A puck of mass m moving at speed v on a horizontal, frictionless surface is stopped in a distance d because a hockey stick exerts on an opposing force of magnitude F on it.

F = (mv^2) / 2d

If the stopping distance d increases 40%, by what percent does the average force needed to stop the puck change, assuming that m and v are unchanged?

(Fnew - F) / F = ? %

Respuesta :

Answer:

[tex]\% Change = [\frac{1}{2.8} -\frac{1}{1}]*100 = -64.29\%[/tex]

Explanation:

Assuming that m and v are unchanged.

For this case we have the following formula for the force:

[tex] F = \frac{mv^2}{2d}[/tex]

For this case the new force would be given:

[tex] F_{new}= \frac{mv^2}{2*(1.4 d)}[/tex]

[tex] F_{new}= \frac{mv^2}{2.8 d}[/tex]

And for this case we can calculate the % like this:

[tex] \Change = \frac{\frac{mv^2}{2.8d} -\frac{mv^2}{2d}}{\frac{mv^2}{2d}} *100[/tex]

And doing the algebra we got:

[tex]\% Change = \frac{\frac{mv^2}{2d}}{\frac{mv^2}{2d}} [\frac{1}{2.8} -\frac{1}{1}]*100[/tex]

[tex]\% Change = [\frac{1}{2.8} -\frac{1}{1}]*100 = -64.29\%[/tex]

So then the force decrease 64.29 percent respect the original force.

The change in the average force needed is -64.29%

Percentage difference:

Originally, the force required to stop the puck is given as;

[tex]F=\frac{mv^2}{2d}[/tex]

where d is the stopping distance

Now, if the stopping distance is increased by 40% then the new distance becomes:

[tex]d'=d+\frac{40}{100}d\\\\d'=\frac{7}{5}d[/tex]

So, the force required to stop the puck at a distance d' is given by:

[tex]F_{new}=\frac{mv^2}{2d'}=\frac{5mv^2}{14d}[/tex]

The percentage difference is given by:

[tex]\frac{F_{new}-F}{F}\times100\% = \frac{\frac{5mv^2}{14d}-\frac{mv^2} {2d}}{\frac{mv^2} {2d}}[/tex]

(Fnew - F) / F = -64.29%

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