Chapter 1 part 3: Mass, Weight and Density

                                            General Physics

• 1.3: Mass and Weight.

All objects composed of matter have mass, anything comprised of atoms can have mass. Weight is just the effect of gravity on a mass. So if a 1 kilogram iron bar is weighed on earth compared to in outer space , the weights will be different ( due to different gravitational fields) while the Mass will be exactly the same.

E.G.
A car weighs 4900 newtons on earth and has a mass of 500 kg . In a zero-gravity environment the car will still be 500 Kilograms but weight 0 newtons. this is because weight is a gravitational force that requires a gravitational field, like on Earth !

So Force is equal to Mass . Acceleration , F=M.A. Since weight it a gravitational force it can be calculated using the same equation , with the acceleration due to gravity being 9.8 or 9.81 , (sometimes 10 for simplicity) , We simplify this into Weight is equal to Mass . Gravity , W=M.G

• 1.4: Density

So mass is the amount of matter in an object in total , density describes how closely packed that matter is together on average. Density is equal to Mass/Volume. as the equation tells us , it is the amount of mass there is per cubic meter or centimeter. To put this into comparison We will ask a question

Q&A:
Q: Which is heavier , 1 kilogram of feathers or 1 kilogram of steel ?
A: the answer is , they have the exact same weight ! , but due to different densities , you need a large volume of feathers to make 1 kilogram , while you only need a small sample of steel in order to make 1 kilogram

So how do we measure densities ? , well we consider the density of water to be 1000 kilograms per meter cubed. if we know the mass of an object but not the volume , we can perform a simple experiment, we simply submerse the object in a known volume of water in a graduated flask or measuring cylinder. we take note of the initial volume (before adding the object) and the final volume ( after adding the object) the difference will equal the volume of the object and the density can be found by dividing mass by the volume found.
If an object is less dense than water , it will float when put into water . That is why oil stays on the surface of water and do not mix. for the same reason a rock sinks in water as it is more dense.