# Homework Help: Change in Momentum of shopping cart

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1. Feb 6, 2016

### watermarked

1. The problem statement, all variables and given/known data
A 12kg shopping cart rolls due south at 1.5m/s. After striking the bumper of a car, it travels at 0.80m/s at 30. degrees E of South. What is the magnitude and direction of the change in momentum sustained by the shopping cart?

2. Relevant equations
p = mv

3. The attempt at a solution
initial py = mv = 12kg * 1.5m/s = -18N
initial px = 0
final py = mv = 12kg * sin30 * 0.8m/s = -4.8N
final px = 12kg * cos30 * 0.8m/s = 8.31N
change in py = -4.8 + 18+ = 13.2N
change in px = 8.31N
magnitude of change in p = square root of (13.2N2 + 8.31N2) = 16N
angle of change in p = tan-1(13.2N/8.31N) = 58 degrees North of East

What am I doing wrong?

2. Feb 6, 2016

### Nathanael

Have you drawn a diagram? Are you sure your "final py" and "final px" are correct? Remember that your y direction is south.

3. Feb 6, 2016

### watermarked

I did draw a diagram, but it turned out to be incorrect -- I was thinking south of east instead of east of south. Thanks for your help!