User interface language: English | Español

Date November 2020 Marks available 2 Reference code 20N.3.SL.TZ0.2
Level Standard level Paper Paper 3 Time zone 0 - no time zone
Command term Explain Question number 2 Adapted from N/A

Question

A student studies the relationship between the centripetal force applied to an object undergoing circular motion and its period T.

The object (mass m) is attached by a light inextensible string, through a tube, to a weight W which hangs vertically. The string is free to move through the tube. A student swings the mass in a horizontal, circular path, adjusting the period T of the motion until the radius r is constant. The radius of the circle and the mass of the object are measured and remain constant for the entire experiment.

© International Baccalaureate Organization 2020.

The student collects the measurements of T five times, for weight W. The weight is then doubled (2W) and the data collection repeated. Then it is repeated with 3W and 4W. The results are expected to support the relationship

W=4π2mrT2.

In reality, there is friction in the system, so in this case W is less than the total centripetal force in the system. A suitable graph is plotted to determine the value of mr experimentally. The value of mr was also calculated directly from the measured values of m and r.

State why the experiment is repeated with different values of W.

[1]
a.

Predict from the equation whether the value of mr found experimentally will be larger, the same or smaller than the value of mr calculated directly.

[2]
b.

The measurements of T were collected five times. Explain how repeated measurements of T reduced the random error in the final experimental value of mr.

[2]
c(i).

Outline why repeated measurements of T would not reduce any systematic error in T.

[1]
c(ii).

Markscheme

In order to draw a graph « of W versus 1T2 »
OR

to confirm proportionality between «W and T-2 »

OR

to confirm relationship between «W and T »

OR

because W is the independent variable in the experiment

 

OWTTE

a.

ALTERNATIVE 1

W+friction=4π2mrT2

OR

centripetal force is larger «than W» / W is smaller «than centripetal»

«so» experimental mr is smaller «than calculated value»

 

ALTERNATIVE 2 (refers to graph)

reference to «friction force is» a systematic error «and does not affect gradient»

«so» mr is the same

 

MP2 awarded only with correct justification.
Candidates can gain zero, MP1 alone or full marks.

OWTTE

b.

mention of mean/average value «of T»

this reduces uncertainty in T / result
OR
more accurate/precise

 

Reference to “random errors average out” scores MP1

Accept “closer to true value”, “more reliable value” OWTTE for MP2

 

c(i).

systematic errors «usually» constant/always present/ not influenced by repetition

 

OWTTE

c(ii).

Examiners report

Most candidates scored. Different wording was used to express the aim of confirming the relationship.

a.

Most successful candidates chose to consider a single point then concluding that the calculated mr would be smaller than the real value as W < centripetal force, or even went into analysing the dependence of the frictional force with W. Many were able to deduce this. Some candidates thought that a graph would still have the same gradient (if friction was constant) and mentioned systematic error, so mr was not changed which was also accepted.

b.

Most candidates stated that the mean of 5 values of T was used to obtain an answer closer to the true value if there were no systematic errors. Some just repeated the question.

c(i).

Usually very well answered acknowledging that systematic errors are constant and present throughout all  measurements.

c(ii).

Syllabus sections

Core » Topic 1: Measurements and uncertainties » 1.2 – Uncertainties and errors
Show 56 related questions
Core » Topic 1: Measurements and uncertainties
Core

View options