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YOUR PARTICIPATION FOR THE GROWTH OF PHYSICS REFERENCE BLOG

Saturday, March 17, 2018

Which statement defines force?


Question 4
Which statement defines force?

A When a force acts on a body that is free to move, the force is the product of the mass of the body and its acceleration.

B When a force acts on a body that is free to move, the force is the rate of change of
momentum of the body.

C When a force acts on a body that is free to move, the force is the work done by the force divided by the distance moved by the body.

D When a force acts on a lever and causes a moment, the force is the moment divided by the perpendicular distance of the force from the pivot.





Reference: Past Exam Paper – March 2016 Paper 12 Q9





Solution:
Answer: B. 

Force is DEFINED from Newton’s second law of motion. This tells us that the force is proportional to the rate of change of momentum of a body.

F = Δp / Δt = Δ(mv) / Δt


The formula ‘F = ma’ is only a consequence of the second law if we assume that the mass is constant.

F = m(Δv/ Δt) = ma


Choices C and D only deal equations that contain ‘force’. These do not defined force.

Sunday, March 11, 2018

A 23892U nucleus decays in two stages to a 23491Pa nucleus. What was emitted in these two stages?



Question 4
A 23892U nucleus decays in two stages to a 23491Pa nucleus.

What was emitted in these two stages?
A α + β          B α + γ          C β + β          D β + γ





Reference: Past Exam Paper – November 2008 Paper 1 Q40





Solution:
Answer: A.

During the decay: 23892U        - - >             - - >  23491Pa + ???

For any decay, the proton number and mass number must be conserved.
Proton number: 92 = 91 + x
Mass number: 238 = 234 + y

Thus, x = 92 – 91 = 1              and y = 238 – 234 = 4


3 types of radiation can be emitted during radioactive decay: alpha, beta (minus) and gamma. We know that alpha particle is 42α and beta particle is 0-1β. Gamma is an EM wave and does not have a mass or proton number.


We need to identify which of the combinations provided in the multiple choice would give an overall proton number of +1 and overall mass number of 4.

Consider choice A:     [α and β]
Proton number = 2 + –1 = +1
Mass number = 4 + 0 = 4

So, A is correct

Monday, March 5, 2018

The luminosity L of a star is given by L = 4πr2σT4 where r is the radius of the star



Question 5
The luminosity L of a star is given by
L = 4πr2σT4
where
r is the radius of the star,
T is the temperature of the star,
σ is a constant with units W m-2 K-4.

What are the SI base units of L?
A kg m2 s-1                        B kg m2 s-2                        C kg m2 s-3                       D kg m2 s-4





Reference: Past Exam Paper – June 2016 Paper 12 Q2





Solution:
Answer: C.

L = 4πr2σT4
[L] = [r2] [σ] [T4]
4π is a constant that has no unit. {Note that some constants may have units.}


[L] = (m2) (W m-2 K-4) (K4) = W
The unit W (watt) is not an SI base unit.


Watt is the unit of power.
Power = Work done / time = (Force × distance) / t = (ma × s) / t

Power = (ma × s) / t


[L] = W = (kg) (m s-2) (m) / s = kg m2 s-3
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