Help physics Q

[Edward Elric]

So basically I have a diagram of a girl jumping on a trampoline, where it shows how shes about to land on it, in other words shes in the air. The Q asks what is the direction of her acceleration at  the time shes about to land. Explain your answer.

Initially i thoight that its obvioulsy downwards, as acceleration is a force of attraction towards the centre of the Earth. But when i checked the answer, i was surprised to find that "Maria is moving down and 'slowing down'". Hence her net acceleration is upwards. -_- WTF. Sorry if this is a stupid Q but can someone explain this concept to me. Thanks in advance.

[b^3]

I think they're taking "at the time she is about to land" as the time that she is in contact with the trampoline when she is coming back down. During this period, the gravitational potential energy of the girl is being transformed into elastic potential energy in the trampoline. As this is happening, the girl will lose velocity, that is she is slowing down. To get this decrease in velocity we need the acceleration of the girl to be in the opposite direction she is travelling.

You can think of it another way with the situation of a car taking off from a rest and then slowing down. As the car takes off and increases it's velocity, it's acceleration is in the direction of travel. When the car starts to brake, the velocity of the car decreases, and to get this decrease we need the car to 'accelerate in the opposite direction' (which you might have heard this described as decelerating in this [particular situation before).

Hope that helps! Sometimes the wording in these questions can be a bit tricky to work out which instant they are trying to refer to.

[PB]

Hmm, well I think this is really just poor phrasing of the question. If it really asks to find "the direction of her acceleration at  the time shes about to land" then the answer seems wrong to me. Because "about to land" implies that she has not yet made contact with the surface of the trampoline, meaning that (as you said) her acceleration would still be downwards due to gravity.

However, the answers seems to indicate that they are referring to when she is already in contact with the trampoline and is being deccelerated, hence her acceleration would be upwards! (deccelaration is simply acceleration in the opposite direction!)

So yeah, its a dodgy question I think. VCAA wouldn't be too ambiguous about this so you shouldn't have to worry

[hobbitle]

I agree with the above. I was going to write the same thing but physics isn't my strongest suit. But yeah. Agreed with ^^

[Edward Elric]

Thanks everyone who replied, I think I understand now haha bit of an ambiguous question after all.

[PB]

dang it.. I just realised b^3 beat me to the post by 6 secs :''(