By
vector addition.
In fact, velocity is a vector, with a magnitude intensity, a direction and a verse, so we can't simply do an algebraic sum of the two (or more velocities).
First we need to decompose each velocity on both x- and y-axis (if we are on a 2D-plane), then we should do the algebraic sum of all the components on the x- axis and of all the components on the y-axis, to find the resultants on x- and y-axis. And finally, the magnitude of the resultant will be given by

where Rx and Rx are the resultants on x- and y-axis. The direction of the resultant will be given by

where

is its direction with respect to the x-axis.
To calculate the specific heat capacity of an object or substance, we can use the formula
c = E / m△T
Where
c as the specific heat capacity,
E as the energy applied (assume no heat loss to surroundings),
m as mass and
△T as the energy change.
Now just substitute the numbers given into the equation.
c = 2000 / 2 x 5
c = 2000/ 10
c = 200
Therefore we can conclude that the specific heat capacity of the block is 200 Jkg^-1°C^-1
A reversible change is a change that can be undone or reversed
There are two conditions necessary for total internal reflection, which is when light hits the boundary between two mediums and reflects back into its original medium:
Light is about to pass from a more optically dense medium (slower) to a less optically dense medium (faster).
The angle of incidence is greater than the defined critical angle for the two mediums, which is given by:
θ = sin⁻¹(
/
)
Where θ = critical angle,
= refractive index of faster medium,
= refractive index of slower medium.
Choice C gives one of the above necessary conditions.