Two blocks connected by a string

In summary, the problem involves two 2.4-kg blocks connected by a string, with one block on a table and the other hanging off the edge. The potential energy of the first block is found by multiplying its mass, acceleration due to gravity, and distance from the table. The kinetic energy equation is then used to find the velocity of the blocks, with the potential energy of both blocks factored in since they are connected by the string. The reason for this is that only changes in potential energy are important, and the block on the table does not experience a change in potential energy.
  • #1
Sarah Kenney
10
1
Two 2.4-kg blocks are connected by a string draped over the edge of a slippery table, so that one block is on the table and the other is just hanging off the edge. A restraint holds the block on the table in place, and the string is 0.50 m long. After the restraint is removed, what speed does the pair of blocks have at the instant the upper one is pulled off the table?

Ok, so I figured out how to solve this problem, but I just need help understanding the solution.

How I solved it was by finding the potential energy of the first block which is:
PE=(2.4kg)((9.8m/s)(0.5m)=11.76 Joules

Then I took the equation for Kinetic energy and found he velocity like so: 11.76 J= (1/2)(2.4m)*v^2. Except, I had to plug in for both the blocks this time, so: 11.76=(2.4)*v^2 So the answer is that the initial velocity of the blocks is 2.2m/s.

So I think my main question is, why do we just find the potential energy of the first block, but then plug in for both blocks when using the Kinetic energy formula? Can anyone shed any light on this? I really don't understand the solution.
 
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  • #2
Only changes in potential energy are important. The block on the table does not change its gravitational potential energy because it does not change its height. The change in potential energy of the hanging block affects the kinetic energy of both blocks since they are connected by the string.
 
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  • #3
brainpushups said:
Only changes in potential energy are important. The block on the table does not change its gravitational potential energy because it does not change its height. The change in potential energy of the hanging block affects the kinetic energy of both blocks since they are connected by the string.
Ok, that makes sense. Thanks!
 

1. What is the concept of "Two Blocks Connected by a String"?

The concept of "Two Blocks Connected by a String" refers to a simple physics experiment where two blocks of different masses are connected by a string or rope. The string is passed over a pulley and one block is hung vertically while the other block is on a horizontal surface. This experiment is used to study the relationship between the masses, the tension in the string, and the acceleration of the blocks.

2. What is the purpose of conducting this experiment?

The purpose of conducting the "Two Blocks Connected by a String" experiment is to understand the principles of Newton's Second Law of Motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This experiment also helps in studying the concept of tension in a string and its effect on the motion of the blocks.

3. How is the acceleration of the blocks related to the masses and the tension in the string?

The acceleration of the blocks is directly proportional to the net force acting on them, which is the difference between the tension in the string and the weight of the hanging block. Therefore, as the mass of the hanging block increases, the tension in the string also increases, resulting in a higher net force and acceleration. Similarly, as the mass of the block on the horizontal surface increases, the tension in the string decreases, resulting in a lower net force and acceleration.

4. What happens if the mass of one of the blocks is significantly higher than the other?

If one of the blocks has a significantly higher mass than the other, the tension in the string will be unequal, and the block with the higher mass will experience a greater force and acceleration. This is because the tension in the string is directly proportional to the mass of the hanging block.

5. How does the inclusion of a pulley affect the experiment?

The pulley is used to redirect the force of gravity acting on the hanging block in a horizontal direction, creating a difference in the tension in the string. Without the pulley, the tension in the string would be equal, and the blocks would not experience any acceleration. Therefore, the pulley plays a crucial role in this experiment by allowing the blocks to experience different tensions and accelerations.

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