F.4 Physics 物理,20Marks 位移

9. Let O be the position of HK
Distance of typhoon from HK when it is at OA = 800 km
Distance of typhoon from HK when it is at OB = 400 km
Angle between OA and OB = 45 degrees
By the cosine rule,
(AB)^2 = 800^2 + 400^2 - 2 x 800 x 400 x cos(45) (km)^2
i.e. AB = 589 km

10.(a) Distance travelled = (90 + 22/7 x 70/2) m = 200 m
Displacement = square-root[90^2 + 70^2] m = 114 m

(b) After running for 2 more laps, the final position of Jospeh is still at B. Hence, the displacement remains the same, i.e. 114 m

11.(a) Just join the 4 dots, A, B, C and D with straight lines.
(b) Total displacement = length of line AD
= square-root[(8x10)^2 + (5x10)^2] m = 94.3 m

12. (a) The route is along a longitude running from the north pole to the south pole.
The displacement is the diameter of the earth joining the two poles.

(b) Distance travelled = pi x radius of earth (where pi = 3.14159...)
hence, distance travelled = pi x 12740/2 km = 20 000 km
Displacement = earth diameter = 12740 km