# Total acceleration from angular acceleration

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1. Dec 13, 2015

### droidofthevoid

1. The problem statement, all variables and given/known data
A discus thrower ( with arm length of 1.2 m) starts from rest and begins to rotate counterclockwise with a constant angular acceleration of 2.5 [rad/s^2]. What is the magnitude of the total acceleration of the discus when its angular velocity is 9.0[rad/s]?

2. Relevant equations
I'm not really connecting the dots here. Do I treat the discus thrower as a rigid body and give a simple moment of inertia, which I then plug into a torque equation tau = I alpha?

3. The attempt at a solution

2. Dec 13, 2015

### JeremyG

Not necessary. The question is interested in the acceleration of the discus, so there is only a need to consider the motion of the discus.

3. Dec 13, 2015

### droidofthevoid

So then I treat it as a particle going in a circle and use a = R alpha? Do I neglect the omega= 3.0 rad/s?

4. Dec 13, 2015

### JeremyG

Yes in this case you can treat it as a particle going round in a circle. $a = r\alpha$ will give you what kind of acceleration? As a hint, see the comment below as well.

You mean 9rad/s? No you do not ignore this.

5. Dec 13, 2015

### droidofthevoid

6. Dec 13, 2015

### JeremyG

Yes, the question wants the total acceleration, so that should give you a clue that it is not just tangential acceleration at play here. What else?

7. Dec 13, 2015

### droidofthevoid

Well then, I would presume it would have something to do with torque and perhaps treating the thrower as a rigid body? or radial acceleration, which would be omega squared times r.

8. Dec 13, 2015

### JeremyG

Torque by the thrower is the cause for the tangential acceleration/angular acceleration of the discus. And as mentioned above, the thrower himself need not be considered in this problem.

The discus is travelling in a circular motion, yes? Tangential acceleration is not sufficient to ensure that the discus is travelling in a circular motion, uniform or not. What about considering the radial acceleration?