Concept of Relative Speed
Movement of one body with respect to another moving body is called the relative speed of each other.
Case 1:
Two bodies are moving in opposite direction at a speed S1 and S2 respectively. Then
Relative speed = S1 + S2
Case 2:
Two bodies are moving in same direction at a speed S1 and S2 respectively. Then
Relative speed= |S1 - S2|
In this case, we take positive value of difference of speeds.
Acceleration:
Acceleration is defined as rate of change of speed. Acceleration can be positive (speed increases) or negative (speed decreases, also known as deceleration). Unit of acceleration is m/S2
Final speed= Initial speed + Acceleration × time.
Average speed:
If a certain distance is covered in parts at different speeds, the average speed is given by
Average speed = $$\frac{{{\text{Total}}\,{\text{distance}}\,{\text{covered}}}}{{{\text{Total}}\,{\text{time}}\,{\text{taken}}}}$$
Suppose a car goes from A to B at an average speed S1 and then comes back from B to A an average speed of S2. If you have to find the average speed of the whole journey, then average speed is given by,
Average speed = $$\frac{{2{S_1} \times {S_2}}}{{{S_1} + {S_2}}}$$
Average speed = $$\frac{{2 \times {\text{products}}\,{\text{of}}\,{\text{speeds}}}}{{{\text{sum}}\,{\text{of}}\,{\text{speeds}}}}$$
Example 1:
Ramu climbs a mountain with 10 km/h speed and returns with 20 km/h speed then what is his average speed?
Solution:
Average speed
$$\eqalign{
& = \frac{{2{S_1} \times {S_2}}}{{{S_1} + {S_2}}} \cr
& = \frac{{2 \times 10 \times 20}}{{10 + 20}} \cr
& = 13\frac{1}{3}\,{\text{km/h}} \cr} $$
Example 2:
A car travels at 60 km/h from Mumbai to Poona and at 120 km/h from Poona to Mumbai. What is the average speed of the car for the entire journey?
Solution:
Average speed
$$\eqalign{
& = \frac{{2{S_1} \times {S_2}}}{{{S_1} + {S_2}}} \cr
& = \frac{{2 \times 60 \times 120}}{{180}} \cr
& = 80\,{\text{km/h}} \cr} $$
Join The Discussion