Consider the total work done on an object that has several forces acting on it.

Question

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 (Figure 1) . 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 10.0 kJ. The mass of the sled is m=(14,700N)/(9.80m/s2)=1500kg. Suppose the sled’s initial speed vi is 2.00 m/s.

Part A

If the tractor pulls horizontally and the friction force has a magnitude of 4900 N , what is the magnitude of the force the tractor must exert to make the sled move with constant speed? (in Newtons)

Explanation / Answer

If the sled is initially moving with 2 m/s and if the tractor wants to move the sled with the same speed, then tractor needs to provide exact friction force in opposite direction (to overcome friction) such that net force is zero (constant speed).

F = 4900 N

tractor force minus friction force should yield zero net force.

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