A trolley T moves at speed 1.2 m s–1 along a horizontal frictionless surface. The trolley collides with a stationary block on the end of a fixed spring. The mass of T is 250 g. T compresses the spring then expands and causes T to move in a direction opposite to its initial direction. At the time that T loses contact with the block, it is moving at a speed of 0.75 m s–1.

Why is the change in momentum= m(v+u) and not m(v-u) since change in momentum is final momentum - initial.

Question

A trolley T moves at speed 1.2 m s–1 along a horizontal frictionless surface. The trolley collides with a stationary block on the end of a fixed spring. The mass of T is 250 g.  T compresses the spring then expands and causes T to move in a direction opposite to its initial direction. At the time that T loses contact with the block, it is moving at a speed of 0.75 m s–1.

Why is the change in momentum= m(v+u) and not m(v-u) since change in momentum is final momentum - initial.

irishboy123 · Answer

$\large u_i = 1.2 m/s, u_f = \color{red}{-0.75}m/s  $

$\large \Delta u = 1.2 - (-0.75)$ .....

maths99 · Answer

Then shouldnt it be -0.75 - ( 1.2) @irishboy123

irishboy123 · Answer

you get the same magnitude, just a different sign

so yes, that's ok too

maths99 · Answer

Thanks @irishboy123 really helped me alot

irishboy123 · Answer

great!