Trouble understanding simple physics problem, acceleration.?

Question

A car accelerates at 5 m/s/s. Assuming the car starts from rest, how much time does it need to accelerate to a speed of 20 m/s?

i think there is suppose to be some sort of formula, but i just can't remember it. can anyone please explain how I can get the answer of this problem? I don't just want the answer, i want to know how to get it. thanks!

Answer 1

Vfinal=Vinitial+at

20=0(starting from rest)+5(t)

20=5(t)

t=4

4 Seconds.

Answer 2

Your final edited problem:

This is a good vector problem, if you pay careful attention to direction and signs.

Let the direction upward be +ve. Then:

v - final velocity - what you are solving for

u = initial velocity in this case +30m/s

a = acceleration In this case -9.81m/s² (-ve because acceleration is downward.)

t = time which has no direction, but taken as +ve 9 seconds.

Substitute into equation:

v =u+at

v = 30 +(-9.81)9

V = 30-88.29

v = -58.29m/s

The negative sign tells you that the velocity is downwards:

Answer: the stone has a velocity of 58.3m/s downwards

Answer 3

Well every second the car travels 5m/s faster than the previous second. So after 2 seconds it is 10 m/s, third second 15m/s and fourth second it travels at 20m/s.

So very easy question, 4 seconds