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# Daily Archives: September 13, 2011

## More problems coming in; Try to answer now!

Stuti posted these:

1. a particle is thrown with  any velocity vertically upward. the distance traveled by particle in last sec of ascent is 1)g 2)g/2 3)g/4 4)cant be calculated
2. a ball is dropped from a bridge of 122.5 metre above a river. after ball has been falling for 2 sec,a second ball is thrown straight down after it. initial velocity of second ball so that both hit water at sme time is 1)49 m/s 2)55.5 m/s 3)26.1 m/s 4)9.8 m/s.
3. two cars are moving in same direction with a speed of 30 km/hr. they are separated from each other by 5 km. third car moving in opposite direction meets the two cars after an interval of 4 minutes. the speed of third car is 1)30 km/hr 2)25 km/hr 3) 40 km/hr 4)45 km/hr
4. a balloon starts rising from ground from rest with an upward acceleration 2 m/s^2. jst after 1 sec , a stone is dropped from it. time taken by stone to strike ground is nearly 1)0.3 sec 2)0.7 sec 3)1 sec 4)1.4 sec

## Problem from relative velocity

A Boat covers certain distance between two spots in a river taking t1 hrs going downstream and t2 hrs going upstream. What time will be taken by boat to cover same distance in still water?

posted by STUTI

$t=\frac{2t_{1}t_{2}}{t_{1}+t_{2}}$

## Collection of Problems received from Class 12 Physics

These problems were posted by Geena. Hope that we will be able to post the answers to these questions soon; each in a separate post. By the time visitors can attempt to post their answers as comments to this post. (Only selected posts will be published)

Please note that the answers are not published until the Physics teachers are not satisfied with the answers. So, please be patient to get the correct answers rather than approximations in a haste.

If you want real time answers to your queries by a group of Physics Lovers, you can post them at the Physics Fans group with facebook at https://www.facebook.com/groups/askphysics/

1. A series battery of 10 lead accumulators each of emf 2 V and internal resistance 0.25 ohm is charged by a 220 V D.C. mains to limit the charging current a resistance of 47.5 ohm is used in series in the charging circuit. What is a) the power supplied by the mains and b) power dissipated as heat? Account for the difference of powers is (a) and (b).
2. A potentiometer wire has a length L and a resistance Ro. It is connected to a battery and
a resistance combination as shown. Obtain an expression for the potential drop per unit length of this potentiometer wire. What is the maximum emf of a ‘test cell’ for which one can get a ‘balance point’ on this potentiometer wire? What precaution should one take, while connecting this ‘test cell’ , in the circuit?
3. A cell, of emf 4 V and .internal resistance 0.5 Ω, is connected across a load of resistance (i) 7.5 Ω, (ii) 11.5 Ω. Calculate (i) the ratio of the differences in the emf of the cell and the potential drop across the load, and (ii) the ratio of the currents in the two cases.
4. In the figure shown, calculate the total flux of the electrostatic field through the spheres S1 and S2 The wire, AB, shown here, has a linear charge density, λ given by λ = kx where x is the distance measured along the wire, from the end A.
5. A straight wire, of length L, carrying a current I, stays suspended horizontally in mid air in a region where there is a uniform magnetic field B . The linear mass density of the wire is λ. Obtain the magnitude and direction of this magnetic field.
6. Two cells of same emf E, but different internal resistance rl and r2 are connected to an external resistance R as shown. in the figure. The voltmeter V reads zero. Obtain an expression for R in terms of rl and r2. Calculate the voltage across the cell of internal resistance r2. (Assume that the voltmeter V is of infinite resistance).
7. A galvanometer with a coil of resistance 120 ohm shows full scale deflection for a current of 2.5 mA How will you convert the galvanometer into an ammeter of range 0 to 7.5 A ? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer.
8. Figure shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph
showing the variation of induced e.m.f (E) with time (t). What does the area enclosed by the
E – t curve depict ?
9. The electron in the hydrogen atom circles around the proton with a speed of 2.18 x 106 m/s in an orbit of radius 5.13 x 10 -11 m. What magnetic field does it produce at the centre?
10. A proton moves with a speed of 7.45 x 105 m/s directly towards a free proton originally at rest. Find the distance of the closest approach for the two protons. (Given: mass of proton = 1.67 x 10–27 kg and e = 1.6 x 10 –19 C)
11. Figure (a), (b) and (c) show three a.c. circuits in which equal currents are flowing. If the frequency of emf be increased, how will the current be affected in these circuits? Give reason for your answer.

## The problem of a rock thrown vertically up

A rock is thrown vertically upward from the ground with an initial speed 15m/s.

a. how high does it go
b. how much time is required for the rock to reach its maximum height?
c. what is the rock’s height at t=2.00s?

(Posted by Merhawi)

(a)

u=15m/s

a=-10m/s2

v=0 m/s (at the max height)

S=?

v2-u2=2aS

S=v2-u2/2a

=225/20

=11.25 m

(b) From the above case

using v=u+at

t=v-u/a=1.5s

(c) Use S = ut + 1/2 at2

put t=2s, u = 15m/s, a=-10m/s2

S=30 – 20 = 10 m

(If you use g = 9.8 m/s2 The answers will be slightly different)

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