Motion under gravity - AS Mechanics

Help with this Mechanics AS question on Motion under gravity, please

http://31.media.tumblr.com/750137536580606960db5a7e3c520531/tumblr_ndn912fJGV1rplobmo1_500.jpg

Thank you :frown:

really struggling to get my head round it

Reply 1

TenOfThem

18

Original post by buttered

Help with this Mechanics AS question on Motion under gravity, please

http://31.media.tumblr.com/750137536580606960db5a7e3c520531/tumblr_ndn912fJGV1rplobmo1_500.jpg

Thank you :frown:

really struggling to get my head round it

what have you done so far?

Reply 2

buttered

OP

3

Original post by TenOfThem

what have you done so far?

I tried to work out the final velocity of the vertically downwards motion,
by using the equation:
v^2 = u^2 + 2as
v = root (25-19.6)
I used -9.8 as the acceleration

However, this got 2.32, which I then halved to get 1.16. This, when re substituting into the same equation with s=?, u=1.16, v=0, a=-9.8
I got 0.068m

The answer in the back of the book says 0.569m :frown:

Reply 3

SteveMcs

1

You need to use the SUVAT equations. If the ball rebounds at half the speed it impacts, then you need to find the velocity that the ball hits the floor and half it. That's then your initial velocity for another suvat equation where you want to find the displacement.

As for the over-estimate or under-estimate... Think about things such as air resistance which you're not taking into consideration... would not including air resistance make your result an over-estimate or under?

Reply 4

SteveMcs

1

Original post by buttered

I tried to work out the final velocity of the vertically downwards motion,
by using the equation:
v^2 = u^2 + 2as
v = root (25-19.6)
I used -9.8 as the acceleration

However, this got 2.32, which I then halved to get 1.16. This, when re substituting into the same equation with s=?, u=1.16, v=0, a=-9.8
I got 0.068m

The answer in the back of the book says 0.569m :frown:

you shouldn't use -9.8 for acceleration going downwards... -9.8 implies a deceleration, but when travel down, gravity would make it accelerate so you should use +9.8 when travelling down, -9.8 when traveling up

Reply 5

buttered

OP

3

Original post by SteveMcs

you shouldn't use -9.8 for acceleration going downwards... -9.8 implies a deceleration, but when travel down, gravity would make it accelerate so you should use +9.8 when travelling down, -9.8 when traveling up

Thank you so much! I have the correct answer now

Reply 6

borysek01

9

When questions are in terms of gravity, always calculate vertically. When something moves up, use negative acceleration of 9.8 and therefore when moves down, use 9.8. This is because acceleration is a vector, and since gravity acts downwards - the sign is very important.

Good luck!

Motion under gravity - AS Mechanics

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