Unit 1 - Introduction

Transcription

1 Unit 1 - Introduction Units and Measurements In Physics, we are constantly measuring PHYSICAL QUANTITIES using MEASURING INSTRUMENTS. When we measure something we need to pay attention to the following: 1) To choose the right instrument for the right job. 2) To write down the unit after our measurement. Using a Measuring instrument: Make sure that you have the right instrument to measure your quantity. Always read the instrument at eye level to avoid errors. Pay attention to the unit on the scale of your instrument Three important physical quantities are Length, Mass and Time. Every physical quantity has its own UNIT. For example the unit of length is the METRE, symbol (m). Mass is measured in kilograms (kg) while Time is measured in seconds (s). Length Area and Volume Length is the long dimension of any object. The length of a thing is the distance between its ends. It is measured in metres. For example the length of the classroom you are in right now is approximately 2.5 metres. Length can be measured using several measuring instruments. Which instrument we use depends on the accuracy with which we want to measure the length. Form 3 Unit 1 - Measurements Page 1

2 Micrometer Screw Ruler Tape Gauge Also, to measure long objects we do not use the centimetre (cm). We can either use the metre (m) or the kilometre (m). 1 Km = 1000m 1 m = 100 cm 1 cm = 10 mm Area: The area of an object tells us how big the surface of that object is. We can find the area of an object by using formulae: Area of a square or rectangle. Area = length x breadth Units cm 2 or m 2 Area of a circle Area = πr 2 Volume: Units cm 2 or m 2 The volume of an object tells us the capacity of that object. 1 litre = 1000 cm 3 We can find the volume of a regular shaped object using a formula. A regular shaped object is an object which is symmetrical in its shape. Volume of a cube = Length x Breadth x Height Units of volume cm 3 or m 3 For irregularly shaped objects we cannot use a formula to find their volume but we can use one of the following methods. Form 3 Unit 1 - Measurements Page 2

3 Measuring Cylinder Method 1) Fill a measuring cylinder to a known volume of water (V1). 2) Drop the object inside the measuring cylinder without spilling any of the water. 3) Take the new reading (V2). 4) Volume of object = V2 V1 Using a displacement Can 1) Place a measuring cylinder below a displacement can. 2) Fill the displacement can with water until it overflows. 3) When the water stops overflowing, place a clean measuring cylinder below the displacement can and drop the object inside the can. Take the reading of the displaced water inside the cylinder. To determine the volume of an object that floats, first attach a metal sinker to the object. Next, submerge the metal sinker and measure the over-flow. Then submerge the object and measure the total overflow. The volume of the object equals the difference between the measurements. The volume of a liquid is often measured using a measuring cylinder A measuring cylinder Form 3 Unit 1 - Measurements Page 3

4 Volume shown = 16ml 1. Keep the measuring cylinder vertical 2. Place your eyes level with the surface of the liquid 3. Read to the lower surface of the meniscus (A meniscus is the tiny curve on the water surface where the water touches the edge of the container) Mass Mass refers to the amount of matter or material present in an object. It is measured in kilograms. Mass can be measured using several measuring instruments. Which instrument we use again depends on the accuracy with which we want to measure the mass. Time 1 Kg = 1000 g Time is part of a measuring system used to sequence events and to measure the duration of the events. It is measured in seconds. For example, a lesson lasts 45 minutes which is equal to 45 x 60 = 2700 seconds. Time is measured using a stopwatch. Electronic stopwatches can be very accurate. 1 day = 24 hours 1 hour = 60 minutes 1 minute = 60 seconds Density Density depends on the weight of the individual atoms and molecules making up the object, and how much space there is Form 3 Unit 1 - Measurements Page 4

5 between them. Density also depends on whether the object is solid, filled with airpockets, or something in between. Very dense substances such as lead and gold have large densities whereas gases, which are less dense, have smaller densities. Objects will change size, and therefore density, in response to a change in temperature or pressure. Which is heavier - plastic or lead? If we were given two identical blocks of plastic and lead, we would notice that the block made from lead is heavier than that made from plastic. This means that lead has a higher DENSITY than plastic. The particles are more closely spaced (more packed ). We can find the density of an object using the formula: Density = mass volume mass We measure density in g/cm 3 or kg/m 3. density x volume ρ(rho) is the symbol for density M is the symbol for mass V is the symbol for volume Floating and sinking ρ = m / V If the density of an object is MORE than the density of water (or whatever the liquid is) it will sink. If its density is less than water it will float. The density of some oil is 0.8 g/cm 3. The density of water is 1g/cm 3. This means that oil will float on water. Some plastic is found to have a density of 0.9 g/cm 3. This means some plastic will sink in the oil but float on the water. Form 3 Unit 1 - Measurements Page 5