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Work And Energy — Science STD 9 — Question

Gujarat BoardEnglish MediumSTD 9ScienceWork And Energy4 Marks
Question
 A more powerful vehiclewould complete a journey in a shorter timethan a less powerful one. We talk of the powerof machines like motorbikes and motorcars.The speed with which these vehicles changeenergy or do work is a basis for theirclassification. Power measures the speed ofwork done, that is, how fast or slow work isdone. Power is defined as the rate of doingwork or the rate of transfer of energy. If anagent does a work W in time t, then power isgiven by

P= work/time

P= W/T. The unit of power is watt.

(i) The rate of doing work is defined as

(a) Energy

(b) Force

(c) Power

(d) None of these

(ii) Total energy consumed divided by total time taken is called as

(a) Average power

(b) Instantaneous power

(c) Both a and b

(d) None of these

(iii) Let A and B having same weight start climbing the rope and reach height of 10m. Let A takes 10sec while B takes 12sec then work done

(a) By both will be same

(b) By A is more than work done by B

(c) By B is more than work done by A

(d) None of these

(iv) Define 1 Watt of power

(v) An electric bulb of 20W is used for 5h per day. Calculate the ‘units’ of energy consumed in one day by the bulb.

✓

Answer

(i) c

(ii) a

(iii) a

(iv) A power is said to be 1 watt when 1 joule of work is done within 1 second of time.

(v) Power of electric bulb = 20 W

= 0.02 kW.

Time used, t = 5 h

Energy = power × time taken

= 0.02kW × 5 h

= 0.10 kW h

= 0.10‘units’.

The energy consumed by the bulb 0.10 units

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W = m x g

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g= acceleration due to the gravitational force

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(b) John Dalton

(c) E. Rutherford

(d) Neilsbhore

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(b) Molecule

(c) Atom

(d) Protons

(3) Identify the correct statement

Statement 1 – Positively charged centre in an atom called the nucleus.

Statement 2 – The electrons revolve around the nucleus in circular paths.

Statement 3 – Nearly all the mass of an atom resides in the nucleus.

Statement 4 – The size of the nucleus is very small as compared to the size of the atom.

(a) Only 2

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(c) Both 1 & 2

(d) All of the above

(4) Write the features of Rutherford’s nuclear model of an atom?

(5) Define Nucleus.

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