We use a ruler to measure the length of a table, a stopwatch to measure the time it takes for a ball to fall, and a thermometer to measure the temperature of water. A different instrument is used for each quantity. 3.1. Measuring mass The mass of a body is the amount of matter it contains. It is measured with scales. Mass and weight are two dif ferent quantities. Weight is the force with which a body is attracted by the celestial body where it is located . The weight of a body on the Earth is dif ferent to its weight on the Moon , although its mass does not change. 3. Measuring instruments 28 What measuring instruments do you use when you buy loose fruit, rather than packaged fruit? 29 How do you know how much water or soft drink is in a container? With the scales at rest, place the object on one of the pans. Add weights to the other pan . Press the lever and when the needle points to zero, the mass of the body is equal to the sum of the mass of the weights. Precision scales or pan balance Can we recycle disposable drink cups? Disposable cups for cof fee or other drinks are containers made of cardboard . They are also coated with a thin layer of plastic so they can hold liquids. As most of the cup is made of cardboard , they can be recycled . Used paper and cardboard cups should be recycled in the paper bin. Turn on the scales and wait for zero to appear. Place the empty container on top and press the tare button . The display will read 0 g even though the container is on the scales. Without touching the scales, remove the container and put the substance you want to weigh inside it. Put it back on the scales and read its mass. Electronic scales 30 A pan balance is in equilibrium when we place a tangerine on one pan and these weights on the other. What is the mass of the tangerine? 2 g 1 g 31 If you place this object on one of the pans of a pan balance, which weights do you have to put on the other pan for it to be in equilibrium? A C T I V I T I E S CHALLENGE T R U E OR FALSE ? 20
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