# Homework Help: Kinematics Acceleration

1. Jul 27, 2016

### Nanu Nana

1. The problem statement, all variables and given/known data
A sled starts from rest and slides down an inclined plane with constant acceleration. In three seconds, it has travelled 9m. How long does it take to reach a velocity of 24m / s?

2. Relevant equations
d = vi * t + ½ * a * t^2, vi = 0

vf = vi + a * t, vi = 0

3. The attempt at a solution
d = vi * t + ½ * a * t^2, vi = 0
9 = ½ * a * 3^2
a = 18 ÷ 9 = 2 m/s^2
To determine the time, we need to determine the sled’s acceleration.

vf = vi + a * t, vi = 0
24 = 2 * t
t = 12 seconds.

I solved this problem with help of someone else . The thing I don't understand is why can't we use 9,81 m/s2 for acceleration . Because the sled is sliding down . Doesn't it has to do with gravity ?

2. Jul 27, 2016

### rpthomps

You can't use 9.81 because the ramp is not completely vertical. Only a portion of the acceleration due to gravity is accelerating the sled

3. Jul 27, 2016

### Nanu Nana

But this can be a 2D motion right ? with horizontal and vertical components ?

4. Jul 27, 2016

### rpthomps

yes, it can.

5. Jul 27, 2016

### rpthomps

To explore it using horizontal and vertical components you would need the angle of the plane though

6. Jul 27, 2016

### Nanu Nana

Oh thats right . The angle is indeed not given . Thank you very much . I know it was a very dumb question .

7. Jul 27, 2016

### rpthomps

not a dumb question at all