Question

A stone of mass 0.4 kg is thrown vertically upward with speed 9.8 m/s. Find the kinetic energy and potential energy after half second.

Answer 1

KE = 1/2 * m * v^2

KE = 1/2 * 0.4 * 9.8^2 = 19.208 J

PE = mgs

Assuming, the ball was thrown from 0 distance from surface,

s = 9.8 * 0.5 + 1/2 * -10 * 0.5^2 = 3.65 m

PE = 0.4 * 10 * 3.65 = 14.6 J

Please comment if you find anything odd, or have a doubt.

Answer 2

The kinetic energy and potential energy after half second will be 4.802 J and 14.406 J respectively.

Explanation:

Given:

A stone of mass 0.4 kg is thrown vertically upward with speed 9.8 m/s.

To Find:

The kinetic energy and potential energy after half second.

Solution:

As given,a stone of mass 0.4 kg is thrown vertically upward with speed 9.8 m/s.

Mass  of a stone m = 0.4 kg

Speed u =9.8 m/s.

t=0.5 second

a= -g

Speed of stone after half second  $v= u+at$

                                                        $v=9.8+(-9.8)(0.5)$

                                                            $=9.8- 9.8\times .5$

                                                             $=9.8\times0.5$

                                                              $=4.9 m/sec^{2}$

The kinetic energy $KE=\frac{1}{2} mv^{2}$

                                         $=\frac{1}{2} \times 0.4\times 4.9^{2}$

                                         $= 4.802 J$

                                                             

After half second stone  height will be $h=ut-\frac{1}{2} g t^{2}$

                                                                     $=9.8\times 0.5-\frac{1}{2}\times9.8\times (0.5)^{2}$

                                                                      $= 4.9 -1.225$

                                                                     $=3.675 m$

The  potential energy $PE=mgh$

                                            $=0.4 \times 3.675 \times 9.8$

                                             $= 14.406J$

Thus,the kinetic energy and potential energy after half second will be 4.802 J and 14.406 J respectively.

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