• Support PF! Buy your school textbooks, materials and every day products Here!

Truck pulling trailer

  • #1
2
0
User informed about keeping the headings of the homework template.
I have a truck (m1,27000 kg) pulling a trailer (m2, 8500 kg) with a chain holding the two together (T). The overall acceleration is 0.78 m/s^2.
Find f1 force on the truck, f2 force on the trailer and T, tension force.

Here is what I did: since the force comes all from the truck, f1 = (m1+m2)(a) = 35500*0.78 N
T = -f2 = 8500 * 0.78 N

But apparently this is how you do it:

f2 = m2a=T = 27000*.78 N = T
f1 = m1a = F-T
F = f1+f2 = (m1+m2)a = 35500*.78
f1 = 35500*.78 - 27000*.78;

I just dont understand how f2 which is just being pulled can have force greater than f1 which is doing all the work.

What am I doing wrong?

Thank you
 

Answers and Replies

  • #2
2
0
The main thing that I dont understand is F = f1+f2, how can a powerless trailer have a positive force. Wouldn't F = f1-f2?
 
  • #3
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
16,669
6,453
Find f1 force on the truck,
This part is a bit unclear, the force on the truck from where? Is it the total force or the force from the ground on the truck or the force from the chain on the truck?
 
  • #4
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,295
+1

Find f1 force on the truck, f2 force on the trailer and T, tension force.
Do you have a diagram showing where f1 and f2 are measured?

As far as I can see these could all be the same. The chain is pulling the trailer so the tension _is_ the "f2 force on the trailer". The force f1 on the truck could be the tension in the chain or the friction force with the ground??
 
  • #5
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,295
But apparently this is how you do it

f2 = m2a=T = 27000*.78 N = T
That's not consistent with the problem statement (m2 is 8500kg not 27000kg). It should be....

f2 = m2a = 8500 * 0.78 = T = 6630N .................................(1)

f1 = m1a = F-T
That implies f1 is the force required to accelerate the truck alone without the trailer. That's OK.

F = f1+f2 = (m1+m2)a = 35500*.78
Ok so F is the total force required to bull both truck and trailer. That's OK

f1 = 35500*.78 - 27000*.78;
That doesn't make sense due to the error above at (1). Should be

f1 = 35500*.78 - 8500*.78

but there is a much easier way to calculate f1...

f1 = m1*a = 27000 * 0.78

I don't see why they need to make it so complicated.
 

Related Threads on Truck pulling trailer

Replies
1
Views
3K
  • Last Post
Replies
15
Views
8K
Replies
6
Views
3K
  • Last Post
Replies
4
Views
4K
Replies
24
Views
3K
  • Last Post
Replies
2
Views
3K
Replies
1
Views
3K
Replies
14
Views
1K
Replies
2
Views
2K
Top