Find Velocity of 4.3kg Mass Using Spring Constant 480 N/m

[lizlyn]

Homework Statement

A mass and spring are arranged on a horizontal, frictionless table as shown in the figure below. The spring constant is k = 480 N/m, and the mass is 4.3 kg. The block is pushed against the spring so that the spring is compressed an amount 0.38 m, and then it is released. Find the velocity of the mass when it leaves the spring.

Homework Equations

energy = (1/2)kx ---> k being 480 N/m
---> x being .38 m
energy = (1/2)mv^2 --- solving for v

The Attempt at a Solution

E = (1/2)(480)(.38) = 91.2
91.2 = (1/2)(4.3)(v^2)
v = 6.51 m/s

I'm not sure what I'm doing wrong?

 

[gneill]

energy = (1/2)kx ---> k being 480 N/m
---> x being .38 m
energy = (1/2)mv^2 --- solving for v

Check the units in your energy expressions.

 

[lizlyn]

I'm not sure what units to switch, would it be kg to N?

 

[gneill]

Make sure that both of your formulas for energy actually produce results in energy units.

 

[intwo]

Lets try some dimensional analysis. In your first equation for energy, you have:

E = .5(k)(x)

If we break this down into units, you have:

Newtons*Meters = (Newtons/Meters)*Meters

The meters cancel out on the right side, therefore making the equation incorrect. Newtons*Meters does not equal Newtons. So what do you need on the right side of the equation to balance it out?

 

[lizlyn]

oh ok! .. once i squared the meters and substituted the values I got the correct answer. Thank you!