1. Grade 7 Unit 4 Topic 3 Mass and Forces

2. Mass Mass: the amount of matter in a substance; often measured with a balance. The metric system measures mass by comparing objects to a standard mass.

3. Mass-cont. Primary Standard: The name given to a small cylinder of metal on which the kilogram (kg) is based; equivalent to 1 kg. Kilogram (kg): the primary measurement of mass in SI, equal to 1000 g; 1 kg is the primary standard for mass.

4. Mass-cont. Exact copies of the primary standard kilogram are kept in various countries, including Canada. Smaller masses are usually expressed in grams (g). “Kilo” means thousand; therefore 1 kilogram is a thousand grams.

5. Mass-cont. “Milli” means “one thousandth”; therefore 1 milligram (mg) is a thousandth of a gram (0.001 g) It would take 1000 milligrams to make one gram.

6. Mass-cont. Balance: A device to measure mass; many balances work by using the force of gravity. Many balances compare the pull of gravity on the object being measured with the pull of gravity on standard masses.

7. Mass-cont. An object’s mass stays constant no matter where it is. An object’s mass will only change if additional matter is added, or some matter is taken away.

8. Forces and Weight While mass stays constant, weight will change if an object moves from the Earth to the Moon. To understand why weight changes you need to understand that weight is a force.

9. Forces and Weight-cont. ForceForce: A push or pull. Newton: The standard unit of force in the Système international d`unitès (SI). 1 N (Newton) is just enough to lift a D-cell battery.

10. Forces and Weight-cont. Forces need to be quantified, measured, in order to better understand what forces affect structures. Force Meter: A scientific device used to measure force; also called a spring scale. A bathroom scale is a type of spring scale

11. Forces and Weight-cont. Force meters are low cost devices, common in laboratories; however they are not that accurate compared to other electronic sensors. Very large forces, or difficult to measure forces, are often calculated by observing their effect on the motion of an object.

12. Forces and Weight-cont. To completely describe a force, you need to determine both its direction and its size.

13. Weight Weight is a force, and like all forces is properly measured in newtons. Isaac Newton, 1642-1727, theorized that there is a force between any two objects, anywhere in the Universe, that tries to pull them together.

14. Weight-cont. Newton Newton mathematically examined the size of this force, which he called gravity. Gravitational Force: The force exerted by gravity on an object; measured in newtons (N); the preferred scientific term for the everyday term “weight”.

15. Weight-cont. The Gravitational Force between two objects depends on the masses of the two objects and the distance between them. The greater the mass and the closer the distance, the greater the gravitational force.

16. Weight-cont. Weight: The force of gravity exerted on a mass. Weight and gravitational force are the same thing. Weight can be stated as: “A 1 kg mass has a weight of 10 N on Earth” (It’s actually 9.81 N, but 10 is close enough for most purposes).

17. Weight-cont. Since gravitational force depends on distance between objects, your weight changes depending where you are. Gravitational Gravitational force also depends on the mass of the object; therefore your weight changes depending on what very large mass you are close to. You weigh 1/6 th of your Earth weight on the moon.

18. Picturing Forces Force Diagram: A drawing that uses arrows to represent the direction and strength of one or more forces. The circle stands for the object Each force is shown with an arrow The length of the arrow shows the size of the force The direction of the arrow indicates the direction of the force.

19. Picturing Forces-cont. The arrow, in a force diagram, is usually drawn pointing away from the place where the force is acting. Mathematicians are able to use scale drawings and calculations to predict what will happen when many forces are acting on a given object.

20. Picturing Forces- An example As a cannonball is fired the force of the cannon’s firing is the greatest force acting on the cannonball. A. B. C. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity

21. Picturing Forces- An example As the cannonball continues its flight the force of the firing begins to decrease compared to the force of gravity A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

22. Picturing Forces- An example At the top of the cannonball’s flight arc the force of firing and the force of gravity become equal. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

23. Picturing Forces- An example The cannonball then begins to descend, as the force of gravity becomes the strongest force. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

24. Picturing Forces- An example The force of firing continues to decrease compared to the force of gravity, and the cannonball picks up speed as it heads towards the ground. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

25. Picturing Forces- An example When the cannonball comes to rest, on the ground, the only force still acting on the cannonball is the force of gravity. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity C.

26. Picturing Forces- An example A force diagram of the whole flight would look like this. A. B. C. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity