How Do Newton's Laws Apply to Multiple Objects in Motion?

[tigerwoods99]

Homework Statement

Three crates are arranged. The masses of the crates are m1= 29.8kg, m2= 15.3kg, and m3=27.3kg. The applied force of 910 N is directed horizontally, and the coefficient of friction is u= .277. what is the acceleration of the three masses and the magnitude of the contact force between m1 and m2, and m2 and m3

Homework Equations

The Attempt at a Solution

Weight of m1= 292.04
normal force on m2 = 150
FF on m1 = 80.9
FF on m2 = 150
Ff on m3= 74.1

 

[rl.bhat]

FF on m2 is wrong.
Net force = applied force - FF.
Acceleration = Net force/ mass

 

[tigerwoods99]

Thanks! Thats what I tried mathematically but resulted with the wrong answer becuase I didn't multiply M2 by the resistance..

 

[tigerwoods99]

And what about the contact forces?

 

[rl.bhat]

If T1 is the contact force between M1 and M2, them
T1 - FF(M1) = M1a.
Similarly you can find T2.

 

[tigerwoods99]

If T1 is the contact force between M1 and M2, them
T1 - FF(M1) = M1a.
Similarly you can find T2.

ok so FF of M1= 80.89
FF of M2= 41.5
and the acceleration would equal 9.85m/s/s

so T1- 122.4(29.8)= (29.8)(9.85) ? That seems way to ghih.

 

[rl.bhat]

From where did you get 122.4?
It should be
T1 - 80.89 = 29.8*9.85.
In the formula FF(M1) is the frictional force on mass 1.

 

[tigerwoods99]

so T1 would equal 374.42 The question asks what is the contact force between m1 and m2. 374.42 does not answer any of these questions. I have a feeling it has something to do with the friction source but I am not sure.. Thanks!

 

[tigerwoods99]

Additionally, I have another question regarding a different problem.
Three crates are connected with ropes and arranged. The masses of the crates are m1= 20kg m2= 16.2kg and m3= 26.6kg. The crates accelerate to the right at 4.05m/s/sand the fiction is 0.279.

what is the magnitude of the tension force on the rope between m1 and m2 = 293.22
T= 2MA
T= 2(16.2)(4.05
T= 293.22N

What is the magnitude of the tension force in rope 2 between m2 and m3 ( m2====m2=====m1)
?

 

[rl.bhat]

In the first problem it is not clear to which mass the force is applied. Direction of the force decides the contact force.
In the second case how did you get T = 2MA?
Show the arrangement of the masses and the direction of the force clearly.