# Homework Help: Bar Swinging Up After Collision - Strategy

1. Aug 18, 2013

### postfan

1. The problem statement, all variables and given/known data

A uniform bar of mass m and length l is suspended on a frictionless hinge. A horizontally launched blob of clay of mass m strikes the bottom end of the bar and sticks to it. After that, the bar swings upward. What is the minimum initial speed v of the blob of clay that would enable the rod to swing a full circle?
Which concepts/laws would be most helpful in solving this problem? Select the best answer from the options below.
CHOICES
kinematics of rotational motion; conservation of energy
conservation of momentum, conservation of energy
conservation of angular momentum, conservation of momentum
conservation of angular momentum, conservation of energy
conservation of energy, Newton's laws
Newton's laws, conservation of angular momentum
conservation of angular momentum, kinematics of rotational motion
Newton's laws, kinematics of rotational motion

2. Relevant equations

3. The attempt at a solution
I though the answer was momentum and linear momentum because the linear momentum converts into the angular one ,but I am wrong. So, what it the right answer?

2. Aug 18, 2013

### Staff: Mentor

There are multiple ways to solve this, but conserved angular momentum in the collision process is certainly interesting. Linear momentum is not conserved (the hinge can absorb some of the momentum of the clay).
Afterwards, you need some way to find out how high the bar will swing.. what would you use for that?

3. Aug 18, 2013

### postfan

Would you use conservation of energy?

4. Aug 18, 2013

### Staff: Mentor

I would use conservation of energy as part of the solution, yes.

5. Aug 18, 2013

### postfan

So the answer is conservation of energy and conservation of angular momentum?

6. Aug 18, 2013

### Staff: Mentor

That is probably the answer I would give.

7. Aug 18, 2013

### postfan

Ok Thanks!

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