A cross- country runner runs 5.0km east along the course then turns around and r
ID: 1691802 • Letter: A
Question
A cross- country runner runs 5.0km east along the course then turns around and runs 5.0 km west along the same path.She returns to the starting point in 45 minutes.a) What is her average speed in Km/minute? b)What is her average speed in m/minute? c)WHat is her average speed in m/s? b)Whta is her average velocity?
Please help I know that average velocity in V=distance/time which the distance i believe is 5.0 and time is 45 minutes but who do i make it to km/minute,m/minute,m/s,and then they ask for average velocity am confused and i don't know where to start.
Explanation / Answer
Keep in mind that speed is a scalar quantity and velocity is a vector quantity (taking into account direction). For part a) speed = distance/time. Her TOTAL distance run was 10.0km, because she ran 5.0km east and 5.0km west, with a total time of 45 minutes. In km/min it's 10km/45min or 0.22km/min. For part b) you need to make a converstion of km to m. There are 1000m in 1km, therefore you need to multiply 0.22km/min by 1000m/km, cancelling the km units leaving you with 220m/min. For part c) you then need to convert minutes to seconds. There are 60 seconds in 1 minute, so you need to multiply 220m/min by 1min/60sec in order to cancel out the min units and have seconds on the bottom, resulting in: 3.7m/s For part d) you need to recognize the difference between a scalar and a vector quantity. A scalar quantity has a magnitude but no direction, we don't know where the person is running, for all we know the person could have run 10.0km in the same direction, but since we're given vector quantities (5.0km east and 5.0km west) we can determine the average velocity as a sum of the vector quantities / time. If you define east to be in the positive direction and sum the velocities you get 5.0km ***minus*** 5.0km (divided by time, but that doesnt matter since the displacement is 0), because west is then the negative direction. Therefore her average velocity is 0. (remeber average velocity = **DISPLACEMENT** / time. Since she returned to her original starting position her displacement is 0.)
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