Science Student - Stephen Smith

## Can you do my mock exam?

• 20 Dec 06, 04:26 PM Here are a couple of questions from the paper, set one term into my physics A-level. Good luck!

1. (a) (i) Define the moment of a force.

(ii) State the principle of moments.

(b) The diagram below shows a pillar (lying horizontally) made of two uniform sections X and Y each of cross-sectional area 3.5 x 10-2. m2. The sections are made from two different materials. The weights of X and Y are shown acting through the centre of gravity of each section. Show that the average density of the pillar is about 1800 kg m-3. .

(c) The pillar in (b) will balance horizontally supported vertically below the point P.

(i) Show, using the principle of moments, that the point P is 1.2m from the end B.

(ii) State the significance of point P.

2. (a) (i) Explain the concept of work and relate it to power.

(ii) Define the joule.

(b) A cable car is used to carry people up a mountain. The mass of the car is 2000 kg and it carries 800 people, of average mass 60 kg. The vertical height travelled is 900 m and the time taken is 5 minutes.

(i) Calculate the gain in gravitational potential energy of the 80 people in the car.

Gravitational potential energy gain = ……….J

(ii) Calculate the minimum power required by a motor to lift the cable car and its passengers to the top of the mountain.

Power =…..… unit………..

The exam questions were taken from an OCR past paper from 2001. If readers want to attempt more exam papers for fun, there are some specimen papers available at the OCR website.

• 1.
• At 01:45 AM on 21 Dec 2006,
• Mark Goodall wrote:

1 a(i) The moment of a force is product of the force and the distance perpendicular to the direction of the force to the point about which the force is acting.

(ii) I don't know the terminology but presumably it is something to do with with two equal moments acting in opposition about a point cancel one another out.

1 b

Volume = 3.5*10^-2*(1.2+0.8)
Mass = 1250N/9.81

Density = Mass/Volume = 1250/9.81/(2*3.5*10^-2)= 1820.3 kg m-3

1 c(i)
The point P is the point at which the moment of each centre of gravity of the two blocks is equal.

The centres of gravity are separated by 1m

For the moments to be equal, measuring the position of p from the centre of gravity of block X

1000*p=(1-p)*250
1000/250=1/p-1
5=1/p
p=1/5m from centre of X

From right hand end
p= 0.4 + 1 - 0.2
= 1.2m

2) a (i) Work is the potential energy change of an object that occurs when an object is moved by a force and is defined by the product of force and distance and is measured in Joules.

Power is the rate at which work is done and is defined as the work per unit time and measured in Watts or Joules per second.

(ii) Joule is the energy required to apply a force of one newton for a distance of one metre.

2 b The question is mistyped , it refers to 80 and then 800 people!

I will assume 80 people

(b) (i)
Energy gain mgh= 80*60*9.81*900
=4.23792*10^7 Joules

(ii) Power, Watts, is Joules per second

Energy gain for people and car
=(80*60+2000)*9.81*900
=6.00372*10^7 Joules

Power
=6.00372*10^7/(5*60)
=200124 Watts or 200kW

• 2.
• At 02:48 AM on 22 Dec 2006,
• Gav wrote:

Just a quick question....is Stephen going to be doing the full A-level over the 2 years or just this module, or just the AS-level modules?

• 3.
• At 10:56 AM on 29 Dec 2006,
• Semantics wrote:

I trust that, like me, most have been enjoying Xmas and have consequenty been too busy to monitor the site. Hopefully there will be maintained interest in the topic.

The "principle of moments" is not a term I also am familiar with although the clue is in its use in 1c)i).

M. G. did not include a response to the trivial 1c)ii), although his statement at the commencement of 1c)i) could be taken as such. I assume the term centre of gravity for the combined black was being sought.

Again we have a question which contains an error, albeit a typo which could have been introduced at any stage in posting the questions. As humans, we all make mistakes and checking one's own work does not remove them all because we see what we want to see. It has been demonstrated that even quite complex text can easily be read if misspelt provided that the first and last letters are correct. Having worked in requirements capture and specification for complex real-time software systems, I am perhaps exceptionally sensitive to the need to remove errors at each stage before passing work to the next stage. However, since I hardly ever read a newspaper without spotting several typos or grammatical errors, I would appreciate a little more care being shown by the media.

• 4.
• At 02:37 PM on 29 Dec 2006,
• Semantics wrote:

Hoist by my own petard or deliberate error to underline a point (black for block in my earlier post)? Mea culpa!

I should add a clarification that the misspelling to which I referred consisted of the correct letters for each word but in a wrong order, apart from the first and last.

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