Acceleration & velocity using instantaneous centre of zero velocity

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Homework Help Overview

The discussion revolves around a mechanical problem involving a link connected by bearings A and B, where the motion is analyzed using concepts of velocity and acceleration. The problem is situated within the context of kinematics and rotational motion.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the relationship between the velocities and accelerations of bearings A and B, using geometric relationships and Pythagorean theorem. There are attempts to derive angular velocities and clarify the concept of the center of zero velocity.

Discussion Status

Some participants have provided calculations for angular and linear velocities, while others express confusion regarding the reasoning and definitions used. There is an ongoing exploration of the relationships between the components of motion, with no explicit consensus on the correctness of the approaches taken.

Contextual Notes

Participants note potential inaccuracies in the dimensions used for calculations and question the definitions of terms like "center of zero velocity." There is also a mention of the constant velocity of bearing A, which influences the acceleration considerations.

Ctom101
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1. A link is connected by two bearings A and B. The bearings run along slots which are at right angles to each other. The length of the link AB (length between the centres of bearings A and B) is 0.2m. At an instant of time, height H, is 0.05m, and the bearing A has a constant velocity V, of 0.1m/s upwards.
At the instant of time calculate;
The velocity of bearing B.
The angular velocity of the link AB.
The acceleration of bearing B.
The angular acceleration of the link AB


2. velocity B = distance from rotational centre x angular velocity of link AB.
angular velocity of link AB = velocity A / distance from centre of zero velocity

3. from the working out in my picture changing it into a triangle I think I have worked out the velocity of bearing B and the angular velocity of link AB, but I don't know how to find the acceleration of B or the angular acceleration of AB.
Could somebody please help me with this and let me know if what I've done so far is incorrect?

The Attempt at a Solution


a = 0.05 c = 0.2
a2 + b2 = c2
0.22 / 0.052 = 16 root 16 = 4 b = 4
velocity A / distance from centre of zero velocity
0.1 / 0.05 = 2rad/s angular velocity of link AB
velocity B = distance from rotational centre(b) x angular velocity link AB
4 x 2 = 8m/s

ωv AB = 2rad/s
v B = 8m/s
 

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You have to exploit the relationship between A and B ... i.e. the dimensions of the triangle.
I don't understand your working though.

If I put c=|AB|, with the position of B as x and the position of A as y, then

c^2=x^2+y^2

...differentiate through by time to get the relationship between the velocities, and again to get the accelerations.
what is the acceleration of A?
 
It says A has a constant velocity of 0.1m/s, so I think acceleration is 0

Looking over it again I think I worked out the length b incorrectly.

link AB = 0.2m
Height = 0.05m

Pythagoras: a^2 + b^2 = c^2

link AB = c
Height = a

c^2 - a^2 = b^2

0.2^2 - 0.05^ = 0.0375
root 0.0375 = 0.1936
b = 0.19
 
angular velocity of link AB = velocity A / distance from centre of zero velocity

velocity A = 0.1m/s
distance from centre of zero velocity (a) = 0.05m
0.1/0.05= 2rad/s

velocity B = distance from centre of zero velocity x angular velocity of link AB

angular velocity of AB = 2rad/s
distance from centre of zero velocity (b) = 0.19

0.19 x 2 = 0.38m/s
velocity B = 0.38m/s

I think this is now right but I don't know how to move on for;
The acceleration of bearing B.
The angular acceleration of the link AB
 
I still don't see how your reasoning arrives at the required quantities.

What does "center of zero velocity" mean?
You seem to be giving the velocity of A in radiens per second as if it were moving perpendicular to the corner.
Is this correct?

What's wrong with the suggestions I already gave you?
 

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