# Physics Lab 3 (Falling Ball)

Physics Laboratory 3

Falling Ball Kinetic and Potential Energy

http://physics.weber.edu/amiri/director/dcrfiles/energy/FallingBallS.dcr

We will use several simulations to explore the relationship between kinetic and potential energy in a few different systems. Kinetic and potential energy represent the forms of mechanical energy.  As you learned in Chapter 3, kinetic energy is a property of an object based on its mass and its speed.  On the other hand potential energy is associated with the position of objects within a system.  The two types of potential energy that are most common are gravitational potential energy and elastic potential energy.  The mechanical potential energy signifies that a system is capable of doing work.  Often the work being done by the system results in an increase in the kinetic energy of the objects in the system.  If we ignore friction (forces that would cause energy to be changed from mechanical energy to another form of energy – general heat) in the system, then the total energy of the system will remain the same.  Since the total energy remains the same, yet the kinetic (or potential) energy of the system changes in value, then the other (kinetic or potential) form of energy must also change accordingly to keep the total amount of energy constant.  Now let’s use the simulations to explore these concepts.

The first simulation is a falling ball in which the type and amount of energy are tracked as a ball falls from rest to a point as before contacting the ground.  If the ball did contact the ground, then we would have other areas of physics to worry about and account for in the motion of the ball, such as momentum, impulse, and energy loss.  The simulation tracks kinetic, potential, and total mechanical energy with bars and numbers.  Use the simulation to answer the following questions.  You can adjust the height from which the ball drops by clicking and dragging the red arrowhead near the ball.

1.   Release the ball from a height of 55.0 m and fill out the table below.
Ball Location Potential Energy (J)Kinetic Energy (J)Total Energy (J)Release Point1078.0 J0 J1078.0 Just Before Ground0 J1078.0 J1078.0 J

2.   Release the ball from a height of 30.0 m and fill out the table below.
Ball Location Potential Energy (J)Kinetic Energy (J)Total Energy (J)Release Point588.0 J0 J588.0 Just Before Ground0 J588.0 J588.0 J

3.   What equation is used to calculate the gravitational potential energy?
$Ep = m*g*H$, where m is the mass of the ball, g is the gravitational acceleration and $H$ the height of the ball

4.   What equation is used to calculate the kinetic energy?
$Ec =m*v^2/2$,  where m is the mass of the ball, v the speed of the ball

5.   What equation can be used to calculate the speed of the ball during the fall? Calculate the speed of the ball just before it hits the ground in both cases above: (a) 55.0 m and (b) 30.0 m.
$V^2 =V0^2 +2*g*H$

where V is the final speed, V0 the initial speed, g the gravitational acceleration and H the initial height of the ball

in our case V0 = 0 m/s

$V =\sqrt{2*g*H}$

for $H = 55.0 m$,  $V = \sqrt{2*9.8*55} =32.83 m/s$

for $H =30.0 m$,  $V = \sqrt{2*9.8*30} =24.25 m/s$

6.   If the mass of the ball were changed, how would the speed of the ball change in each case?
The fall of the ball does not depend on its mass. Therefore if the mass of the ball is changed its final speed will not change.

7. Calculate the mass of the ball in the simulation.  The lab tells you what it is, how would you calculate it.

$Mass = Potential-energy/(g*H) = 2*kinetic energy/V^2 =2*1078/32.83^2 =2 kg$