# Kinematics - identifying and describing types of motion

1. Mar 21, 2016

### alexandria

1. The problem statement, all variables and given/known data

2. Relevant equations

acceleration equation:
a = v2 – v1 / t2 – t1

3. The attempt at a solution

21.

22.
to calculate the velocity, i determined the slope of the tangents, postioned at 0.50s, 0.75 s, and 1.00 s

Using this information, i created a velocity time graph for the orange

Acceleration:

a = v2 – v1 / t2 – t1

a = 8.5 m/s (down) – 6.7 m/s (down) / 1.00 s – 0.75 s

a = 7.2 m/s2 (down)

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2. Mar 21, 2016

### haruspex

Not bad, but which of all the available datapoints for velocity should be the most reliable?

3. Mar 21, 2016

### alexandria

what do you mean? are you referring to the velocity i calculated in my time-velocity data table? and what do you mean by most reliable? arent they all reliable?

4. Mar 21, 2016

### haruspex

They're not wonderfully reliable because you are drawing tangents by eye and finding the velocities from their slopes. I can see, for example, that your tangent at 0.5s is a bit too steep.
There is one completely reliable velocity that you have not used. You do not even have to draw a tangent to find it!

5. Mar 22, 2016

### alexandria

i dont understand :P

6. Mar 22, 2016

### haruspex

Which bit don't you understand, the inaccuracy inherent to the method you have been told to use to find velocities, or what data point you have not used?

7. Mar 22, 2016

### alexandria

both
i was just told to draw tangents and then calculate the slope of each tangent to determine the velocity
here's an example from my lesson:

8. Mar 22, 2016

### haruspex

Yes, I understand that, but you must realise that drawing tangents by eye like that is open to substantial error. The tangent you drew at 0.5s looks to me more like the tangent at 0.55s.
Since you are following a worked example, I suppose you cannot do much better than that. But had I prepared the example I would have taken the velocity at time 0 as an obvious known value. Note that using that value together with your t=1 tangent gives a rather more accurate value for g.

9. Mar 22, 2016

### alexandria

ok i understand, should i fix the tangent at 0.5 s, will that make it more accurate?

10. Mar 22, 2016

### haruspex

Yes, a bit.

11. Mar 22, 2016

### alexandria

ok thank you.