1. Physics

2. PHS 5043 Forces & Energy Gravitational attraction Force: Any agent capable of changing the shape of an object or changing its state of rest or motion Symbol: F Units: N (newton)

3. PHS 5043 Forces & Energy Gravitational attraction There two main types of forces: _Forces acting at a distance _Forces acting upon contact

4. PHS 5043 Forces & Energy Gravitational attraction Main forces acting at a distance: _Gravitational force _Electromagnetic force _Weak nuclear force _Strong nuclear force

5. PHS 5043 Forces & Energy Gravitational attraction Law of universal gravitation: “Every object in the universe attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centres”

6. PHS 5043 Forces & Energy Gravitational attraction Gravitational attraction F g : (Gravitational force) F g = G m 1 m 2 / d 2 _ F g is gravitational attraction force (unit : N) _ m 1 & m 2 are the masses of the two objects (unit: kg) _ d is the distance between the centers of each object (unit: m) _ G is the universal gravitational constant (value: 6.67 * 10 -11 unit: Nm 2 /kg 2 )

7. PHS 5043 Forces & Energy Gravitational attraction Example: Calculate the gravitational force between two people with masses of 60 and 70 kg respectively, separated by a distance of 2 meters. F g = G m 1 m 2 / d 2 F g = (6.67 * 10 -11 Nm 2 /kg 2 ) (60kg) (70kg) / (2m) 2 F g = 7 * 10 -8 N

8. PHS 5043 Forces & Energy Mass & Weight Mass of an object: Amount of matter an object contains Symbol: m Units: kg *The mass of an object does not change

9. PHS 5043 Forces & Energy Mass & Weight Weight of an object: (universal gravitation) Defined by the product of the mass of the object and the strength of the gravitational field (g) in which it is located Symbol: F g Formula F g = m g Units: N *The weight of an object changes with its location

10. PHS 5043 Forces & Energy Mass & Weight Obtaining “g”: Measuring the weight of an object, is measuring the gravitational attraction that Earth exerts on it. Thus: F g = G m 1 m 2 / d 2 F g = m 2 (G m 1 / d 2 ) m 1 : mass of Earth (5.98*10 24 kg) d: radius of Earth (6.38 * 10 6 m) G: universal gravitational constant (6.67 * 10 -11 Nm 2 /kg 2 ) F g = m 2 (6.67 * 10 -11 Nm 2 /kg 2 ) (5.98*10 24 kg) / (6.38 * 10 6 m) 2 F g = m 2 (9.8 N/kg) F g = m 2 g

11. PHS 5043 Forces & Energy Mass & Weight The weight of an object changes with its location: F g = m g _Higher latitudes, higher “g” since Earth radius is smaller, therefore objects are heavier _Higher altitudes, lower “g” since longer distance between object and Earth center, therefore objects are lighter _Planets with more mass than Earth will have higher “g”, therefore objects are heavier _Planets smaller than Earth will have higher “g” since their radii would be smaller, therefore objects are heavier

12. PHS 5043 Forces & Energy Mass & Weight Weightlessness: State by which objects appear to have no weight, due to continuous falling (free fall). Example: astronauts in space stations Regardless of appearances objects in state of weightlessness are acted on by gravitational forces (by Earth or other planets)

13. PHS 5043 Forces & Energy Mass & Weight Weightlessness: PROS: Can produce interesting results for scientific research (e.g. crystallization and synthesis) due to the lack of gravity CONS: Affects blood circulation, and the structure of bones and muscles