# ENGG 311 Lecture Notes - Lecture 3: Spring Scale, Weighing Scale, Specific Volume

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15 Jan 2017

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ENGG 311 Lecture 3

Relating Weight and Mass

Weight is the force exerted on an object by gravity

Weight and force are related by Newton’s Second Law

o F=ma

Mass is the same everywhere, weight changes with location

Different measuring devices are used to measure mass and force

o A spring balance measures weight, but it is only good for the location at which it was

calibrated

o A pan balance measures mass, so it works at any location

EXAMPLE: A spring stretches 0.2cm per N of applied force. An object is suspended from this

spring and the deflection is 3cm. If g=9.81 m/s2, what is the mass of the object?

F=kx (Hooke’s Law)

1N=k(0.2cm)

k=5N/cm

F=kx

F=(5N/cm)(3cm)

F=15N

m=weight/g=15N/9.81m/s2 = 1.53 kg

Systems and Control Volumes

A system identifies either the amount or the location of the material which is being studied