# Exam Review: Force - Which Answer is Correct?

• nwowen
In summary, the question is asking for the force on the trailer due to the tractor, given that the tractor is pulling with a constant velocity of 20 m/s and the mass of the trailer is 200kg while the mass of the tractor is 500kg. The correct answer is B, as there is no change in acceleration and therefore no net force acting on the trailer. However, the previous person who took the exam put C as their answer, possibly due to confusion with the concept of net force and the assumption of frictionless motion.

#### nwowen

Hi, I am reviewing for an exam on Saturday and I'm looking at some old exams for the course and I came across the question.
A tractor (t) is pulling a trailer (m) with a constant velocity. If the velocity is 20 m/s and m = 200kg and t = 500 kg the the force on the trailer dou to the tractor is:

A 200 N backward
B 0
C 600 N forward
D 35 N forward
E 450 N backward

Now I personally believe that the correct answer is B, because there is no change in acceleration, so with f=ma, there is a value of a=0 because there is no change. However, whoever took the exam before me put C and I'm somewhat hesitant to second guess the other person.

Thanks.

Why are you hesitant to second guess somebody who you don't know, and could have been completely retarded?

Try to see if you can somehow get their answer...

A note for the future, you've got to remember that F=ma is the sum of forces on the trailer.

So the tractor could be pulling with a force of 500N forward but the drag and rolling resistance is 500N backwards. Net force = 0 therefore no acceleration.

Zero acceleration does not necessarily mean that the tractor is not pulling the trailer with a certain force.

Now in this case there is no information on drag or friction so you have to assume its frictionless. Meaning that for the forces to sum to zero the tractor must be pulling with zero force.