# Using acceleration to find tension (concept problem)

Homework Statement:
Two boxes of masses M1= 2kg and M2 = 1 kg are pulled up a frictionless ramp by a force P=25N. Calculate the tension in the rope connecting the boxes. Theta is 40 degrees.
Relevant Equations:
F = m* a
Hi guys, I feel that this question has a very straightforward answer but I am just not quite grasping it. The first picture is the homework question, the second is the free body diagram I drew.

I know that a = F/m and I know the solution should be:

a = (P - m1*gsinθ - m2*gsinθ) / m1 + m2

What I don't understand is why isn't tension (T) included in the above equation? It's clearly along the x-axis with the others and P is used, so why not tension as well?

The equation that seems logical to me to use is:

a = (T + P - m1*gsingθ - m2*gsingθ ) / m1 + m2

but this is wrong. Can you help me understand why T doesn't belong?

#### Attachments

nasu
Gold Member
The tension acts on both objects not just on one of them. It is an internal force if you consider the system of the two objects.

• SkyOfMyOwnLight
Chestermiller
Mentor
If you we’re doing a force balance on each body individually, the tension T would come into each of the force balances. What would those two force balance equations be?

SammyS
Staff Emeritus
Homework Helper
Gold Member
Problem Statement: Two boxes of masses M1= 2kg and M2 = 1 kg are pulled up a frictionless ramp by a force P=25N. Calculate the tension in the rope connecting the boxes. Theta is 40 degrees.
Relevant Equations: F = m* a

Hi guys, I feel that this question has a very straightforward answer but I am just not quite grasping it. The first picture is the homework question, the second is the free body diagram I drew.

I know that a = F/m and I know the solution should be:

a = (P - m1*gsinθ - m2*gsinθ) / m1 + m2

What I don't understand is why isn't tension (T) included in the above equation? It's clearly along the x-axis with the others and P is used, so why not tension as well?

The equation that seems logical to me to use is:

a = (T + P - m1*gsingθ - m2*gsingθ ) / m1 + m2

but this is wrong. Can you help me understand why T doesn't belong?

Hello, @SkyOfMyOwnLight . The given solution treats the two boxes along with the connecting rope as one object.

By the way: the sum of the masses, m1 + m2, should be enclosed in parentheses if you mean for it all to be in the denominator as in the following.

a = (P - m1⋅g⋅sinθ - m2⋅g⋅sinθ) / (m1 + m2)

This acceleration can then be used to determine the tension, T . 