physics question - momentum/ WEP

lavastar SG

A steel ball of mass 0.020kg is released from a height of 1.2m above a steel plate and rebounds vertically to a height of 1.1m. The ball is in contact with the surface for 0.9ms

im stuck on one part of this question.that is the change of the momentum of the ball in its collision with its plate ( the answer is 0.19Ns )

The answers for the 1st two parts are

Velocity of ball as it arrive at the plate = 4.9m/s
momentum of ball as it arrive at the plate = 0.097

i tried alot of ways but cannot reach the answer? any help?

eagle SG

We use back vi = -4.85 m/s

vf = sqrt(2 * 9.81 * 1.1) = 4.65 m/s

So change in momentum = (0.020) (4.65 - (-4.85)) = 0.19N

Because we take momentum is a vector, velocity is a vector, and taking upwards as positive direction, hence vi = -4.85

Regards,
Eagle

lavastar SG

omg thanks alot.. realised that i disregarded the minus sign even though momentum is a vector quantity.

eagle SG

students always make this same careless mistake

Tip: everytime, when you answer questions that contain vectors, always declare which direction is positive. This way, it is easier on both you and the marker.

^tamago^ SG

for me i always declare the 2 axes for kinetics involving vectors, and they are usually along the axis of movement, or vertical-horizontal.

eagle SG

Yep

To add on another tip, every diagonal vector can be easily resolved into a vertical and a horizontal vector for overall easier calculations and understanding