can someone please walk me through how to do this question please? my exam is in two days!
I love this question, it is a great way to see how an object in uniform circular motion is accelerating.
Start by drawing the two velocity vectors separately in the grid. Pay close attention to their size and direction. Label them A and B.
When calculating the change in a quantity we use:
change in … = final … - initial …
In our case this is:
change in velocity = final velocity - initial velocity
So we need to subtract the initial vector (A) from the final vector (B).
It can be tricky to see how to subtract vectors, but effectively we just reverse the direction of our initial vector (A) and all it -A
Given that:
change in velocity = B - A
is mathematically the same as:
change in velocity = B + -A
We can now add the B and -A vectors (remember to join them nose to tail - it doesn’t matter which one you start with).
Now draw a dotted line from the tail of your first vector to the tip of your second vector. This dotted line shows the change in velocity, and it should make your vectors into a triangle. Now you an use the scale of the boxes, or pythagoras theorem to work out the magnitude (size) of the change in velocity.
Finally, if you were to draw the change in velocity on the circle diagram, where would you place it? Where would it then point?
would it be possible for you to show me in a diagram what you mean please, as it doesn’t really make much sense to me as just words?
In the following diagram, I’ve done these steps for you (from my earlier response):
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Start by drawing the two velocity vectors separately in the grid. Pay close attention to their size and direction. Label them A (red) and B (green).
-
It can be tricky to see how to subtract vectors, but effectively we just reverse the direction of our initial vector (A) and all it -A (blue)
-
We can now add the B and -A vectors (remember to join them nose to tail - it doesn’t matter which one you start with).
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Now draw a dotted line from the tail of your first vector to the tip of your second vector. (shown in orange)
To finish the question, work out the size of the orange dotted arrow. You could do this using the a scale diagram, but I would recommend using pythagoras.
Once you’ve done that, here are two more questions to help your understanding:
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Where would you place the orange arrow on the original circle to show the change in velocity between A and B?
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What is another term for a change in velocity?
the size of the dotted arrow is 7.1m/s.
I would place the orange arrow between the tip of the red arrow and the bottom of the green arrow.
another term for a change in velocity is an acceleration
@Sarah.Roycroft
Absolutely right! If the orange arrow is placed as you’ve described, it should point towards the centre of the circle. You’ve effectively shown how an object with uniform speed can be accelerating, and shown the direction of this (centripetal) acceleration.
If the object is accelerating towards the centre, it must be acted upon by an unbalanced force in that direction as well. These are the principles of circular motion you may need in your exam.
thank you so much once again Doug! My physics exam is my last exam for the year and it is on Monday morning so I can’t wait for it to be over with!
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Your efforts today should prove fruitful in your exam on Monday. Keep up the great work! Not long to go now.
Thanks Doug, you are a huge help when it comes to this sorta stuff
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