A 0.100-kg toy car is propelled by a spring with force constant 250.0 N/m, compressed 4.00 cm. Assuming negligible friction and that the spring's energy converts completely to kinetic energy on level ground, what is the car's speed just after leaving the spring?

1. With height unchanged, conservation of mechanical energy gives one half k xi squared equal to one half m vf squared. 2. Solving for the final speed, vf equals the square root of (k/m) multiplied by xi. 3. Substituting the values, vf = the square root of (250.0 N/m divided by 0.100 kg), times 0.0400 m. 4. The square root of 2500 is 50.0, and 50.0 times 0.0400 m gives vf = 2.00 m/s. 5. 4.00 m/s would come from forgetting to take the square root of the stiffness-to-mass ratio, and 0.400 m/s and 1.00 m/s come from misplacing a decimal point in the compression distance. _Source: OpenStax College Physics (CC BY 4.0), Ch 7 "Work, Energy, and Energy Resources", section 7.4 Conservative Forces and Potential Energy_