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4. 3.

1. What is the minimum force required to open a door when the torque is 3.1 N∙m and the force is applied at 0.94 m from the hinges at an angle of 90.0° to the face of the door? (Ans. 3.3 N)

*F 2 F 1*

50.0° 90.0° 2. Two helmsmen, in disagreement about which way to turn a ship, exert forces shown below on a ship’s wheel. The wheel has a radius of 0.74 m and the two forces have magnitudes *F* *1* = 72 N and *F*

*2*

= 58 N. Find the magnitude and direction of the net torque on the wheel from the two forces. (Ans. 2 N∙m, cw) 3*L/*4* * *L/*4* *

*F 1 mg F 2*

Mass *m* is supported on a light plank by two forces *F*

*1*

and *F*

*2*

as shown above. In terms of the given variables, find expressions for each of the forces that keep the plank in static equilibrium.

*L *

oval-shaped diver

*d *

A 5.00 m long diving board of negligible mass is supported by two pillars which are 1.50 m apart. Find the force exerted by each pillar when a 90.0 kg diver stands at the end of the board. Caution: what is the direction of the force exerted on the diving board by the pillar on the left? (Ans. *F* *R* = 2940 N, *F*

*L*

= 2060 N) (*over please*)

5. A cat walks along a uniform plank that is 4.00 m long and has a mass of 7.00 kg. The plank is supported by two sawhorses, one 0.440 m from the left end of the plank and the other 1.50 m from the right end. When the cat reaches the right end, the plank just begins to tip. What is the mass of the cat? Hint: when the plank just begins to tip, what is the force from the left sawhorse? (Ans. 2.33 kg)

6. When spiking a volleyball, a player changes the velocity of the ball from 4.2 m/s to −24 m/s along a certain direction. If the impulse delivered to the ball by the player is −9.3 N∙s, what is the mass of the volleyball? (Ans. 0.33 kg)