Work, Power and Energy assignment marked wrong somehow? :(

In summary, the conversation revolves around a problem with calculating time in a physics problem. The individual has already calculated the acceleration and initial velocity, but is struggling with using the distance and velocity at impact to determine the time. The tutor explains that the time cannot be calculated using d=vt and suggests using the equation for constant acceleration instead. They also mention that the deceleration at impact is not constant and suggest using the distance, speed, and acceleration to solve the problem instead of trying to calculate time.
  • #1
jordanbeaton
2
0
Homework Statement
A press machine has a tooling mass of 42.5kg which falls from a height of 2.3 metres onto an aluminium alloy work piece. If the depth of indentation into the block is 8.25mm calculate the following:
a) KE at impact
b) the impact velocity
c) retarding force on the tool in producing the indentation
d) work done in producing the indentation (assuming the energy is exhausted)
Relevant Equations
potential energy (PE) = mgh
PE = KE (kinetic energy)
acceleration formula
time formula
velocity break down formula
All of my working out is attached with the photos, I'm struggling to do anymore as the tutor hasn't explained anything like this yet. Any help would be much appreciated
 

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  • #2
It goes wrong when you calculate the time. First, what time did you want to calculate? The time to stop the tool, right? You used the velocity at impact, which is fine, but you used the distance it fell which has nothing to do with the deceleration at impact. Even if that had been the right distance, you can’t use d=v t. That is only true if the velocity is constant.

I believe what you want to do here is assume the acceleration is constant. Can you relate constant acceleration, initial velocity, distance, and time?
 
  • #3
1582657946544.png

The time to fall is wrong because it does not fall at constant speed. You know ##s=\frac12 a t^2 ## ?
But you don't need this. You've done a, b, now you're looking at the deceleration over the very short distance of the dent.

This will also not be constant speed.
Don't bother to work out the time - you don't need it - and there is an equation that doesn't involve time. Just use the distance, speed and acceleration.
 

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There could be several reasons for your assignment to be marked wrong. Some possible reasons could be incorrect calculations, not following the instructions, or not providing enough supporting evidence for your answers.

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4. How can I ensure that my "Work, Power and Energy" assignment is free of errors and will not be marked wrong?

To ensure that your assignment is free of errors and will not be marked wrong, you should carefully read and follow the instructions provided by your teacher. Double-check your calculations and make sure you provide enough supporting evidence for your answers. You can also ask a classmate or a teacher to review your assignment before submitting it.

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