There are two spinning disks A and B arranged so that they initially are separat

Question

There are two spinning disks A and B arranged so that they initially are separated but can be pushed into contact. Disk A is made of a lighter material, so it has a moment of inertia that is one third that of disk B. Disk A starts off spinning at some ?i when the two disks are separated. The two disks are then pushed together, coupling their motion. 2400 J of heat energy were produced in this process. What was the initial kinetic energy of disk A?

The Answer provide by my professor was 3200 J but i have no idea how he got this =( If someone could explain how i can get this answer that would be awesome =D

Explanation / Answer

Angular Momentum remains Constant

So

I1W1 = I2W2

Here i am using i as W1

So

I1*W1 = (I1 + 3I1)*W2

Therefore

W2 = W1/4

Therfore

Loss in Kintic Energy = 0.5*I1W1^2 - 0.5*I2W2^2 = 2400

0.5*I*W1^2 - 0.5*4I*(W1/4)^2 = 2400

Therefore

I1W1^2 = 6400

Therefore

Initital Kinetic Energy = 0.5*I1W1^2

= 6400/2

= 3200 J

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.