Lab Assignment 9: Gravity

Instructor’s Overview

As you continue with your study of physics, you’ll learn that there are four fundamental interactions in nature. These interactions are summarized in this table:

Interaction	Comments	Relative strength	Effective range
Strong nuclear force	Binds protons and neutrons in nuclei	1	10-15 m
Electromagnetic force	Force between charged particles. Follows inverse-square law.	10-2	Infinite
Weak force	Responsible for certain types of radioactive decay	10-13	10-18 m
Gravitational force	Force between all objects. Determined by magnitude of masses and separation. Follows inverse-square law.	10-38	Infinite

So gravity is one of the four fundamental forces of nature. We will learn about the other three in Physics II. Some interesting things to note about the gravitational force:

• In a relative sense, it is the weakest of the four fundamental interactions. Even so, gravity is largely responsible for the dynamics of our solar system, galaxy, and the Universe as a whole.
• Gravity acts over an infinite distance. Objects may be separated by thousands of light years, but they experience gravitational attraction.
• Gravitational interactions obey the inverse-square law.
• Coulomb’s Law dictating the force between two charged particles is also an inverse-square relationship.

In this lab, you will perform experiments that illuminate the concept of gravitational interaction.

This activity is based on Lab 10 of the eScience Lab kit.

Our lab consists of two main components. These components are described in detail in the eScience manual. Here is a quick overview:

• Experiment 1: In the first part of the lab, you will subject a variety of objects to the force of gravity and discuss your observations.
• Experiment 2: In the second part of the lab, you will use a flashlight as an analogy to demonstrate the inverse-square law of gravity.

Take detailed notes as you perform the experiment and fill out the sections below. This document serves as your lab report. Please include detailed descriptions of your experimental methods and observations.

Date:

Student:

Abstract

Introduction

Material and Methods

Results

Based on your results from the experiments, please answer the following questions:

Experiment 1: Falling in a Gravitational Field

1. The following expression for the acceleration due to gravity works well for objects near the Earth’s surface (G is the gravitational constant, M_e is the mass of the Earth, and R_e is the radius of the Earth):

Use the above equation to calculate the gravitational acceleration at an altitude of 100,000 meters above the Earth. By what percentage is this acceleration different from that on the Earth’s surface?

1. How does air resistance alter the way we perceive falling objects?

1. Using the universal law of gravitation, show that the gravitational acceleration experienced by an object is independent of its mass.

Experiment 2: Inverse Square Law

Data table

Distance from wall	Diameter of light circle	Radius of light circle	Area of light circle

NOTE: Be sure to specify your units.

• How did the intensity (brightness) of the circle of light seem to changes as you increased the source distance?
• How did the area of the circle of light change as you moved farther away? How does this relate to the gravity field of a body?
• Make a plot of distance vs. area for the light shining from the cup to the wall (plot the area data on the y‐axis).
• Draw a line of best fit through your data points, and comment on the general shape of your plot. What kind of relationship does this indicate (i.e. linear, exponential, logarithmic, etc.)?
• If the amount of light shining from the flashlight is constant, the intensity of light should decrease as the area it shines on gets larger. Using your plot from question 2, sketch an approximate plot for light intensity vs. distance.
• If you moved too far away from the wall the circle of light probably became very difficult to see. Is the intensity of the light circle ever zero? Is it correct to say that gravity doesn’t exist between objects on opposite sides of the galaxy?

Conclusions

References