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A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum.

Q A4.     A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.

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The initial and final velocities are :

                                                          u\ =\ 90\times \frac{5}{18}\ =\ 25\ m/s

and                                                    v\ =\ 18\times \frac{5}{18}\ =\ 5\ m/s

Using equation of motion we can write :

                                                    v\ =\ u\ +\ at

or                                                5\ =\ 25\ +\ a\times 4

or                                                a\ =\ -5\ m/s^2

Now the force is given by :

                                                F\ =\ ma

or                                                    =\ 1200\times (-5)\ =\ -\ 6000\ N

And the change in momentum is :           \Delta p\ =\ m\Delta v

or                                                                       =\ 1200(-20)\ =\ - 24000\ Kg\ m/s

                                               

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