1. An airtight box, having a lid of area 120 cm 2 , is partially evacuated. Atmo

Question

1. An airtight box, having a lid of area 120 cm2, is partially evacuated. Atmospheric pressure is 1.01 x 105 Pa. A force of 120 lb is required to pull the lid off the box. What was the pressure in the box (in Pa)? Round your answer to the nearest hundred.

2. A hoop (I = MR2) of mass 3 kg and radius 1 m is rolling at a center-of-mass speed of 10 m/s. An external force does 810 J of work on the hoop. What is the new speed of the center of mass of the hoop (in m/s)? Round your answer to the nearest whole number.

3. A 19-g paper clip is attached to the rim of a phonograph record with a diameter of 47 cm, spinning at 2.6 rad/s. What is the magnitude of its angular momentum (in kg m2/s)? Round your answer to the nearest ten-thousandth.

Explanation / Answer

2)

Assuming that the hoop is not skidding then the kinetic energy of rotation is exactly equal to 1/2 m v^2
and so is the kinetic energy of translation.
ie HALF of the energy goes into accelerating the centre of mass.
1/2 * 3 * 10^2 +810 = 1/2 * 3 * v^2
or, 1.5x10^2 + 810= 1.5xv^2
or, v^2=640

or, v= 25.30 m/s..................................................................................................ans

3)

angular momentum = I x omega
Inertia (I) = mr^2 = ((19/1000)x0.3^2) = 0.019 kg x 0.47^2 = 0.0041971 kg/m^2
omega = 2.6 rad/s

so, the magnitude of its angular momentum

=0.0041971 x 2.6 = 10.91 x 10^(-3) kg-m^2/s .......................................................ans

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