Finding time taken to stop a skater given mass, velocity and force

AI Thread Summary
The discussion revolves around calculating the time taken to stop a skater using the formula Δt = p/F, where p is momentum and F is force. The initial calculations yield a momentum of 480 kg·m/s and a time of 4 seconds, but the accuracy of this result is questioned. It is emphasized that the force opposing the skater's motion must be considered, leading to the conclusion that the force should be treated as negative. Additionally, the importance of defining the positive direction in the problem is highlighted, suggesting that the skater's initial velocity should be considered positive. Overall, the solution requires careful consideration of direction and opposing forces for accuracy.
Anmol Dubey
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Homework Statement
An ice skater with a mass of 60 kg moves with a constant speed of 8 m/s in a straight line. How long
will it take a force of 120 N to stop the skater if it were applied so as to oppose the motion? What would
the speed of the skater be if the force were applied for twice as long?
Relevant Equations
p=mv
kinetic energy = 1/2mv^2
I derived the formula for t but don't know how it works
Δt = p/F

I got
p = mv
= 60*8
= 480kgms-1

Δt = 480/120N
= 4s

Is that correct?
 
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You could check your answer by using ##F = ma##.
 
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PeroK said:
You could check your answer by using ##F = ma##.
Oh so a = v/t
= 8/4 = 2m/s2 (?)
F = ma
= 60*2 = 120N
 
Anmol Dubey said:
Oh so a = v/t
= 8/4 = 2m/s2 (?)
F = ma
= 60*2 = 120N
That's not a complete answer. And, it doesn't take into account that the force is opposite to the motion. Neither did your original solution.

You should start the solution by deciding which direction is positive. It would make sense to me that the skater's initial velocity (and momentum) are positive. I.e. ##v_0 = +8 \ m/s##. This means that the force is ##F = -120 \ N##.
 
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