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Physicshelpline

. March 2017

Sound Waves

Q- The graph bellow represents a travelling wave. Each interval on the vertical axis corresponds to 5 cm, and each interval along the horizontal axis corresponds to 0.2 s. Find the amplitude, time period, frequency and wavelength of the wave. 

Solution:

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Electromagnetic Induction

Q- A circular loop with radius a = 0.35 m and N = 21 turns lies in the plane of the page. The wire used in constructing the loop has a resistance per unit length of dR/dl = 0.11 W/m. A spatially uniform magnetic field points into the page. In the interval between t = 0 and 10 s, the strength of this…

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Rotational Motion: Angular Momentum

Q- Big ben the parliament tower clock in London, has hour and minute hands with lengths of 2.70 m and 4.50 m and masses of 60.0 kg and 100 kg respectively. Calculate the total angular momentum of these hands about the center point. Treat the hands as long thin uniform rods.

Solution:

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Radiation Pressure

Q- Calculate the work done on a 1000 square meter, 1 kg solar sail during a trip from Earth to Mars. Assume the sunlight is normal to the sail during the trip and is totally reflective. Consider forces of gravity of the sun and its radiation pressure only. 

Solution:

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Motion: Variable Mass

Q- a small body of mass m0 is projected vertically upwards in a cloud. Its initial speed is √(2gk). During its motion the body picks up moisture from the stationary cloud. Its mass at height x above the point of projection is m0(1+ ρx),where ρ is a +ve constant. Show that the greatest height h sa…

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Ray optics: Convex Mirror

Q- If the focal length of a convex mirror be written as -|f|, show that the magnification m of an object at a distance d0 from this mirror is given by m = |f|/(d0+|f|). Based on this relation, explain why your nose looks bigger than the rest of your face when looking into a convex mirror.

Solution:

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Wave Motion

Q- The displacement of a wave traveling in the negative y-direction is D(y, t) = (5.0 cm) sin (5.5y + 74t) where y is in m and t is in s. What are each of the following for this wave?

(a) Frequency

(b) Wavelength

(c) Speed

Solution:

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Alternating Current: Phasor diagram

Q- Represent current wave-forms I1 = 4 cos(ωt) and I2 = 3 sin (ωt) in a phasor diagram, using sin (ωt) as the reference phasor. Now represent the resultant current waveform on the phasor diagram in the same way.  

Solution:

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Rotational Motion

Q- A top has a moment of inertia equal to 4.00 x 10-4 kg.m2 and is initially at rest. It is free to rotate about the stationary vertical axis AA'. A string wrapped around a peg along the axis of the top, is pulled in such a manner as to maintain a constant tension of 5.57 N. If the string does not s…

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Coulomb's Law: Force and Field

Q- Two charges placed on the x axis. Qa = 4 µC at x = 0 and Qb = - 4 µC at x = 22cm.

(a) Find the net electric field at point P at x = - 6 cm. 

(b) Find the force on Qb due to Qa

(c) The charges Qa and Qb are now attached to the ends of a spring whose un-stretched length is s0 = 22 cm and…

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Electromagnetic induction

Q- A circular loop of radius a = 20 cm and N=168 turns is fixed in the x-y plane. A spatially uniform magnetic field with only -component covers the entire area of the loop. The plot bellow shows Bz measured in Tesla versus time t measured in seconds. The +z direction is OUT of the screen.

(a) …

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Wave Optics: Diffraction

Q- A diffraction grating has 20 slits per mm. The distance between the slit centers is twice the width of each slit. The grating is uniformly illuminated at normal incidence. The transmitted light passes through a lens of focal length 1 m and then onto a screen that lies 1 m from the lens. On the sc…

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Newton's Laws of Motion

(on request - Shantanu)

Q- A man with mass m=75 kg is standing on a scale in an elevator. Find what the scale shows in the following two situations.
(a) the elevator is going up while speeding up at 2 m/s2
(b) the elevator is going down while slowing down at 2 m/s2

Solution

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Electric and Magnetic forces

(On request - Rajesh)

Q- (a) A positive ion of mass m1 and charge q1, initially at rest is accelerated through a potential difference V. Find its speed in terms of given quantities.

(b) The ion than enters a region of uniform magnetic field B, normal to its motion, and moves in a semicircular orbi…

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Motion in 1D:

(On request - Tamanna)

Q- A falling flowerpot takes 0.20 s to fall past a window that is 1.9 m tall. From what height above the top of the window was the flowerpot dropped? 

Solution:

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Electric and Magnetic Forces

Q- An electron is moving northward at 5*105 m/s in a uniform electric field of magnitude 20 N/C directed vertically downward. Due to presence of a magnetic field as well the electron moves in a straight line un-deviated. 

(a) Find the magnitude and direction of the force on the electron due to ele…

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