Can anyone help me with physics here?

Question

Can I e-mail anyone?

The constant speed of a runner was 30 km/hr, they slowed down to 5 km/hr at a rate of -6km/hr/s...How long should it take the runner to get to the 5 km/hr walk?

What in the world does this mean? How do I solve it? I already tried to google velocity and acceleration...I don't get it.

Answer 1

The change in velocity is 25 km/hr (start speed of 30km/hr minus end speed of 5 km/hr).

Every second, the runner is going 6km/hr slower. (change in acceleration)

So how many seconds would it take to change the velocity by 25 km/hr?

T+0 (before slowing down) = 30

T+1 = 24

T+2 = 18

T+3 = 12

T+4 = 6

T+5 = 0

Just by this method, you know it has to be between 4 and 5 seconds.

Divide your change in velocity by your acceleration (both values will be negative since both speed and acceleration are decreasing).

-25/-6  = 25/6  = 4.16667

It would take the runner 4.16667 seconds to reach the 5km/hr speed.