The relative speed of a car is 20km/h relative to an observer moves at speed of 40 km/h in the same direction, so the actual speed of this car is...........................km/h
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Answer:
“Relative” means “in comparison to”. Thus, the concept of relative speed is used when two or more bodies moving with some speeds are considered. To make things simpler, one body can be made stationary (i.e. Speed =zero) and take the speed of the other body with respect to the stationary body, which is the sum of the speeds if the bodies are moving in the opposite direction and the difference if they are moving in the same direction. This speed of the moving body with respect to the stationary body is called the relative Speed. Relative Speed of 2 bodies = Sum of their individual speeds if they are moving in the opposite direction = Difference of their individual speeds if they are moving in the same direction.
Consider A and B starting at the same time, travelling in the opposite direction with a speed of 20 kmph and 30 kmph and they have to travel a distance of 200 km. Now the concept of relative speed can be used to find the time of their meeting.
Two bodies moving towards each other are A and B with speeds 20 kmph and 30 kmph. Now make body A stationary and take the speed of B in reference to A, so relative Speed of B = sum of their respective speeds = 20 + 30 = 50 kmph ( as they are travelling in opposite directions).
Now A can be assumed to be stationary and the total distance has to be travelled by B which he will do with a Speed = 50 kmph. So he will meet A after 200/50 = 4 hrs.Relative speed is one of the most interesting topics which is central to most physics concepts. When do you say that an object is in motion? If an object is lying completely still on the ground, is it in motion or at rest? To understand these questions one must learn the relative speed concept.
What is Relative Speed?
We can define relative speed as the speed of a moving body with respect to another. When two bodies are moving in the same direction, the relative speed is computed by their difference. But when to bodies are moving in the opposite direction, the relative speed is calculated by adding the speed of both the bodies.
The difference between relative speed and relative velocity is that relative speed is the scalar quantity whereas relative velocity is the vector quantity.
How to calculate relative speed?
Suppose two bodies are moving at a different speed in the same direction.
Let the speed of 1st body be x km/hr
And the speed of the 2nd body be y km/hr.
So, their relative speed is = (x – y) km/hr [x > y]
Then, The time after which both the bodies meet = distance travelled / relative speed = d km/ (x – y) km/hr
Now, we already know that relative speed is the speed of one object with respect to another.
So, when time after which both the bodies is given,
Suppose, time = t hrs.
Then, the distance covered in ‘t’ hours = relative speed * time = (x – y) km/hr * t hrs.