Tran ties a string to a doorknob 0.70 m from the hinge as shown in the figure. T

Question

Tran ties a string to a doorknob 0.70 m from the hinge as shown in the figure. The door is 0.80 m in length (l) and its mass (m) is 15 kg. The moment of inertia for the door is given by: Idoor = 1/3 ml2 . At the instant shown, the Robbie applies a 7.0 N force to the string. i. What is the torque on the door?Determine the angular acceleration of the door just as Tran pulls the string. If a doorstop were placed halfway between the hinge and outer edge of the door, what force would it apply to the door to keep it from moving?

Explanation / Answer

Part A)

Torque = Fr = (7)(.7) = 4.9 Nm

Part B)

Fr = I(alpha)

4.9 = .333(15)(.8)2(alpha)

alpha = 1.53 rad/s2

Part C)

Fr = 4.9

F(.4) = 4.9

F = 12.25 N

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