Register to reply

Angular acceleration of an arm

by tristanmagnum
Tags: acceleration, angular
Share this thread:
imiuru
#19
Oct16-13, 11:20 PM
P: 44
F=ma says that an object with mass m will experience acceleration a under force F.

The acceleration you obtained belong to which object? Arm? Ball?

Which mass should you use then?
tristanmagnum
#20
Oct16-13, 11:29 PM
P: 44
the mass of the arm? correct
Periapsis
#21
Oct17-13, 12:02 AM
P: 26
add the two masses up.
imiuru
#22
Oct17-13, 01:01 AM
P: 44
Ops! Part B appeared to be more tricky than I initially thought. You can forget using F=ma to solve it.

May I know where you get this problem?

There are a few assumptions to be made in order to solve this problem. Still, it takes plenty of steps to get the final answer.

First, you must draw the free body diagram with the arm and the ball as the system. This will help you to picture how each components contribute to the τ (net torque) of the system.

With τ=Iα,where I is the moment of inertia, α is the angular acceleration, you will be able to get the value of τ, provided you know how to calculate I. (refer to your textbook on moment of inertia for uniform rod)

τ(tricep) - τ(arm) - τ(ball) = τ(net)

Using the above equation, you will be able to solve for τ(tricep) and in turn, F(tricep).

You will need this in your workout: τ=Fr where r is the distance from the pivot point to the point where force is applied.
Periapsis
#23
Oct17-13, 01:21 AM
P: 26
How can you get the moment of inertia if you dont have the radius of the ball? 2/5mr^2
Periapsis
#24
Oct17-13, 01:23 AM
P: 26
i guess, I = ∫ r2d(m) could work
imiuru
#25
Oct17-13, 01:34 AM
P: 44
Maybe there is no need to calculate moment of inertia. Use τ=Fr to calculate τ(net) will do. F can be obtained through F=ma and r can be obtained through finding the centre of mass of the system.

Anyways, it is up to the original question raiser to do the math.
haruspex
#26
Oct17-13, 12:30 PM
Homework
Sci Advisor
HW Helper
Thanks
P: 9,849
Quote Quote by Periapsis View Post
How can you get the moment of inertia if you dont have the radius of the ball? 2/5mr^2
You can treat the ball as a point mass, making its MI about the arm's axis easy. The Mi of the arm is obtained by treating it as a rod rotated about one end.
haruspex
#27
Oct17-13, 12:33 PM
Homework
Sci Advisor
HW Helper
Thanks
P: 9,849
Quote Quote by tristanmagnum View Post
1. The problem statement, all variables and given/known data

Assume that a 1.20-kg ball is thrown solely by the action of the forearm, which rotates about the elbow joint under the action of the triceps muscle, the figure. The ball is accelerated uniformly from rest to 9.5m/s in 0.38s , at which point it is released. Assume that the forearm has a mass of 3.7 kg and rotates like a uniform rod about an axis at its end.

Part A: Calculate the angular acceleration of the arm.

Part B:Calculate the force required of the triceps muscle.
.
Unless there's more info in the diagram, there's not enough to determine the force from the triceps. All you can do is calculate the torque.


Register to reply

Related Discussions
Help needed to convert angular velocity to angular acceleration General Physics 12
Calculate angular velocity and angular acceleration of a rotating mass Calculus & Beyond Homework 2
Angular acceleration and force with constant angular velocity? Introductory Physics Homework 6
Are angular displacement, angular velocity, and angular acceleration, vectors? Classical Physics 5
Rotational motion -angular energy+angular acceleration Introductory Physics Homework 4