# Forces on 2 particle connected by a string?! [closed]

A van of mass $$3000$$ kg is pulling a trailer of mass $$500$$ kg along a straight horizontal road at a constant speed of $$25$$ms−1. The system of the van and the trailer is modeled as two particles connected by a light inextensible cable. There is a constant resistance to motion of $$300$$ N on the van and $$100$$ N on the trailer.
(i) Find the power of the van’s engine.
(ii) Write down the tension in the cable.

In first part we use total resistance to get the power. Why don't we use $$300$$ N? How do we imagine tension in different area (string, van and trailer)?

## closed as off-topic by Servaes, zipirovich, Alexander Gruber♦Apr 28 at 8:27

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Alexander Gruber
If this question can be reworded to fit the rules in the help center, please edit the question.

• This seems more suited for physicsSE – user1952500 Apr 24 at 20:41
• It's Alevel math 42 – Asad Apr 24 at 20:42

The only forces acting on the van are the resistance on it, the force exerted on it by the cable and the propulsive force supplied by the engine. Since the van is moving at constant speed, these forces must also balance. The reason why you don't use only the resistance, $$300$$ N, on the van in computing the power of the engine is because the propulsive force doesn't merely have to balance that force by itself, but the sum of that force and the force exerted on the van by the cable.