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| Solutions of Savchenko Problems in Physics <br> | | Solutions of Savchenko Problems in Physics <br> |
| <i><b>knowledge must be free</b></i> | | <i><b>knowledge must be free</b></i> |
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| <h3 id="back-link"><a href="../../#2.1">$\leftarrow$Back</a></h3> | | <h3 id="back-link"><a href="../../#2.1">$\leftarrow$Back</a></h3> |
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| <h3> Statement </h3> | | <h3> Statement </h3> |
| <p> | | <p> |
| $2.1.2.$ The puck, which was sliding on the ice, stopped after a time $t = 5$ s after hitting the stick at a distance $l = 20$ m from the place of impact. The mass of the washer is $m = 100$ g. Determine the friction force acting on the washer. | | $2.1.2.$ The puck, which was sliding on the ice, stopped after a time $t = 5$ s after hitting the stick at a distance $l = 20$ m from the place of impact. The mass of the washer is $m = 100$ g. Determine the friction force acting on the washer. |
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| <h3>Solution</h3> | | <h3>Solution</h3> |
| <p> | | <p> |
| $$ 0 = v_0 - at\Rightarrow v_0 = at $$ | | $$ 0 = v_0 - at\Rightarrow v_0 = at $$ |
| Then | | Then |
| $$ l = \frac{at}{2}t = \frac{at^2}{2}\Rightarrow a = \frac{2l}{t^2} $$ | | $$ l = \frac{at}{2}t = \frac{at^2}{2}\Rightarrow a = \frac{2l}{t^2} $$ |
| Making a substitution in (1) | | Making a substitution in (1) |
| $$ F_{fr} = \frac{2ml}{t^2} $$ | | $$ F_{fr} = \frac{2ml}{t^2} $$ |
| Calculations | | Calculations |
| $$ F_{fr} = \frac{2 \cdot 0,1 kg \cdot 20 m}{(5 с)^2} = 0.16 \text{ N} $$ | | $$ F_{fr} = \frac{2 \cdot 0,1 kg \cdot 20 m}{(5 с)^2} = 0.16 \text{ N} $$ |
| It is interesting that acceleration can be obtained even more sifrly by using the “method by contradiction” in solving the problem, if from the stopping point, accelerate the puck back to the stick, then | | It is interesting that acceleration can be obtained even more sifrly by using the “method by contradiction” in solving the problem, if from the stopping point, accelerate the puck back to the stick, then |
| $$ l = \frac{at^2}{2}\text{, and }a = \frac{2l}{t^2} $$ | | $$ l = \frac{at^2}{2}\text{, and }a = \frac{2l}{t^2} $$ |
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| <h4>Answer</h4> | | <h4>Answer</h4> |
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| $$\fbox{$F_{fr} = \frac{2ml}{t^2}$; $F_{fr} = 0.16 \text{ N}$}$$ | | $$\fbox{$F_{fr} = \frac{2ml}{t^2}$; $F_{fr} = 0.16 \text{ N}$}$$ |
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