You are Here: Home >< Maths

# [Mechanics] Confused with problem involving thrust watch

1. Attachment 701274
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).
2. (Original post by FuryBall)
Attachment 701274
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).
i cant see the attachement
3. (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.
4. (Original post by FuryBall)
Attachment 701274
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).

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
5. (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.
6. Thanks for the reply. Yes I understand it now. Why did you write “LOL”? hahah
7. (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.
8. Different time zones haha.
9. (Original post by FuryBall)
Different time zones haha.
true, you live in Japan
10. (Original post by Onlineslayer)
true, you live in Japan
How could you tell!
11. (Original post by FuryBall)
How could you tell!
'Cause I am creepy. Just kidding. Let's just say I did my research.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: November 5, 2017
The home of Results and Clearing

### 1,085

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. SAE Institute
Animation, Audio, Film, Games, Music, Business, Web Further education
Thu, 16 Aug '18
2. Bournemouth University
Fri, 17 Aug '18
3. University of Bolton
Fri, 17 Aug '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams