JRAHS 2020 question (2 Viewers)

[Run hard@thehsc]

How would you do this question - i got multiple equations and am stuck at part ii

 

[c_z_m]

Did you mean to post that physics gravity question as an attachment? What was the answer to (ii), just so I can double check my answer.

 

[Run hard@thehsc]

@c_z_m did not mean that
just the 3u q

 

[Run hard@thehsc]

Did you mean to post that physics gravity question as an attachment? What was the answer to (ii), just so I can double check my answer.

I don't have the answer unfortunately

 

[c_z_m]

I don't have the answer unfortunately

Alright, I guess I'll have a crack at it.

Perpendicular should immediately scream "dot product" at you. Dot product vectors CD and BE which would then equal zero.

So, CD.BE=0, (AD-AC).(AE-AB)=0 -> expand this
Make AD.AE the subject so AD.AE=AB.AD+AC.AE-AC.AB
Since AB || AD, and AC || AE dot product is product of their lengths. Dot product of AC.AB use your formula - lengths should be 2r.
cos(theta)=AD.AE/|AD||AE|=[2r*r+2r*r-cos(theta)(2r)^2]/r^2
5r^2cos(theta)=2r^2+2r^2=4r^2
cos(theta)=4/5
theta=arccos(4/5)

Apologies for the non-latex mess.

 

[jimmysmith560]

@c_z_m is correct. Here are the official solutions to this question:

Part i):



Part ii):



I hope this helps!