Quick Mechanics 1 Question

A car of mass 800kg is pulling a caravan of mass 100kg along a straight horizontal road. Caravan is connected to car by a light, rigid tow bar. Car is exerting a driving force of 1270N. The resistance to the forward motion of the car and caravan are 400N and 600N respectively; you may assume that these resistances remain constant.

i) Show that acceleration of the car and caravn is 0.15 done this part
ii) Draw a diagram showing all forces acting on caravn along the line of its motion. calculate the tension in the towbar. Done this part tension is 750N.

The driving force is removed but the car's brakes are not applies.
iii)Determine whether the tow bar is now in tension or compression??

No idea how to determine this but i think its in tension but u have to calulate the value too. Answer in book tension 44N any help on how they got to this well appreiciated

Reply 1

BibbleJW

The idea here is that you need to work out which os slowing down faster (doesn't really make sense, but you know what I mean). If the caravan is slowing down, in relation to the car, then the car is pulling it, and the towbar would be in tension. Then you would need to calculate the force the towbar (and so the car) would have to apply to the caravan in order to keep the two maving at the same speed.

Reply 2

amh189

still cant get 44N i got 18640N?? does anyone know how to get 44N diagram would help if possible

Reply 3

generalebriety

Post your working. :smile:

We can't help you unless we know what you're doing.

Reply 4

amh189

ok its probably totally wrong but i assumed T - (600+400) - (1000+800)*9.8 = 0
so T = 18 640N
i know this is totally wrong but for the first time in mechanics im stumped!

Reply 5

generalebriety

Sorry, I have no idea where you've got that line from...?

Reply 6

amh189

hi sorry forget that working i knew it was completely wrong but lets try and strt a fesh so is this diagram correct it shows all the resistive forces and am i right in thinking acceleration is 0 and it is moving at a constant speed or is the whole system slowing down?

ive only shown the horizontal forces not the vertical forces as they will cancel each other out

Reply 7

amh189

sorry heres the attachment should i take the force T off as the Q asks to determine whether towbar is in compression or tension so we dont know which direction the force T is acting?

untitled.JPG17.9KB

Reply 8

Dadeyemi

is this from the M1 book if so pleas give the page or a question refernce because i think you might be giving the question wrong as you have been giving multiple values for the mass.

Reply 9

generalebriety

amh189

sorry heres the attachment should i take the force T off as the Q asks to determine whether towbar is in compression or tension so we dont know which direction the force T is acting?

Ugh. You've given us three different masses for the caravan now, please get the question right. :s-smilie:

Reply 10

amh189

sorry mass of caravn is 1000KG not 10000 or 100Kg my bad soz Q is on page 71 of M1 book

Reply 11

Dadeyemi

ok start by working out the acceleration of the whole system

Spoiler

Reply 12

amh189

ok i see how you did it i went completely wrong cos i assumed the car carried on at a constant speed so a = 0 but i knew that was completely wrong pretty simple really dont know why i didnt get that and got the restof exercise right thanks for the help tho well appreciated