# Relative Motion Analysis: Acceleration

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1. Mar 3, 2016

### Smoker

Mentor note: moved to homework forum later, therefore no template.

When doing a rigid body relative motion (accel) problem using vector analysis, how do I know when it's appropriate to substitute sin/cos values vs when to use just the numbers given as the velocity?

Last edited by a moderator: Mar 4, 2016
2. Mar 4, 2016

### Simon Bridge

It sounds like you are trying to use formulas without understanding them.
vis: sine and cosine are usually used to compute components of vectors resolved against a coordinate system, or against other vectors ... so that is when you use them. You should use whichever trig functions you need to in order to work the problem, not just sine and cosine.

The trouble with this sort of question is that the people who can answer you find this stuff intuitively obvious - this means there is a bit more information needed to get us (me) to understand where the trouble lies.

Please provide an example of where you get in trouble deciding what to use.

3. Mar 4, 2016

### Smoker

• Post moved from the technical forums, so no HH Template is shown
I am to determine the velocity of slider block C. Given angular velocity of AB (4.1) and angular rotation of AB (5.5).

4. Mar 4, 2016

### Staff: Mentor

Okay, and which velocity value where do you want to calculate how, where you ran into the problem of post 1?

5. Mar 4, 2016

### Smoker

Basically I want to know the x and y components of the velocity of B, A is zero because it's fixed and C is (Vc)i because it's restricted to the x axis, but what is Vb? and why?

6. Mar 4, 2016

### Staff: Mentor

Okay. Can you find the speed of B, and the direction of motion?
That allows to find the velocity components.

I think this thread fits better to the homework section.

7. Mar 5, 2016

### Simon Bridge

Just a note re initial question about when to use sine and cosine:
If the speed of C is $v_c$ then the x-component is $v_{cx}=v_c\cos\theta$ but, here, $\theta = 180^\circ$ (it is the angle $\vec v_c$ makes to the +x-axis). So you get $\vec v_c = -v_c\hat\imath$ ... which is to say, the sine and cosine values are still used, it's just that they are easy.