Weight and GravityPrinciples of Physics - Foederer

Do Now:

Calculate the acceleration that this object experiences

30 kg150 N

Newton’s 2nd Law (N2L)

When Fnet = 0, acceleration = 0

The object is in equilibrium

All forces acting on the object are balanced

maFnet

Weight, FW

If an object dropped on Earth were able to fall without any air resistance, it would accelerate at a rate of g = 9.8 m/s2

The only force acting on the object would be gravitational force

Therefore, according to N2L

Fnet = FW = ma

Since Earth’s gravitational acceleration is a constant it has its own variable, g

Therefore, FW = mg = m(9.8 m/s2 )

Remember: weight ≠ mass!!!

Mass vs. Weight

Since mass is used to calculate weight, mass and weight can never be equal.› Weight is dependent on location (on Earth,

on the Moon, in space, etc)› Mass only depends on the amount of

matter Your mass would be the same on Earth or on

the moon

Mass vs Weight

Mass is a scalar (same amount no matter where you are)› measured in kilograms

Weight is a force (the force of gravity pulling on an object) and a vector› Weight is measured in Newtons

Free fall and air resistanceAll things fall to Earth with the same acceleration = g

So, FW = mg

 

Why don’t things appear to fall at the same rate??

 Air Resistance  The air must be moved out of the way in order for an object to move through it.

What affects the amount of air resistance?

Air density- more particles to move out of the way

Speed of the object- particles must be moved out of the way faster

Surface area of the object- more particles must be displaced at once.

Terminal Velocity

Terminal velocity – highest speed reached while falling on Earth› Occurs when Fw = Fair

› Fair increases as your speed increases until it equals Fw

› When Fw = Fair the object is in equilibrium

Calculate your Weight in Newtons

1. One kilogram weights 2.2 pounds. Determine your mass using this as a conversion factor

2. Use your mass to calculate your weight

kglbs

kglbs2.68

2.2

150

2.2

1

1

150

mgFW

)8.9)(2.68(WF

NFW 4.668