Using Newtons law and mechanical energy?

[jimmyboykun]

1. Homework Statement [/b]
A car is rolling down a ramp that is inclined at and angle of 9.9 degree with respect to horizontal. There is no friction acting from the road on the tires.


Employ conservation of mechanical energy to find the speed of a car that starts from rest and rolls down the ramp a distance of 52 m.

2. Homework Equations [/b]

1/2mv(initial)^2+mgh(initial)=1/2mv(final)^2+mgh(final)


3. The Attempt at a Solution [/b]
Vi=0m/s
Vf=?
hi=52m
hf=0

52mg=1/2vf

2*52m*9.81m/s^2=vf

√1020.24m^2s^2=√v

31.94m/s=v

I got a 1 out of ten in this problem. most likely got the units right, but my math did I used the right equation?

 

[subzero0137]

The initial height is not 52 m. Draw a diagram to see why this is the case.

 

[jimmyboykun]

The initial height is not 52 m. Draw a diagram to see why this is the case.

ok I drew the free body diagram initial height is not 52m. would the 52 m be considered Δh?

 

[Tsunoyukami]

Not quite - 52m is the distance the car travels down the ramp. It's change in height is not equal to this distance because this distance is composed of both the x and y distances. :)

 

[jimmyboykun]

Not quite - 52m is the distance the car travels down the ramp. It's change in height is not equal to this distance because this distance is composed of both the x and y distances. :)

ok in this equation, where would the 52 m be placed, or am I using the proper equation for mechanical energy?

 

[subzero0137]

ok I drew the free body diagram initial height is not 52m. would the 52 m be considered Δh?

No.
HINT: You have to use the value of the angle of the ramp and 52 m to get your Δh.