The Conical pendulum

Introduction

A conical pendulum is a pendulum that is spun round in a circle instead of swung backwards and forwards. In this experiment a mass is attached to a string and made to spin in a circle of fixed radius, the time period of the motion is related to the length of the string. By varying the length and measuring Time period the acceleration of gravity can be found.

Research question

How does the time period of a conical pendulum depend upon the length of the string?

Independent variable: Length of pendulum

Dependent variable: Time period

Controlled variables: Radius, mass and properties of the string.

Method

Draw a clear circle of about 10cm diameter on a piece of paper. Holding the string above the centre of the circle swing the mass so it travels around the circumference. This isn’t that easy so it might be worth practising with different lengths before you start the measurements. You will notice that the time period is longer when the string is longer.

Theory

Above is a free body diagram for the mass. If the forces are resolved vertically and horizontally:

Vertically the forces are balanced so mg=Fcosθ

Where F is the tension

Horizontally there is centripetal acceleration so mω2r=Fsinθ

Where

m = mass
ω = Angular velocity

Dividing gives

mg/mω2r =Fcosθ/Fsinθ =1/ tanθ

But

tanθ = r/h so ω2r/g= r/h

from the definition

ω = 2π/T so 4π2/gT2 = 1/h

Where T = time period

Squaring gives

h2 = g2T4/16π4

From Pythagoras

L2 = h2 + r2 so h2=L2 - r2

Finally

L2=g2T4/16π4 + r2

By changing L and measuring T keeping r constant use a graphical method to find g

New Conical pendulum

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