# Dynamics help: variable acceleration and angular velocity

1. Feb 20, 2013

### medynamics

I have to do these 2 problems for homework, but I can't figure them out. Any help would be appreciated.

1. The motorcycle is traveling at 1m/s when it is at A(the origin). If the speed is then increased at $\dot{V} = .1 m/s^2$, determine its speed and acceleration at the instant t = 5 seconds. The motorcycle is following the curve $y = 0.5 x^2$.

2. Rod OA rotates counterclockwise with a constant angular velocity of θ = 6 rad/s. Through mechanical means collar B moves along the rod with a speed of $r = 4 t^2 m/s$, where t is in seconds. If r = 0 when t = 0, determine the magnitudes of velocity and acceleration of the collar when t = 0.75 sec.

This is the link to the image for question 2:
http://s1308.beta.photobucket.com/user/jackcastle618/media/Dynamics4_zps5a431c20.jpg.html

Last edited: Feb 20, 2013
2. Feb 20, 2013

### CaptainEvil

For question 1, start with the Kinematic Equations, specifically v = u + at

3. Feb 20, 2013

### medynamics

so using v = u + at:

v = 1m/s + .1(5)
v = 1.5 m/s

now I need a. i believe i need to find radial and tangential a?
so i have a = vdot
so a_r = .1 m/s^2
but how do I get the acceleration that is based on the curve y = .5x^2?

4. Feb 20, 2013

### haruspex

I don't think you can use that here. The rate of increase of speed is constant, but that's not the same as acceleration as strictly defined. Acceleration is a vector, or a component of a vector in a consistent direction.
medynamics, try to write down some equations relating dx/dt, dy/dt etc.

It's not a good idea to put two unrelated problems in the same thread.