Laboratory Activity 6: Gravity

/ Group member names:
-Shawn Brown
-Steve Gormely
-Won-Yong Shin

Objectives

  • Explore how gravity affects different objects.
  • Develop a quantitative understanding of the gravitational force.
  • Learn about proportional and inversely proportional graphs.

Equipment

  • Computer
  • Ultrasonic motion sensor
  • USB Link
  • Basketball
  • Racquetball
  • Tow ball

Activity One: Motion of a falling object

Prediction 1: Using the line tool on the drawing toolbar of Word, draw a line on both the velocity and acceleration graphs below predicting the shape of a curve for a ball being dropped.

Set up the equipment: Obtain a USB Link and attach it to the computer. If there is not already a Motion Sensor mounted on the ceiling that you can use, get one and hook it on the frame of the drop ceiling. Put the detector on Person setting. Open the experiment file called FallingBall.ds to display acceleration and velocity graphs. When you use the activity, you should notice two things. First, the coordinate system in the file has been reversed so that up (towards the detector) is the positive direction. Second, the rate of data collection has been increased, since the motion of the falling ball is faster than that of the carts that we have worked with.

Experiment 1: One partner should hold the ball near the detector, careful to keep it at least 30 cm away, while the other takes data. When you are ready, the first person should let the ball drop straight down as the second starts the data collection. When you have a nice graph, highlight the section of the graph that the ball was falling so that it is marked yellow, copy the graph and paste it in the section below.


-paste graph here-

Question 1: What type of motion is the falling ball (constant velocity, constant acceleration, increasing or decreasing acceleration)? Is the sign of the acceleration and velocity positive or negative? Have you seen any other objects in a lab move with this type of motion?

Constant acceleration. Negative acceleration. Negative velocity. Yes.

Experiment 2: Measure the average acceleration of the ball during the time it is falling freely. Do this by click-dragging a rectangle around the data points with the mouse, and then selecting “mean” from the “Statistics” button at the top of the window. (Don’t forget units.)

Average acceleration / -9.4m/s^2

Now measure the average acceleration by finding the slope of the velocity graph. Select the region of the velocity graph in which the ball is falling freely, and then use the fit button to do a linear fit. As you know, the slope of the line is the acceleration.

Average acceleration / -9.4736m/s^2

Question 2: How do the two values compare? Should they be the same or not?

They should be very similar if not the same. They should be the same.

Experiment 3: Now take the ball and hold it below the detector around waist level. While your partner takes data, carefully through it straight up so it goes up and back down in the view of the detector. It may take several practice tries until you can throw it straight up without it coming too close to the detector. Once you have a good graph, copy it and paste it below.

-paste graph here-

Question 3: Look at your graph and identify with labels or arrows the time in which the ball was being thrown, when it is on the way up, when it is at the top of the flight, when it is coming down, and when it hits the ground.

Prediction 4: Suppose you were given a light object, such as a racquetball, and a heavy object about the same size, such as the ball for towing. If you dropped them at the same time, which object would hit the ground first?

They will land at the same time.

Get a racquetball and tow ball from your teacher and weigh them, recording the masses below

mass racquetball / 41g
mass tow ball / 810g

Experiment 4: Take the two objects, hold them with the bottoms at the same level, and drop them at the same time onto a piece of cardboard.

Question 4: Did one of the objects hit the ground significantly before the other? Does the acceleration due to gravity depend on the mass of the object? Does the force due to gravity depend on the mass of the object?

No they hit at the same time. Acceleration due to gravity does not depend on the mass of the object. Yes the force of gravity is directly proportional the mass of an object.

Summary

The following questions will help you get the main ideas out of this lab. You should find these straightforward questions, but take the time to talk it over with your team and write complete answers to these questions. You may find your answers here to be the most useful part of this lab down the road.

Summary 1:For an object falling near the surface of the earth, how does the force on it change depending on its speed and direction of motion?

The force will stay the same.

Summary 2: For an object falling near the surface of the earth, how does the force on it change depending on its mass? Using what you know from Newton’s laws, can you write down a mathematical expression that gives the force of gravity for an arbitrary object?

Fg = M(Ag)

Summary 3: How do you know your above statements are true? What experimental observations and/or logical reasoning can you give to justify what you said in summary questions 1&2?

All objects fall at a rate of –9.8 m/s^2. The trailer hitch ball and the racquetball landed at the same time. The trailer ball hit the ground with a greater force then the racquetball because it has a larger mass.