Question

One speed skater starts across a frozen lake at an average speed of 8 m/s. Ten seconds later, a second speed skater starts from the same point and skates in the same direction at an average speed of 9 m/s. How many seconds after the second skater starts will the second skater overtake the first skater?

Answer 1

Answer: Hello mate!

Let's define the variable t as the time, and define t = 0 as the moment when the first skater starts to move:

We know that the speed of the first skater is 8 m/s, and we need to find the position as a function of time, then we need to integrate the velocity over time

if v1(t) = 8m/s

then p1(t) = (8m/s)*t

now we also know that the second skater has a velocity of 9m/s and enters in the frozen lake at t= 10s.

then the velocity of the second skater is: v2(t) = 9m/s, and the position is:

p2(t) = (9m/s)*(t - 10s)

now we want to know how many seconds after the second skater starts are needed for the second skater to overtake the first one.

this is equivalent to see when his positions will be equal.

so p1(t) = p2(t):

(8m/s)*t = (9m/s)(t - 10s) = (9m/s)*t - 90m

(8m/s)*t - (9m/s)*t = -90m

(-1m/s)*t = 90m

t = 90m/(1m/s) = 90s

Then in t = 90 seconds, the second skater will overtake the first one, and knowing that the second skater started at t = 10 seconds; there are 80 seconds after the second skater started needed to overtake the first skater.