Lab 1: Measuring of the Acceleration Due to Gravity. Geology 202: Earth s Interior

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1 Lab 1: Measuring of the Acceleration Due to Gravity Geology 202: Earth s Interior Introduction: In order to make constraints on Earth s average density o,weneed to know its mass M and volume V. The mass is the volume integral of the density, which can be written as an integral over Earth s radius, a, assuming that density varies only with depth: The average density M = 4 a (r)r 2 dr. 0 o is found simply by dividing the mass of Earth by its volume using the approximation that Earth is a perfect sphere of radius a. o = M /(4/3) a 3. (1) As shown above, aprimary constraint on Earth s density is its mass, which can be found from the acceleration of gravity at the surface using Newton s law of gravitation. This law states that the force F exerted by a body of mass m 1 on another with mass m 2 depends on the two masses, the distance R between them, and the gravitational constant G: F = Gm 1m 2 R 2. Using Newton s law of motion, that force also equals mass multiplied by the acceleration. We see that an object of mass m at the Earth s surface is subject to an acceleration due to gravity g: g = F m = GM a 2. (2) We can derive the acceleration due to gravity using two experiments, dropping a ball and swinging a pendulum. Once we determine g we can then compute o, and use this to gain insight of Earth s internal structure.

2 -2- PART 1: The Ball-Drop Experiment Aball is dropped between two infrared sensors (gates) A and B connected to a timer. This system yields: 1) the time the ball takes to pass through gate A, t A 2) the time the ball takes to pass through gate B, t B 3) the total time the ball takes to go from gate A to gate B, t AB. The ball s velocity at each gate is the diameter of the ball, D divided by the time it takes to pass through the gate. Thus, the velocity of the ball at each gate is: v A = D/t A v B = D/t B. Because acceleration is the time derivative of velocity, a = dv/dt, the ball s acceleration due to gravity can be found by approximating the derivative with the differences: g = v B v A t AB = D t B D t A t AB = 1 t AB D(t A t B ) t A t B

3 -3- Here are a few things to think about before performing the experiment: i. Using two balls with different weights, which would fall faster? ii. How would positioning the time gates further apart affect the estimate of g? Experiment: 1. Measure and record t A, t B and t AB using the steel ball. Repeat for 3 trials. Record your results in the table below. Find and record the average for these measurements. 2. Repeat step 1. using the plastic ball. The diameter, D,ofboth balls is 1.9 cm (0.75 in). Steel Ball Plastic Ball Trial: t A : t B : t AB : t A : t B : t AB : Avg. Questions: iii. How dothe times for the two balls compare? iv. Calculate g using the values from the average times for the steel ball. How well do your measurements of g agree with the accepted value of 9. 8 m?what might be some causes for a difference? s2

4 -4- PART 2: Pendulum Experiment One day while attending Mass, Galileo noticed a chandelier above him was swaying in a draft. He noticed that for large and small swings the lamp had the same period of motion. The period, T is the amount of time taken for the swinging motion of the lamp to repeat. Galileo then confirmed his observation by noticing that the period of oscillation of any pendulum was dependent only on the length of the string used to suspend the mass, and not on the mass. The arc-path of the mass, s is related to the product of the length of the string l, and the angle,ofthe swing: s = l The force acting on the pendulum tangential to the arc-path of the mass is: where mg is the weight of the mass. F = mgsin, Using Newton s law of motion, this force is also equal to the product of the mass and the tangential acceleration, d 2 s/dt 2 : which can also be written as: F = mgsin = ma= m d2 s dt 2 d 2 s = g sin. dt2

5 -5- By using the small angle approximation (when angle is small, sin )), we now have: d 2 s dt 2 = g = g s l 2 is equal to g/l. Using a relationship between the angular fre- The square of the angular frequency quency and the period, T : T = 2 / = 2 We can rearrange this equation to find the value of g: g = 4 T 2 2 l l g. Experiment: 1. Set the pendulum to a length of 0.5 meters. 2. Time how long the pendulum takes to make 10 oscillations. Record the time in the table below; repeat the trial five times. 3. Find the value of the period for each trial, average the values for time and period. 4. Calculate the acceleration due to gravity, g for the averaged value of the period. Trial Length Time Period m m m m m Avg. 0.5 m v. How well do your measurements of g agree with the accepted value of 9. 8 m s 2? List two possible explanations for the variations in your calculated value of g between the two methods used to calculate it.

6 -6- Interpretation: Using the value we determined for g, the Earth s average radius of 6371 km, and the gravitational constant G of 6.67 x N m 2 /kg 2 we can find Earth s mass and average density. Byrearranging equation (2), Earth s mass is found using: M = ga 2 /G vi. Using the above relationships, calculate the mass, M and the average density (see equation 1), o of Earth. How dothese results compare with the accepted values of mass and average density of 5.98 X kg and 5.5 x 10 3 kg/ m 3,respectively? vii. Crustal Earth rocks have an average density of c =3g/cm 3. How does this compare with o? What does this imply about Earth s density structure? Assignment: (due in one week) Completely fill in the data tables, and answer questions i - vii (make sure toshow all work with your calculations). Write a brief (one paragraph) summary of this lab which includes: a statement of purpose, methods, observations/results, and interpretations.