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enter image description here Suppose we use a trailer to pull a car, which has a damaged motor. The car moves at a constant speed, assume that air drag is negligible and the wheel does not slide on the road, it only rotates. So,as shown in the diagram, the tension and the frictional force both point in the direction of movement. So how can the car moves at a constant speed?

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    $\begingroup$ The frictional force always opposes the direction of motion - you'll find that your diagram is not correct. $\endgroup$ – Sam Blitz Jan 20 '16 at 3:43
  • $\begingroup$ This answer may be helpful! $\endgroup$ – lucas Jun 20 '16 at 18:43
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You can have constant speed.

The frictional force of the floor on the wheels of the car is backwards not fowards as you have drawn. This differs from the frictional force of the floor on the wheels of the trailer (which has the running engine).

The wheels of the trailer exert a backwards force on the floor, which pushes it forward (which could balance the tension in the back).

Now the wheels in the car (which has no engine running) try to drag the floor along with it (forward force). The floor hence exerts a backward force on the wheels. This is what causes the wheels in the back car to rotate. The tension in the cable is forward. The friction is backwards. They could balance each other.

So the wheels in the trailer vs car, act differently.

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  • $\begingroup$ you do realise, you're talking about different forces acting on different bodies!!! $\endgroup$ – don_Gunner94 Jan 20 '16 at 15:46
  • $\begingroup$ Yes. What's your point? $\endgroup$ – Ameet Sharma Jan 20 '16 at 15:49

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