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# Physics Acceleration

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016

1. For the question in the picture. Is the correct answer not A because the rock starts from rest so acc = 0. As it begins to fall the acc increases. However the mark scheme says C can someone explain?

Thanks
2. Graph A is a graph of the velocity. The *acceleration*, however, is constant, as the only force acting on it (ignoring air resistance) is its weight, which doesn't change, hence the answer is C. Remember, F=ma, so constant force means constant acceleration.
3. how do we know that the force is constant?
4. Hmm...

Well, in a velocity acceleration time graph, any upward curve represents an increasing acceleration.

However, if air resistance was present, the line would still be a curve because it is accelerating but the curve would start to become more shallower as it starts to reach terminal velocity would you agree because air resistance decreases the rate of acceleration.

In this graph (graph C), it shows an exponential growth in acceleration instead of a decreased growth in acceleration because there is no air resistance to decrease the acceleration.

Honestly, I looked at this my self and had to look at my revision guide because it stumped me.
5. understood thanks
6. (Original post by uzer)
how do we know that the force is constant?
Gravity is always constant on earth which affects weight hence why weight is constant. For our GCSE purposes, the force of earth's gravitational pull is 10N/Kg
7. It's the bottom left one as you know drag can be ignored as said in the question, so terminal velocity can be ignored too. This means that only GPE turning into KE is the force affecting the object, so acceleration will continue to increase over time as more GPE is lost as it falls, and more KE is gained

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8. so confirm in graph C acceleration is zero because the gradient of the line is zero and acceleration is delta v over delta t?

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