How do you calculate initial velocity?

Question

Hi,

I am a "young" kid in 7th grade, so your answers should be simple. In a form like Speed = distance/time. Like an understandable and well explained form.

So how do I calculate initial speed if I have got:
acceleration : 8 m/s 2
time: 5 s
final velocity: 30 m/s

Thanks!

Answer 1

initial velocity = final velocity - acceleration * time
so
initial velocity = 30 m/s - (8m/s2)(5s)
= -10-m/s

Answer 2

What bdizzy21... said is correct, but you asked for explanations -- that's my specialty.

The basic equation covers a situation where you know initial velocity, acceleration and time,
final velocity = initial velocity + acceleration*time

What that formula does: With a = 8 m/s^2 that means for each second, the velocity increases by 8 m/s. After 2 sec, total increase is 16 m/s, after 3 sec: + 24 m/s, etc, so after 5 sec the total increase in velocity is 40 m/s.

But what you know is final velocity, acceleration and time. So rearrange the equation, to the form bdizzy21... gave it to you:
initial velocity = final velocity - acceleration * time
initial velocity = 30 m/s - (8 m/s^2)*5 s
initial velocity = 30 m/s - 40 m/s
initial velocity = -10 m/s

OK, you might be asking: What's it mean that the sign is negative? The object started out going one way and the acceleration made it come to a stop and go the other way.