Why is electromagnet made of soft iron?

Electromagnet is a temporary magnet made of soft iron. Soft iron is magnetised by the magnetic field of a coil or solenoid.
Soft iron has high magnetic permeability due to which it gets magnetised quickly and it loses magnetic properties easily due to the fact that it has low magnetic retentivity .

Temperature doesn't change during phase change( melting, boiling etc..). Why?

The heat added to the solid is absorbed by the particles and it is utilised in overcoming the intermolecular force of attraction between the particles. So heat absorbed in solids is changed to potential energy without any change in kinetic energy of the molecules. As temperature in Kelvin scale is the measure of kinetic energy of particles, there is no change in temperature during melting.

CLOUDS APPEAR WHITE, WHY?

Cloud appears white due to the phenomena of scattering of white light. The water particles in the cloud are big in size as compared to air molecules and wavelength of light. Hence all the colours get scattered in equal intensity and mixture of all colours gives white light. This white light reaches our eyes giving the impression that colour of a cloud is white.

CALORIMETER

It is a device to measure the heat lost or gained by one substance mixed with another at different temperature.
It is based on the principle of mixtures or principle of calorimetry.
Calorimeter is made of thin sheet of copper kept in a wooden jacket to with other insulating materials so that the process exchange heat by conduction is checked. Its surfaces are polished to check the process of radiation. Wooden or asbestos lid is used to minimise the loss of heat by convection.

Thin sheet is used to lower the heat capacity.

Temperature falls as altitude increases, Why?

The volume of rising warm air increases with height due the availability of more space above. This expansion (Joule-Thompson effect) results in cooling effect. Hence hillstations have low temperature. This phenomena helps in the formation of clouds and various forms of precipitation.

FUSE WIRE

Fuse wire is a safety device to limit the current in an electric circuit. It gets melted when excess current flows in a circuit. It works with the principle of heating effect of electric current. It is made with an alloy of lead and tin (solder wire).
Its material has two important properties
1. It has low melting point (around 250 degree celcius).
2.  It has high resistivity.

It is advantageous to run before taking a long jump. Why?

When we run, our body gains inertia of motion due to which it wants to be in motion continuously. When we take long jump our body has the tendency to be thrown forward more.

NEWTON'S SECOND LAW OF MOTION

The Second law states that the rate of change of momentum is directly proportional to the applied force and the change takes place in the direction of the force applied.

If u= initial velocity,
v=final velocity,
t= time taken to change the velocity from u to v,

Then
Initial momentum = mu
Final  momentum=mv

So change in momentum = mv-mu =m(v-u)
Hence the rate of change of momentum = m(v-u)/t

Therefore F= m(v-u)/t
  F=ma.