How to find acceleration from force and time?

SpiraRoam

1. Homework Statement
A coin is dropped into a well and the time taken to hit the bottom is 2.48 seconds. Calculate the mass of the coin if the water slows it to a stop in 0.72 seconds exerting an average force of 0.41N on the coin

2. Homework Equations
F=ma m=f/a

3. The Attempt at a Solution
Finding the mass from m=f/a but first finding the acceleration with just the time and force values

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BvU

Homework Helper
Hi day,

It is clear you'll need another relevant equation: in yours there is no time variable present. Any idea what to do with the 2.48 seconds, for example ?

SpiraRoam

Hmm I didn't think the 2.48s was relevant to the mass but it very well could be. The other two parts of the question before this ask for the depth of the well and the final velocity of the coin - which I derived from the suvat equation s=ut+1/2at^2 which I rearranged for the acceleration of 0.806m/s^2. I also got the displacement of the fall as 4.959m and the final velocity as 1.999m. These are probably relevant to the final question and the mass but i'm not sure how..

SpiraRoam

Ahh, I think I've forgotten to include gravity 9.81ms^2 in the original equation as acceleration.

Buffu

Finding the mass
Can you find the deceleration caused by the water ?
and then use $f = ma$.

SpiraRoam

Possibly. I'm not sure how to calculate acceleration in water - would the 0.41N of force from the water be a clue? Once I've got that I can find a new velocity with the 0.72s. I may have to just use the final velocity from the fall and carry it over? It is just 0.72s afterall...

gneill

Mentor
Draw an FBD for the coin in water. What are the forces acting and in which directions?

Buffu

Possibly. I'm not sure how to calculate acceleration in water - would the 0.41N of force from the water be a clue? Once I've got that I can find a new velocity with the 0.72s. I may have to just use the final velocity from the fall and carry it over? It is just 0.72s afterall...
try $v-u=at$ to find declaration.

BvU

Homework Helper
Hmm I didn't think the 2.48s was relevant to the mass but it very well could be. The other two parts of the question before this ask for the depth of the well and the final velocity of the coin - which I derived from the suvat equation s=ut+1/2at^2 which I rearranged for the acceleration of 0.806m/s^2. I also got the displacement of the fall as 4.959m and the final velocity as 1.999m. These are probably relevant to the final question and the mass but i'm not sure how..
${1/2}\;gt^2$ (an expression that does include a time!) gives me something else for $t=2.48$ s. And 1.999 m is not a speed. Could you show the complete exercise text and post your working (instead of some answer numbers)?

"How to find acceleration from force and time?"

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