- #1
TomK
- 69
- 14
- Homework Statement
- ENGAA 2017 - Question 54
- Relevant Equations
- SUVAT
Please scroll-down to the end (Question 54): https://www.undergraduate.study.cam.ac.uk/files/publications/engineering_s1_qp_2017.pdf
I have also been referring to unofficial worked solutions (http://www.engineeringadmissionsassessment.com/2017-solutions.html), but I didn't understand how it gives the answer.
The correct answer is 'A' (speed = 4, height = 0.8). This is the working I did:
u = 0, v = ?, t = 0.5, a = 10
v = u + at = 0 + (10 x 0.5) = 5: speed = 5
From here, I said the starting speed immediately after the first impact is 5 (and the speed at max. height = 0), but, depending on the equation/information I use, I seem to get two different height values:
u = 5, s = ?, a = -10, t = 0.4.
s = ut + (1/2)at² = 2 + (-5 x (0.4)²) = 2 - 0.8
s = 1.2m
Or: u = 5, s = ?, a = -10, v = 0
v² = u² + 2as: s = (v² - u²)/2a = -25/-20
s = 1.25m
Either way, it gives me the wrong answer of 'D' (speed = 5, height = 1.25).
Would someone be able to tell me what has gone wrong here? Am I not allowed to assume the speed is the same after hitting the ground?
I have also been referring to unofficial worked solutions (http://www.engineeringadmissionsassessment.com/2017-solutions.html), but I didn't understand how it gives the answer.
The correct answer is 'A' (speed = 4, height = 0.8). This is the working I did:
u = 0, v = ?, t = 0.5, a = 10
v = u + at = 0 + (10 x 0.5) = 5: speed = 5
From here, I said the starting speed immediately after the first impact is 5 (and the speed at max. height = 0), but, depending on the equation/information I use, I seem to get two different height values:
u = 5, s = ?, a = -10, t = 0.4.
s = ut + (1/2)at² = 2 + (-5 x (0.4)²) = 2 - 0.8
s = 1.2m
Or: u = 5, s = ?, a = -10, v = 0
v² = u² + 2as: s = (v² - u²)/2a = -25/-20
s = 1.25m
Either way, it gives me the wrong answer of 'D' (speed = 5, height = 1.25).
Would someone be able to tell me what has gone wrong here? Am I not allowed to assume the speed is the same after hitting the ground?