Learn How to Calculate Average Speed in Physics - Simple Problem Solved

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In summary, the problem involves finding the average speed of a car travelling from point A to point B and back, with different speeds for each direction. The distance between A and B is not given, but we can use it as a variable (D) to solve the problem. The average speed is found by dividing the total distance by the total time, which we can calculate by finding the individual times for each speed and adding them together. The final average speed is 68.27 km/h.
  • #1
Hi everyone. I'm taking my very first physics course and can't seem to get a grasp on it. I can solve some problems without problems but get stuck at simple things. I need a little hint on this problem:

A car drives from A to B, half the time at 55 km/h and the other half at 90 km/h. On the way back, the car travels half the distance at 55km/h and the other half at 90km/h. What is the average speed from B to A?

Thanks in advance!
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  • #2
Cute problem.

The problem doesn't tell us what the distance between A and B is so let's call it "D" km.
Notice that the problem only asks for the average speed driving back from B to A. All the first part, about how the person got from A to B is irrelevant.

The person drives back from B to A (presumably the same distance D!) going half the distance at 55 mph. If we let T1 be the time at 55 km/h, then 55T1= D/2 so T1= D/110.
He then drives half the distance at 90 km/h. If we let T2 be the time at 90 km/h, we have 90T2= D/2 so T2= D/180.

To find "average speed", we need to divide the distance by the total time so we need to find total time:
The total time is T1+ T2= D/110+ D/180= ((18+ 11)/1980)D= (29/1980)D (180= 18*10 and 110= 11*10 so the least common denominator is 18*11*10= 1980). You could, of course, just use a calculator to get 0.00909+ 0.00555= 0.01465= 29/1980.

The average speed is D/((29/1980)D)= 1980/29= 68.27 km/h.

What is average speed in physics?

Average speed in physics is the overall rate of an object's motion over a specific period of time. It is calculated by dividing the total distance traveled by the total time taken.

How do I calculate average speed?

To calculate average speed, you need to know the total distance traveled and the total time taken. The formula for average speed is: Average Speed = Total Distance / Total Time.

Can average speed be negative?

Yes, average speed can be negative. This means that the object is moving in the opposite direction of its initial direction of motion. For example, if an object moves 10 meters forward and then 5 meters backward in 2 seconds, its average speed would be -2.5 meters per second.

What units are used for average speed?

The units used for average speed depend on the units used for distance and time. For example, if distance is measured in meters and time is measured in seconds, then average speed would be measured in meters per second (m/s).

How is average speed different from instantaneous speed?

Average speed is calculated over a period of time, while instantaneous speed is the speed of an object at a specific moment in time. Average speed takes into account the overall motion of an object, while instantaneous speed reflects the speed at a specific point in time.

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