[ALI ALZAHRANI]

3. A boy holds a 40-N weight at arm’s length for 10 s. His arm is 1.5m above the ground. The work done by the force of the boy on the weight while he is holding it is: A. 0 B. 6.1J C. 40 J D. 60 J E. 90 J Ans:a 5. An object moves in a circle at constant speed. The work done by the centripetal force is zero because: A. the displacement for each revolution is zero B. the average force for each revolution is zero C. there is no friction D. the magnitude of the acceleration is zero E. the centripetal force is perpendicular to the velocity Ans:e 6. An object of mass 1 g is whirled in a horizontal circle of radius 0.5m at a constant speed of 2m/s. The work done on the object during one revolution is: A. 0 B. 1 J C. 2 J D. 4 J E. 16 J ans: A

الأسئلة الثلاثة الأولى لا يوجد شغل (الشغل يساوي صفر) لأن القوة عمودية على الازاحة (الزاوية بينهما تساوي 90) وحيث إن

W = F d cosQ = F d cos 90 = 0

لان كوساين 90 تساوي صفر

10. A 2-kg object is moving at 3m/s. A 4-N force is applied in the direction of motion and then removed after the object has traveled an additional 5m. The work done by this force is: A. 12 J B. 15 J C. 18 J D. 20 J E. 38 J ans: D

الشغل يساوي

W = F d cosQ = 4*5*cos0 = 20

13. Camping equipment weighing 6000N is pulled across a frozen lake by means of a horizontal rope. The coefficient of kinetic friction is 0.05. How much work is done by the campers in pulling the equipment 1000m if its speed is increasing at the constant rate of 0.20m/s2? A. −1.2 × 106 J B. 1.8 × 105 J C. 3.0 × 105 J D. 4.2 × 105 J E. 1.2 × 106 J

أولا نوجد قوة الشد، حيث ان محصلة القوى على الجسم تساوي

T-fk = ma

T = ma + fk = ma + umg = 6000/9.8*0.2+60000.05 = 422.45 N

وبالتالي سيكون الشغل

W = T d cosQ = 422.45*1000*cos0 = 4.2 × 10^5 J