Question

A ball moving with momentum with 5 kg m/s strikes against a wall at 45 and is reflected at the same angle. Calculate the magnitude of change of momentum.

Answer 1

Convert the momentum into components as parallel to the wall and perpendicular to the wall.

Initial momentum = 5 kg m/s
perpendicular component = 5 cos 45 = 5/√2 kg m/s
parallel component = 5 sin 45 = 5/√2 kg m/s

After hitting the wall, it reflects at same angle.
So magnitude will the same = 5 kg m/s
parallel component = 5 sin 45 = 5/√2 kg m/s
perpendicular component = -5 cos 45 = -5/√2 kg m/s
(- sign comes due to change in direction)

So you can see that there is no change in parallel component.
Only perpendicular component changes.
So change = 5/√2 kg m/s - (-5/√2 kg m/s)
                  = 5/√2 kg m/s+5/√2 kg m/s
                  = 10/√2 kg m/s
                  =5√2 kg m/s

Answer 2

Answer:

Initial momentum = 5 kg m/s

perpendicular component = 5 cos 45 = 5/√2 kg m/s

parallel component = 5 sin 45 = 5/√2 kg m/s

After hitting the wall, it reflects at same angle.

So magnitude will the same = 5 kg m/s

parallel component = 5 sin 45 = 5/√2 kg m/s

perpendicular component = -5 cos 45 = -5/√2 kg m/s

(- sign comes due to change in direction)

So you can see that there is no change in parallel component.

Only perpendicular component changes.

So change = 5/√2 kg m/s - (-5/√2 kg m/s)

                 = 5/√2 kg m/s+5/√2 kg m/s

                 = 10/√2 kg m/s

                 =5√2 kg m/s

Explanation: