Physics 4A Lab 6 Newton’s 2nd Law

Parts List

Part / Quantity
Track / 1 /
Cart / 1 /
“LabPro” interface / 1 /
Motion Detector / 1 /
Force Probe / 1 /
Weight Set and 500gr weight bar for the cart. / 1 /
Weight Hanger / 1 /
Table Clamp
(only for station #1. Also add: 2 short rods and 2 right angle clamps for this station / 1 /
Pulley / 1 /
Pulley clamp / 1 /
Digital Scale / 1 on the front bench /

Physics 4A Lab 6 Newton’s 2nd Law

Overview

Imagine that all the mass scales in the world disappeared and the only way to find the mass of an object was by using Newton’s Law and the motion track and motion detector in this lab. You have a set of weights with the mass stamped on them and you are fairly confident in their values. You’re job is to determine the mass of a cart. You will do this with three different methods and compare your results. In both cases, the goal is to find the MASS of the CART. Have Fun!

Part I. Find the Mass of the Cart with a Varying Applied Force

In the set up shown below, the weight of the hanging mass M accelerates the system. We can measure the acceleration of the cart, a, with the motion detector. Assuming that the tension in the string and the acceleration of the cart and the hanging mass are the same we can apply Newton’s Second Law to both the and derive an equation for the mass of the cart:

·  Derive equation (1). Show free body diagrams for each object. Inlcude a copy in your lab report. You can do this derivation by hand.

·  Construct the set up as shown here.

·  Open file :”4A_NEWTON_2_A”

·  Place 200 grams on the hanger. This is the mass M. Hit “collect” and release the cart and mass M and let it go!

·  From the graphs find the average acceleration and record it in your data sheet.

·  Repeat the procedure for all the masses, M, listed in table I.

·  Complete the calculations for the hanging mass M in kilograms, and the mass of the cart, m.

·  Find the average value of the mass of the cart and its average deviation. Express your final answer for the mass with its uncertainty in standard form.

·  Now measure the actual mass of the cart on the scale and enter the value in the table.

·  Calculate the percent difference between the measured (true) value of the mass of the cart your experimental value.

·  Does the true value of the cart fall within your experimental uncertainty? Explain.

Part II. Find the Mass of the Cart with a Varying Mass

This time we will have a constant applied force (constant mass M on the hanger) and we will vary the mass on the cart itself! We will add known masses to the cart. Again, we use Newton’s second law to find the mass of the cart from equation (1) above but this time the mass of the cart includes the added masses so that . You will have to subtract off the added mass to find the mass of the cart. Why do it this way? Wait and see….

·  We will use the same set up as part I.

·  Open file :”4A_NEWTON_2_A”

·  Place a 150gr mass on the hanger and hit “collect”. This will be a constant accelerating mass.

·  Find the average acceleration and record it in your data sheet.

·  Using equation (1) calculate total mass on the cart and then find the mass of the cart from . Put these values in your data table. Be sure to use correct units!

·  Place extra mass (listed in table 2) on the cart and repeat the measurement.

·  Find the average value of the mass of the cart and its average deviation. Express your final answer for the mass with its uncertainty in standard form.

·  Using the measured value of the mass of the cart from Part I, calculate the percent difference between the measured (true) value of the mass of the cart your experimental value from Part II.

·  Does the true value of the cart fall within your experimental uncertainty? Explain.

Part III. Find the Mass of the Cart with a Force Probe.

This time we will use a force probe to measure the force applied and the acceleration of the cart and then find the mass of the cart directly from Newton’s Second Law:

·  Construct the set up as shown here.

·  Open file ”4A_NEWTON_2_B”

·  Calibrate the force probe.

·  Hold the force probe horizontally and zero it.

·  Move the force probe away from the motion detector in such a way to produce a constant acceleration for the cart. You may need to perform several trials to get a satisfactory graph.

Include a copy of the force and acceleration graphs in the lab report.

·  Find the average force and accelerations from the graphs and put them in your datasheet.

o  Calculate mass of the cart, m, from m = F/a and record it in your datasheet. (There is only one measurement to be made here and no uncertainty.)

o  Using the measured value of the mass of the cart from Part I, calculate the percent error between the measured (true) value of the mass of the cart your experimental value from Part III.

Part IV: Analysis

Which method gave you the best results? The worst? Why? Explain in nice detail.