# Kinetic Energy and skateboarder

A50-kg skateboarder design a creative way to gain enough speed to coast up a ramp. By compressing a stiff spring. H e finds that he can go 1.5 m up the ramp?
A) How much kinetic energy does the spring give him?
B) The energy in the spring can be calculated using the expression 1/2kx^2 where k= 1470N/m and is called the spring constant and x is the distance the spring is compressed. How much was the spring compressed in this case?

My Proof

A)
Eg= mgh
Eg=50kg*9.8N/kg*1.5m
Eg=735J
So Eg=Ek
Ek=735J
B)
735J=1/2*1470N/m*x^2
x^2=1470J/1470N/m
x=1m
Is That right or Wrong?

Dick
Homework Helper
Looks fine to me.

Looks fine to me.
r u 100% sure?

Dick
Homework Helper
Assuming 'distance up the ramp' means vertical distance, yes. Why aren't you sure??

Assuming 'distance up the ramp' means vertical distance, yes. Why aren't you sure??
i am confuse in B.

Dick
Homework Helper
Confused about what? Spring potential energy=gravity potential energy.

Confused about what? Spring potential energy=gravity potential energy.

thaxxxxx u

PhanthomJay
Homework Helper
Gold Member
The wording on this problem has me confused also. I guess it's 1.5m vertically up from the compressed position of the spring. So that gives him 735J of gravitational POTENTIAL energy at the top, and the spring compression must be 1 meter, as you and Dick have confirmed. But part A asks about how much KINETIC energy he has. He's got none at the top, none at the start, and some varying amount in between. Are you sure part A asks for Kinetic Energy, or is this a typo??

Dick
Homework Helper
It only makes sense if the kinetic energy referred to is his kinetic energy after using the spring and before ascending the ramp. It could have been written more clearly...

PhanthomJay
Homework Helper
Gold Member
It only makes sense if the kinetic energy referred to is his kinetic energy after using the spring and before ascending the ramp. It could have been written more clearly...
But I don't think there is enuf info given to solve it, since he will have GPE as well as KE just after using the spring, which is unknown without knowing the slope.

Dick
Homework Helper
But I don't think there is enuf info given to solve it, since he will have GPE as well as KE just after using the spring, which is unknown without knowing the slope.

Not if there is a straight run before hitting the ramp. In any event question A should be rewritten to ask "how much energy did the spring give him". Not "kinetic energy". Then the answer is unambiguous and not subject to nit picking.

PhanthomJay
Homework Helper
Gold Member
Not if there is a straight run before hitting the ramp. In any event question A should be rewritten to ask "how much energy did the spring give him". Not "kinetic energy". Then the answer is unambiguous and not subject to nit picking.
Yes, I wasn't envisioning the 'straight run' concept.

here diagram

http://img389.imageshack.us/img389/1103/physicrs1.png [Broken]

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PhanthomJay