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In this lesson, we will learn how to calculate the positions and masses of objects in rotational systems such that the net rotational acceleration is zero.

Q1:

A pole is at a 9 0 ∘ bend in a power line, as shown. The pole is subjected to more shear force than poles in straight parts of the line. The tension in each line is 4 . 0 0 × 1 0 4 N at the angles shown. The pole is 15 m tall, has an 18 cm diameter, and can be considered to have a Young’s modulus of 0 . 7 5 × 1 0 1 0 Pa.

Calculate the compression of the pole.

Find the magnitude of the bending of the pole.

Find the tension in a guy wire used to keep the pole straight. The guy wire is attached to the top of the pole at an angle of 3 0 ∘ with the vertical and is in the opposite horizontal direction to the bend in the power line.

Q2:

The forearm shown below is positioned at an angle with respect to the upper arm, and a 5.0-kg mass is held in the hand. The total mass of the forearm and hand is 3.0 kg, and their center of mass is 15.0 cm from the elbow.

What is the magnitude of the force that the biceps muscle exerts on the forearm for ?

What is the magnitude of the force on the elbow joint for ?

Q3:

A uniform 4.0-m plank weighing 200.0 N rests against the corner of a wall, as shown in the figure. There is no friction at the point where the plank meets the corner.

Find the magnitude of the force that the corner exerts on the plank.

The force exerted by the corner of the wall on the plank acts at an angle from the horizontal. Find the angle.

Find the magnitude of the force that the floor exerts on the plank.

The force exerted by the floor on the plank acts at an angle from the horizontal. Find the angle.

What is the minimum coefficient of static friction between the floor and the plank that is required to prevent the plank from slipping?

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