LAB: CONSERVATION of ENERGY   PE = KE

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INTRODUCTION

 

Potential energy is stored energy. Kinetic energy is the energy of moving object. Gravitational potential energy is the stored energy than an object has because of its position. The amount of gravitational potential energy that an object has is shown by the formula:

PE = _________ , m is the mass on the object, h is its height above ground, g = 9.8m/s/s

 

The amount of kinetic energy that a moving object has is shown by the following formula:

KE = ______________, v is the speed of the object

 

When an object falls to the ground or rolls down a ramp, its gravitational potential energy is converted into kinetic energy if we neglect friction. In that case PE = KE

In reality friction is acting of the rolling object and transfers some of the original energy is lost to heat so: PE _____ KE. In this lab we will check this relation.

 

KILLS: comparing, measuring, analyzing

 

MATERIALS: marbles or balls  meter sticks   time watch    masking tape   

I  incline planes   your enthusiasm

 

PURPOSE: A cart of mass me will roll down a ramp, you will:

· compare the final speed (bottom speed)  for different masses and heights                   

· compare PE to KE for different mass and height

 

PROCEDURE

 

Step0: find the mass of the 3 balls using a digital scale (from chemistry lab). Convert to kg and record the values in table 1

 

Step1: Set the incline plane on a thin book. Mark the departure position with a masking tape and find the height from the table to the mark. H= _______ cm = ________m

 

Step2: Mark the final position, down the ramp, with another masking tape. Find the distance between the 2 tapes. D = _______cm = ________m

Step7: Use your values for the final speed to find the kinetic energy at the bottom of the plane for each ball. Record your value in TABLE 2. (Remember? KE = 0.5 mV² , m in kg)

 

Step8: Use height of the ramp and the acceleration du to gravity to calculate the potential energy of each ball. Record in TABLE 2 (remember? PE = mgH , m in kg and H in m)

 

 

 

 

 

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Step3: Place ball #1 at the top of the ramp and time how long it takes to reach the bottom.

Record the result in table1.

 

Step4: Repeat step3 with the same ball 2or 3 more times. Record in table1

 

Step5: Find the average speed for each ball. Record in table 1.

 (remember ? Average speed = total distance / total time  so Vaverage = D / T

 

Step6: Find the final speed V2 for each ball. Use the formula: V2 =  2 x Vaverage

You can derive this expression using the equations of motion. The speed increases over time because of the force of gravity. The instantaneous is therefore not equal to the average speed. V2 is the speed at the bottom of the plane.

(the average speed is not equal to the instantaneous speed at a given time)

Record in TABLE1

 

TABLE1:

 

 

 

TABLE 1:

ANALYSIS

 

1) For each ball, compare the PE and the KE.

/ball 1: the KE is : the same? Larger ? Smaller ? Than PE_________________________

Ball2: the KE is : the same? Larger ? Smaller ? Than PE_________________________

Ball3:the KE is : the same? Larger ? Smaller ? Than PE_________________________

 

Without ____________ PE = KE , the energy is conserved but in real life ____________________

 

2) Compare the PE of the three balls. As the mass increases the PE _________________

Does it make sense ? ____________________ (knowing PE = mgh)

 

3) Compare the final speed for the three balls. Is there a big difference ? ___________

 

 

4) Suppose there is no friction. PE=KE or mgh = 0.5 m v²                 

Solve for V = ________________  (use g, h, maybe m)

Does V depend on the mass ? Not matter the mass, the final speed is the same as long as there is no friction. Did you get that in 3 ?

 

5) According to the expression of V found in 3), do you expect the speed to increase if you increase the height ? __________

Try to check.