Mass, Weight and Density Mass:
it is the amount of matter in a body.
S.I. unit: = kilogram (kg) Measuring instrument: beam balance, electronic balance, top pan balance.
Weight:
it is the pull of gravity on an object.
S.I unit: = Newton (N) Measuring instrument: spring balance, (Newton-meter)
Differences between mass and weight Mass It is amount of matter Scalar Measured in kg Stays the same everywhere
Weight It is pull of gravity Vector Measured in N Changes depending on gravitational field strength
Formula (for converting mass into weight): = W = weight (N) m = mass (kg) g = gravitational field strength (10N/kg) Inertia:
it is the property of a mass to resist change in its motion.
You may have experienced that when a car takes a turn, you sway to one side. This is because your mass resists change in its direction of motion. It tries to go straight in line but the car doors push you to make you turn as well. Similarly, if you apply sudden brakes, the car skids. This is again due to inertia. (See the COIN TRICK on http://www.physics.com.pk/). It is easy to push a small light car than a big heavy truck because the truck has more mass so more inertia.
mass � inertia More the mass more the inertia.
Density: Formula:
it is mass per unit volume.
=
Unit of density: kg/m3
= Mass is in kg; volume in m3; density in kg/m3.
Measuring density of a regular object 1. Measure the mass of the object. 2. Calculate its volume by mathematical formula (for rectangle:l x b x w, etc.) 3. Find its density using the above formula.
Measuring density of an irregular object 1. Measure the mass of the object. 2. Find its density by the water displacement method as shown below. 3. Find its density using the above formula.
Water displacement method This method is used to find the volume of an irregular object such as a stone.
Step 1: place some water in a measuring cylinder and record its initial volume, V1.
V1
Step 2: place the stone with a thread in the cylinder.
V2
Step 3: flick the cylinder Step 4: record the final volume of stone and water, V2. Step 5: find the volume of the stone by subtracting V1 from V2. Volume of stone, V = V2 –
V1
For objects that float on water
Floating object
Step 1: Find the volume of water + stone as V1. Remember to flick the cylinder before you take the readings.
V1
Step 2: Then tie the object with the stone and place it in water. Record the volume of water + stone + object as V2.
V2
Step 5: find the volume of the floating object by subtracting V1 from V2. Volume of object that floats, V = V2 – V1
Precautions: 1. Flicking the cylinder before taking the second reading will make air bubbles escape out of the water and give you a more accurate value. 2. Drop the object in water with the help of a thread so that water does not splash out.
By Shafaq Hafeez shafaq@physics.com.pk