Practice Problem 7.4 Consider the total work done on an object that has several

Question

Practice Problem 7.4 Consider the total work done on an object that has several forces acting on it. A tractor is hitched to a sled loaded with firewood and pulls the sled a distance of 20.0 m along level frozen ground. Let's determine what actually happened to the work energy produced by the forces acting on the sled. The free-body diagram is shown in . We find that the total work done on the sled by all the forces is 10.0 kJ, so the kinetic energy of the sled must increase by 100 kJ. The mass of the sled is m (14,700 N)/(9.80 m/s) = 1500 kg. Suppose the sods n al speed vi is 2.00 m/s. What is its final speed? -200 ms SOLUTION SET UP The work W done by the weight is zero because its direction is perpendicular to the displacement the cosine of the angle is zero.) For the same reason, the work also zero. So we-11, 0. . (The angle between the two directions is 90" and W, done by the normal force i (which, incidentally, is not equal in magnitude to the weight) as That leaves FrandfFrom W = F.the work w, dore bythe tractoris(with Fi-F,cose, (SOON)(0.800)(20.0 m) = 80,000 N-m .. 80.0 kJ = The friction force f is opposite to the displacement, so, for this force,180°and cosé.-1. The work W, done by the friction force is W, (feos 180' )s=-(3500 N)(20.0 ma) -70,000 N·m=-70.0 kJ The total work Wut done by a·ofthe forces on the sled isthe aigebrac sum uotthe vear sum)ofthe work don, by te ndiadual trees 80.0kJ+0+0+-70.0 kJ) 10.0kJ We found that Wio -10.o kJ. The initial kinetic energy Kis

Explanation / Answer

Any body moving with constant speed its acceleration is zero

So, net force acting on the body is zero

In that case forces balance each other

Magnitude of the force of the tractor is 4500N

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