palaphys
- 235
- 12
- Homework Statement
- A block dropped onto a spring system as shown in the figure. If the spring constant of the longer spring is mg/2h then find the spring constant of the larger.
- Relevant Equations
- Work done= change in kinetic energy
Consider this situation. A mass $$m$$ is dropped onto the spring system from rest , and it is given that the smaller spring gets compressed by a distance $$h$$
My main query in this question is regarding the use of Work Energy theorem, which states that the work done on a body by all the forces acting on it is equal to the change in its kinetic energy.
Additionally it is given that the spring constant of the longer spring is $$mg/2h$$.
Now, applying Work Energy theorem between the top point and the bottom, we get:
$$ 0= 2mgh -(mg/2h) (4h^2 /2) - k_2 h^2 /2 $$
Here the first term is the work done by gravity, second the work done by the longer spring, and third the work done by the smaller spring.
Solving, we get $$ k_2 = 4mg/h $$
However, my main problem is while trying to apply work energy theorem between the point B and A.
I decided to try a different approach to this problem and began by using Work Energy theorem between the top point and B
This gave
$$ Mv_b ^2 /2 = 2mgh- (mg/2h)(h^2 /2) $$
Solving this, I get $$KE_B= 7mgh/4 $$
Now comes the problem. Applying Work Energy theorem between B and the bottom point, I get
$$ 0- 7mgh/4 = W_{sp1} + W_{sp2} + W_g$$
$$ -7mgh/4 =-(mg/2h)h^2/2 - (k_2)( h^2/2) + mgh$$
**Now solving for k_2**,
$$k_2= 5mg/h $$
**Why is there this subtle difference?**
Has there been a misunderstanding or incorrect application of work energy theorem?
Which one is right.
And why?
My main query in this question is regarding the use of Work Energy theorem, which states that the work done on a body by all the forces acting on it is equal to the change in its kinetic energy.
Additionally it is given that the spring constant of the longer spring is $$mg/2h$$.
Now, applying Work Energy theorem between the top point and the bottom, we get:
$$ 0= 2mgh -(mg/2h) (4h^2 /2) - k_2 h^2 /2 $$
Here the first term is the work done by gravity, second the work done by the longer spring, and third the work done by the smaller spring.
Solving, we get $$ k_2 = 4mg/h $$
However, my main problem is while trying to apply work energy theorem between the point B and A.
I decided to try a different approach to this problem and began by using Work Energy theorem between the top point and B
This gave
$$ Mv_b ^2 /2 = 2mgh- (mg/2h)(h^2 /2) $$
Solving this, I get $$KE_B= 7mgh/4 $$
Now comes the problem. Applying Work Energy theorem between B and the bottom point, I get
$$ 0- 7mgh/4 = W_{sp1} + W_{sp2} + W_g$$
$$ -7mgh/4 =-(mg/2h)h^2/2 - (k_2)( h^2/2) + mgh$$
**Now solving for k_2**,
$$k_2= 5mg/h $$
**Why is there this subtle difference?**
Has there been a misunderstanding or incorrect application of work energy theorem?
Which one is right.