CONSERVATION OF ENERGY

GOAL

The goal of this work is to investigate how energy is transformed between potential and kinetic forms.

PREREQUISITES

R.D. Knight, Physics for scientists and engineers, chapters 10, 11.

EQUIPMENT

Rubber band

Ruler

Protractor

Meter stick

Triple beam scale

Precision Weight Scale

String

THEORY

The spring force in string or rubber band can be calculated using Hook’s law

F = -k·s,

where F is the force of the spring (rubber band), k is the spring constant, and s=Δx=x-x₀ is the distance the spring (rubber band) is displaced from equilibrium position x₀. The potential energy of the spring (rubber band) is given by

U = ½ks².

This energy can be converted either in kinetic energy

K = ½mv²,

or into another potential energy, for example gravitational potential energy

U = m·g·h,

where h is the height of the object.

PROCEDURE

1. Determine the spring constant measuring the displacement of the rubber band. NOTE, that the maximum displacement should not exceed 10 cm not to damage the band. Picture below illustrates the procedure. Record your data in the table and determine the pring constant.

s (m)	F (N)	k (N/m)
±	±

2. Weight the rubber band and record its mass ____________________ .

3. Measure the distance the rubber band must be stretched such that it can be shot from some reference point (the floor, table top, etc.) directly up to just reach the ceiling. Clearly show your calculations in the space below. Record this displacement, the potential energy stored in the rubber band, and the change in the rubber bands potential energy when reaching the ceiling.

N of trial	Displacement (m)	Rubber band’s potential energy	Rubber band’s gravitational energy
1
2
3

ANALYSIS

1. Make all necessary calculation to fill both Tables completely.

2. Calculate the fraction of the rubber band’s spring potential energy transferred to the kinetic/potential energy of the rubber band.

3. Explain your results.