}$,$10 \hat{j}$ and $10 \hat{k}$,respectively. The velocity of the centre of mass of the three particles is

What type of relationship is observed between insect-pollinated flowers and their pollinating insects?

$A$ satellite $S$ is moving in an elliptical orbit around the earth. The mass of the satellite is very small compared to the mass of the earth.

$A$ radioactive nucleus (initial mass number $A$ and atomic number $Z$) emits $3$ $\alpha$-particles and $2$ positrons. The ratio of neutrons to protons in the final nucleus will be: