1- The old LP record can spin at 30 rmp. if the linear speed at the rims of the

Question

1- The old LP record can spin at 30 rmp. if the linear speed at the rims of the record is 6 cm/s, what is the linear speed half way between the center and the rims of the record?

2- A person pushes horizontally with constant force of 100N on a box of weight 250 N across a kitchen floor at a constant speed in a straight line . the force of friction between the box and the floor is?

3- A bungee jumper standing at the top of a vertical cliff has a pontential energy of 50 KJ (50000 j) relative to the gound above. after he jumps and is halfway to the ground, his kinetic energy is

4- If 1 kwh (1000wh) of energy costs 10 cents, how much would it cost you to leave a 100 W light bulb on for 20 hours?

Explanation / Answer

1)

radius of the Lp record is R = V/w

v = 6 cm/s

w = 30 rpm = 30*2*pi/60 = 3.142 rad/s

R = 0.06/(3.142) = 0.01909 m

then required linear speed is v = (R/2)*w= (0.01909/2)*3.142 = 0.0299m/s = 3 cm/s

2) since the box is moving with constant speed then accelaration a = 0 m/s^2

Fnet = Fapplied -frictional force f = m*a = 0

Fapplied = frictional force f = 100 N

3) using law of conservation of energy

energy at the top = energy at the half way

m*g*h = (m*g*h/2) + KE

50 KJ = 25KJ + KE

KE = 25 KJ

4) energy consumed is 100*20 = 2000 wH = 2kWh

1 kWh = 10 cents

2 kWh = 2*10 cents = 20 cents

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