Competition - post a photo you've taken of nature >>

[Mechanics] Confused with problem involving thrust

Attachment not found

I’ve done a) b) of this question, where I had to show Acceleration was 0.75 and find the tension in tow bar.

Is there anything wrong with the following diagram? I’m pretty sure that the thrust is pushing the 2 objects inside (together).

I am unsure what the solution of F + 200 - 100 = 900a means (particularly why I have to subtract 100N. I think it’s the thrust but I don’t know how it’s acting on the car).

(edited 6 years ago)

Reply 1

Onlineslayer

Original post by FuryBall

Attachment not found

I am unsure what the solution of F + 200 - 100 = 900a means (particularly why I have to subtract 100N. I think it’s the thrust but I don’t know how it’s acting on the car).

i cant see the attachement

Reply 2

FuryBall

Original post by Onlineslayer

i cant see the attachement

I also realized that the second diagram I drew is incorrect in terms of direction of tension.

(edited 6 years ago)

Reply 3

Onlineslayer

Original post by FuryBall

Attachment not found

I am unsure what the solution of F + 200 - 100 = 900a means (particularly why I have to subtract 100N. I think it’s the thrust but I don’t know how it’s acting on the car).

Great. Do not think too deep about this.

Now, to help your understanding better, try to draw two separate diagram of forces for each vehicle.

Thrust is a push force from the car, it is opposite to a pull force (tension). So if you look at your solution to (a) and (b), those two little arrows facing each other were tension forces. In (c) however, the thrust would be two little arrows facing away from each other.

What I mean is, the Thrust force is pointing toward the right of the trailer, and pointing towards the left of the car.

So, for the trailer: The trailer has two forces acting on it, The 100N usual resistance and the 100N thrust both acting towards the right hand side. therefore deceleration is:

a = (100N + 100N)/300kg = 2/3

As for the car, there are three forces acting on it, the Thrust on the car is now acting towards the left (as I said earlier), The brake force F is towards the Right, The 200N resistance is also towards the right.
Hence:
F + 200 - 100 = 900a

where a=2/3

You do not need to worry about the forces acting on the trailer at this point, because as far as the car is concerned, these are the only forces acting on it. (as you can see from the forces diagram I told you to draw separately).

However, if you must know why we have done so, it is simply because if we wanted to include every force in the diagram, then we could have just had
F + 200 - Thrust + Thrust + 100N = 900a + 300a
and we will get exactly same result because a=2/3
LOL

Reply 4

Onlineslayer

Original post by FuryBall

I also realized that the second diagram I drew is incorrect in terms of direction of tension.

, Nah, that was Thrust, and so the directions was very correct.

Reply 5

FuryBall

Thanks for the reply. Yes I understand it now. Why did you write “LOL”? hahah

Reply 6

Onlineslayer

Original post by FuryBall

Thanks for the reply. Yes I understand it now. Why did you write “LOL”? hahah

hahaha don't mind me, I tend to be very unserious sometimes. And how come you have been awake all through. I actually went to bed and later woke up to solve it.