Page 13 - Q & A STPM 2022 Physics

Physics Semester 1 STPM Chapter 1 Physical Quantities and Units

Section B Structured Questions
Question 4

(a) The force of attraction F between the Sun with mass M and a planet of mass
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m is related by the expression
F = GMm .
r 2
Where r is the distance of separation between the Sun and the planet. Use
the expression to find units for the gravitational constant G in terms of base
units.

Semester
1 (b) The orbital period T of the planet about the Sun is related to the distance
r from the sun as shown in the equation below.
x
y
T = ( 4π 2 )M r z
G
Where M is the mass of the Sun and G is the gravitational constant.
Determine the values of x, y and z.

Answer:
(a) F = GMm
r 2
G = Fr 2
Mm
Unit of F = kg m s kg 2 –2
–2
Penerbitan Pelangi y z
kg m s m
Unit of G =

= m kg s
3

–1 –2
(b) [T] = T
–1
–2
3
[G] = L M T
[r] = L
[M] = M
x
–2 –1
T = (L M T ) M L
–1
3
1
–3
y z
= L M T M L
2
z–3
y+1
2
= L M T
Equating the coefficients:
x = 2
y + 1 = 0, y = –1
z – 3 = 0, z = 3
4
01 STPM Q&A PHY T1.indd 4 17/02/2022 10:03 AM