Posted by **sumi** on Thursday, December 8, 2011 at 1:39am.

Problem 1

A goalkeeper kicks a ball of mass 400 g in the air, and reaches a position of height 15.9 m above the ground corresponding to a horizontal distance 19.7 m from its initial position. During its flight, the ball experiences a horizontal friction force of 3 N from a wind blowing opposite to the horizontal direction of its motion.

1) What is the work done by the gravitational force on the ball?

2) What is the work done by the friction force on the ball?

3) What is the total work done on the ball?

Problem 2

When a mass of 2 kg is hung vertically at the end of two light springs connected in series, the combined stretching of the two springs is 10 cm .

1) If the spring constant of the first spring is k1 = 294 N/m, what is the spring constant of the second spring k2?

2) If the 2 kg mass is removed, how much work should be done by an applied force to stretch the two springs by 20 cm from their unstretched position (see figure)?

Problem 3

An object of mass 5 kg is thrown up on a rough incline of 20o with an initial speed of 2 m/s, and stops at a distance d at the top of the incline . The coefficient of kinetic friction between the object and ground is k= 0.1

(1) Using the work-kinetic energy theorem, determine the distance d.

(2) What is the average power used?

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