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

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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 Newtons 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 (Hookes 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
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