- #1
TomK
- 69
- 14
- Homework Statement
- ENGAA 2019
- Relevant Equations
- Equation of motion
The correct answer is 'C'. Why is my working wrong?
haruspex said:The image of your working is rather faint, and it would benefit from some explanation of what you are doing.
You seem to be considering forces on the car (ma, D1), but that is rather pointless since you have no given info on the tractive force on the car. Yes, you can deduce it, but it is more direct to consider forces on the caravan.
If we take the bar as massless, ΣF=ma tells us that the net force on it must be zero.TomK said:If you pull a bar from both ends, what counts as the resultant force?
haruspex said:If we take the bar as massless, ΣF=ma tells us that the net force on it must be zero.
Tension is not quite the same as a force; it is more a pair of equal and opposite forces at each point along the bar.
See section 2 at https://www.physicsforums.com/insights/frequently-made-errors-mechanics-forces/
Because it is being pulled equally in opposite directions by the car and the caravan. That's tension.TomK said:How can the resultant force on the bar be zero if it extends?
haruspex said:Because it is being pulled equally in opposite directions by the car and the caravan. That's tension.
Separate the system into three bodies, car, bar and caravan.TomK said:I still don't understand what the total force is. If F = D1 + D2, how can it be pulled equally? D2 is pulling left, and F is pulling right.
haruspex said:Separate the system into three bodies, car, bar and caravan.
In general, the car pulls on the bar with force T1, and by Newton's laws the bar pulls on the car with an equal and opposite force.
Similarly a force T2 between bar and caravan.
Net force on the bar is T1-T2=mbara. Since mbar is negligible here, the difference between T1 and T2 is negligible, so call them both T (but in opposite directions).
For the purposes of finding the extension of the bar, we need T.
If F is the total driving force on the system then F-D1-D2=(M+m)a. But that is not helpful since it sheds no light on T. You can combine it with the equation for the car, F-D1-T=ma, to obtain T-D2=Ma, but you can get that directly by considering the forces on the caravan.
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