Question

Brad is working on a speed problem in physics class. The problem tells him that a girl runs from her house to the park 0.05 km away in 10 s. Brad calculates that her speed is 0.005 m/s. Is he correct? If not, explain the flaw or flaws in his problem solving process.

Answer 1

It is corecct becasue it 10 s more added from 0.05

Answer 2

Answer:

He is incorrect!  Her speed was 5m/s.

Explanation:

For calculating the speed, first we shall remember that:

$v=\dfrac{d}{t}$

Where $v$ is the speed, $d$ is the distance travelled and, $t$ is the time it takes to travel distance $d$.

So one migth think that velocity can be easely compute:

$v=\dfrac{0.05}{10}$

$v=0.005\dfrac{m}{s}$

Be carefull, he does not make a proper dimensional analisis!  

Before computing the speed we must know in what dimensions our values are.

$d=0.05km$, distances is measure in Kilometers.

$t=10s$, time is measure in seconds.

If we want our speed to be in $m/s$, first we need to be sure that our values are expressed  in meters and seconds.

Time is already  expressed in seconds, distance is not in Kilometers.

So

$0.05Km=50m$,

now we can compute the speed:

$v=\dfrac{d}{t}$

$v=\dfrac{50m}{10s}$

$v=\5dfrac{m}{s}$