Dr. Aaron Titus | Department of Physics, High Point University PHY1050      Astronomy of Stars, Galaxies, and the Cosmos home | WebAssign | textbook | course calendar

## mass

The mass of an object is related to the number of protons and neutrons (and to a lesser extent, electrons) that make up the object.

But this isn't an operational definition because it doesn't describe how we measure the mass of an object? Mass is measured with an equal-arm balance and a standard unit. Mass of an object is defined as the number of standard units required to balance the object on an equal-arm balance.

The simulation below shows an equal-arm balance. If the mass of the object(s) in the left pan is equal to the mass of the object(s) in the right pan, then the beam will not rotate when it is released from rest (assuming it is not sitting on the yellow bar).

In the simulation, you may move the blocks to and from the pan by clicking and dragging on the blocks. The buttons allow you to play or pause the simulation, reset the simulation (and clear the pans), and level the beam. To move the blocks, you must pause the simulation first. It is usually best to level the beam before running the simulation in order to test whether the beam is balanced.

The SI unit of mass is the kilogram. In this simulation, the red block has a mass of 1 kg. What are the mass of the blue block and green block? Use the equal arm balance to answer the question.

Mass of blue block = kg
Mass of green block = kg

Here are masses of a few objects in kg.

Object mass (kg)
1 liter of water 1
one hydrogen atom 1.67 x 10-27
Earth 5.97 x 1024
Moon 7.36 x 1022
Sun 1.99 x 1030
Dr. Titus 82
universe approximately 1050

## Mass and Weight are not the same thing

By adding kilograms on one side of a balance, one can measure the mass of any object. But would this experiment give the same results on Moon where the gravitational force of Moon on a 1-kg object is 1/6 of Earth's gravitational pull on a 1-kg object?

Mass and weight are not the same thing.

Weight is measured by how much an object compresses or stretches a spring when it is set on top of a spring or when it hangs from a spring. For example, to measure your weight you might stand on a bathroom scale. When you stand on the scale, you compress a spring which turns a dial that shows you your weight in units of pounds.

The picture below shows a 1 kg brick just before it is set on a spring. In one case, it's on Earth, and in the other case it's on Moon.

Which spring do you think will compress the most when the bricks are released?

Click the Run button to play or pause the simulation. Click the Reset button to reset the simulation to the beginning.

Though springs are used to measure weight. The definition of weight is the gravitational force of one body on another. Thus your weight on Earth is the gravitational force of Earth on you. The British unit of force is pound (lb). However, scientists use the SI system of units, and the SI unit of force is newton (N).

My weight on Earth is approximately 180 lb (and getting bigger everyday!). Listed below is my weight on various planets, though standing on Jupiter would be quite difficult and Pluto would be EXTREMELY cold. BRRRRR!

Body weight (N) of Dr. Titus weight (lbs) of Dr. Titus
Earth 800 180
Moon 130 29
Jupiter 2050 460
Mars 300 67
Pluto 8 1.8

Do you want to know what the Titus weight loss plan is?

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