Lab Assignment 3: Newton’s Laws
Newton’s laws of motion are a central component of our understanding of physics. As we discussed in Module 5, Newton’s laws can be summarized as follows:
- 1. Inertia – An object tends to resist changes in its motion.
- 2. Relationship between the mass of an object, the net applied force, and the resulting acceleration – F = m a.
- 3. Action-reaction pairs – Forces come in pairs.
In this lab, you will perform experiments to explore each of the laws of motion.
This activity is based on Lab 5 of the eScience Lab kit. Although you should read all of the content in Lab 5, we will be performing a targeted subset of the eScience experiments.
Our lab consists of three main components. These components are described in detail in the eScience manual (pages 55-61). Here is a quick overview:
- • In the first part of the lab, you will use a bowl full of water to understand the concept of inertia. (eScience Experiment 1)
- • In the second part of the lab, you will recreate a classic physics experiment, the Atwood Machine. This system consists of a pulley holding a string with two unequal masses. Experimenting with an Atwood Machine is an excellent way to understand Newton’s second law of motion. (eScience Experiment 2, Procedure 1)
- • In the final part of the lab you will create a balloon-powered vehicle to elucidate Newton’s third law of motion. (eScience Experiment 4)
Note: Record all of your data in the tables that are provided in this document.
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.
Newton’s First Law – Water in a Bowl
- • I recommend that you perform this experiment outdoors as there most likely will be some spillage of water.
Newton’s Second Law – The Atwood Machine
- • Prior to determining the mass of the washers, make sure to zero your spring scale. To zero your spring scale, hold it vertically with no mass attach and turn the top screw until the scale reads 0 grams. Refer to the following picture:
- • You may want to use the hooks on the pulley to hang your Atwood machine. I placed mine on a hanger:
Newton’s Third Law – Balloon-Powered Vehicle
- • Here is a picture of my balloon-powered vehicle:
- • To add mass, I taped washers to the straw.
Experiment 1 – inertia – Newton’s first law of motion.
See page 119 of Physics by James Walker, 5th edition, for a statement of Newton’s First Law of Motion.
1. Fill the container with a couple of inches of water.
2. Find an open space outside to walk around in with the container of water in your hands.
3. Perform the following activities and record your observations of each motion in Table 1:
a. Start with the water at rest (e.g., on top of a table). Grab the container and quickly accelerate it.
b. Walk with constant speed in a straight line for 15 feet.
c. After walking a straight line at constant speed, make an abrupt right-hand turn. Repeat with a left-hand turn.
d. After walking a straight line at constant speed, stop abruptly.
Experiment 2 – mass and acceleration – Newton’s 2nd Law of Motion
See page 122 of Physics by James Walker, 5th edition, for a statement of Newton’s Second Law of Motion.
A diagram, equations, and free body diagram for the Atwood Machine (a pulley with hanging masses) are shown on page 176 of Physics by James Walker, 5th edition.
You will use the metal washers to make the masses. You can tie the washers to the string or use a hanger, such as a paper clip. If you do use a hanger you will have to include its mass into the total mass, mass of washers + mass of hanger.
You will use 15 washers to make the larger mass and 5 washers to make the smaller mass.
Use enough string to allow a mass to fall to floor when starting from near the pulley. The other mass is going from the floor to near the pulley.
With the masses hanging from the pulley, the greater mass near the top, measure the distance the mass will fall to the floor. Time the fall of that mass.
Calculate the acceleration of the falling mass:
y = ½ a t2 : we assume no initial velocity when you started timing, that is you just let it drop and started timing when you let go. From the above we solve for a (acceleration):
Note: This assumes you do not have a constant velocity. A constant velocity could occur with a significant pulley friction. Having masses with a large difference in value helps reduce the effect of pulley friction.
Record 10 trials.
Data table for the Atwood Machine experiment (Experiment 2, Procedure 1):
Height = __ meters
Mass of 10 washers = __ grams
Mass of 5 washers = __ grams
M1 = __ grams (lighter mass)
M2 = __ grams (heavier mass)
Fall time (sec)
Calculated acceleration (m/s2)
Experiment 3 – Balloon-Powered Vehicle – Newton’s third law of motion.
A statement of Newton’s Third Law of Motion is on page 129 of Physics by James Walker, 5th edition.
This will be easier with an assistant if available. Blow up the balloon similar to that shown in the picture above but do NOT tie the balloon. Attach a straw by taping to the balloon. Thread a string of about 10 ft. length through the balloon. Attach the string to two chairs and separate them until the string is tight.
Release the balloon and observe its motion. Does it appear to accelerate?
When released can you feel the air rushing out the orifice (nozzle if you like).
Tape a washer to the balloon and repeat the experiment noting any observed difference in the balloons motion.
ANALYSIS and DISCUSSION
Based on your experimental results, please answer the following questions:
Explain how your observations of the water demonstrate Newton’s law of inertia.
Draw a free body diagram of your containers of water from the situation in part d (After walking in a straight line at constant speed, stop abruptly). In your free body diagram, draw arrows for the force of gravity, the normal force (your hand pushing up on the container), and the stopping force (your hand decelerating the container as you stop.)
What is the direction of the water’s acceleration.
Describe two instances where you feel inertial forces in a car.
Draw a FBD for M1 and M2 in your Atwood machine. Draw force arrows for the force due to gravity acting on both masses and the force of tension
Copy the Atwood Machine acceleration equation from the text.
Using the masses M1 and M2, use the above expression to calculate the acceleration of the system. Make sure to show your calculation for the acceleration. How does this value compare to your experimentally measured acceleration? What factors may cause discrepancies between the two values?
Explain what caused the balloon to move in terms of Newton’s Third Law.
What is the force pair in this experiment? Draw a free body diagram to represent the (unbalanced) forces on the balloon/straw combination.