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2. 2 Introduction What does this bathroom scale measure? What does this beam balance measure?

3. 3 Question Does weight have the same meaning as mass? =

4. 4 Lesson Objectives At the end of this lesson, you should be able to: 1. State that mass is a measure of the amount of substance in a body. 2. State that mass of a body resists a change in the state of rest or motion of the body. 3. State that a gravitational field is a region in which a mass experiences a force due to gravitational attraction. 4. Define gravitational field strength as gravitational force per unit mass. 5. Recall the relationship weight = mass x gravitational field strength. 6. Recall the relationship density = mass / volume.

5. 5 Mass Definition A measure of the amount of substance in a body. Measured in kilograms (kg). Source: http://bures.hyperlink.cz/ h_kilogram.htmbures.hyperlink.cz/ h_kilogram.htm

6. 6 Mass Would your mass increase or decrease when you are on the Moon? Mass cannot be changed by the object’s location and shape. Would the mass of a lump of plasticine increase or decrease if we were to flatten it?

7. 7 Measurement Instruments Mass is measured using a balance such as the beam balance electronic balance Beam Balance Electronic Balance

8. 8 Some Examples of Masses of Objects ObjectMass in kilogram (kg) Electron A fine grain of sand A pea An apple A medium-sized car Earth Sun 10 -30 10 -6 10 -3 10 -1 10 3 10 24 10 30

9. 9 Weight The force of the earth’s gravitational pull on a body is known as its weight Weight, W = mass x acceleration due to gravity W = mg Weight is a force, therefore, its SI unit is Newton (N)

10. 10 Acceleration due to gravity, g When the speed of an object changes, the object is said to be accelerating. When an object is allowed to fall freely under the earth’s gravitational pull, it does so with increasing speed. Hence, it experiences a constant acceleration. This constant acceleration due to gravity is defined as g. It has a value of 10 ms -2.

11. 11 Example If a book has a mass of 1.5 kg, how much does it weigh (a) on earth and (b) on the moon? (g moon is 1/6 of the earth’s) Take g = 10 ms -2 and g moon = g/6 (a) Weight of book on earth = mg = (1.5)(10) = 15 N (b) Weight of book on moon = mg moon = (1.5)(10/6) = 2.5 N

12. 12 Weight Question  Would you weigh the same at the North Pole and at the equator?  Would you weigh the same on the Moon and on Earth?

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14. 14 Measurement Instruments Weight is measured using a balance such as the spring balance compression balance Spring Balance Compression Balance

15. 15 Question Does weight have the same meaning as mass? = No! Weight does not have the same meaning as mass.

16. 16 Difference between mass and weight Mass Is the amount of matter in the body Has only magnitude but no direction Is measured in kg Is constant regardless of the place or location Is measured by a beam balance or an electronic balanceWeight Is the pull of gravity on the body Has both magnitude and direction Is measured in N Changes from place to place; the weight at the equator is different from the weight at the North or South Pole Is measured by a spring balance or a compression balance

17. 17 Inertia Inertia refers to the reluctance of an object to start moving if it is stationary in the first instance or the reluctance of the object to stop moving if it is moving in the first instance

18. 18 Inertia All objects have inertia A force is needed to overcome inertia Inertia depends on mass The bigger the mass, the greater the force is needed to overcome its inertia

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20. 20 Gravitational Field Region in which a mass experiences a force due to gravitational attraction Force is strongest on the surface of Earth and gets weaker as we go farther away

21. 21 Gravitational Field Strength Gravitational field strength, g, is defined as gravitational force acting per unit mass on an object Since weight, W, is the gravitational force acting on an object, we then have W = mgwhere m is mass of object

22. 22 Question Have you ever wondered why a test-tube containing a certain volume of mercury feels much heavier than a test-tube containing an equal volume of water?

23. 23 Density When we compare masses of equal volumes of different substances, we are actually comparing densities Substance Density / kg m -3 Mercury13600 Water1000 Polystyrene16 Pinewood500

24. 24 Density It actually means how much stuff (mass) we can fit into a space (volume). Example:  The more students (mass) we pack into a class (volume), the denser it would be.  The less students (mass) we pack into a class (volume), the less dense it would be. Density can be calculated from a ratio of mass over volume

25. 25 Density Density is defined as mass per unit volume Density,  = Mass, m / Volume, V SI unit is kg m -3 M DV My Darling Valentine

26. 26 Example A concrete slab 1.0 m by 0.5 m by 0.1 m has a mass of 120 kg. What is the density of concrete? Volume = l x b x h = 1.0 x 0.5 x 0.1 = 0.05 m 3 Density = Mass / Volume = 120 kg / 0.05 m 3 = 2 400 kg m -3

27. 27 Weight, W  = m / V W = m x g Mass, m Density,  related to mass by related to density by related to mass by mass per unit volume amount of substance in a body Inertia g = gravitational force per unit mass acting on an object gravitational force acting on an object kg m -3 kg N related to weight by defined as is a measure of defined as where