How Do You Solve a Physics Problem Involving Multiple Blocks and Tensions?

[raychelle93]

Homework Statement

"Blocks A, B, and C are placed as in the figure and connected by ropes of neligible mass. Both A and B weigh 25.0 N each, and the coefficient of kinetic friction between each block and the surface is 0.35. Block C descends with constant velocity. a) Draw separate free body diagrams shwoing the forces acting on A and B. b)Find the tension in the rope connecting blocks A and B. c) What is the weight of block C? d) If the rope connecting A and B were cut, what would be the acceleration of C?"

Homework Equations

The Attempt at a Solution

I've gotten as far as trying to find "A" :b)
Let ' T1 ' & ' T2 ' be the tensions in the rope connecting blocks AB and BC respct.

Apply ; Newton's 2nd Law
Writing the equation of motion for mB
T1 - mBg Sin 36.9 0 - mk mBg Cos 36.9 0 - T2 = mB a -----------(i)
Writing the equation of motion for mA

T2 - mk mA g = mA a ----------------------------(ii)
Writing the equation of motion for mC

mC g - T1 = mC a ----------------------------(ii)

Add equ(i); equ (ii) & equ(iii) ; we have
to calculate the value of a & put it in equation (i) & (ii) respt, to get the value of T1 & T 2
>>Can't find a though :( Help!

 

[rl.bhat]

Hi raychelle93, welcome to PF.
Your second equation for mass A is wrong. Check it.

 

[kerimek]

I would first commend the student for their attempt at solving the problem and using Newton's Laws to find the solution. I would then provide some guidance and additional information to help them find the value of "a" and continue with their solution.

Firstly, I would suggest that the student draw the free body diagrams for blocks A and B as requested in part a) of the problem. This will help them visualize all the forces acting on each block and make it easier to set up the equations of motion.

To find the value of "a", the student can use the fact that block C is descending with constant velocity. This means that the net force on block C must be zero, according to Newton's First Law. Therefore, the weight of block C must be equal in magnitude to the tension in the rope connecting blocks A and B. This information can be used in the equation for block C to solve for "a".

Once the value of "a" is found, the student can use it in equations (i) and (ii) to solve for the values of T1 and T2, as requested in part b) of the problem.

For part c), the weight of block C can be found by using its mass and the acceleration due to gravity (9.8 m/s^2).

For part d), the student can consider the forces acting on block C after the rope connecting blocks A and B is cut. The only force acting on block C will be its weight, which will cause it to accelerate downwards with an acceleration equal to g (9.8 m/s^2).

Overall, I would encourage the student to practice setting up equations of motion and using Newton's Laws to solve problems like this one. It is a fundamental skill in the field of science and will serve them well in their future studies.