# Homework Help: Change in momentum

1. Oct 11, 2012

### omc1

1. The problem statement, all variables and given/known data An object of mass 4.8 kg is projected into the air at a 55° angle. It hits the ground 3.6 s later. What is the magnitude of its change in momentum while it is in the air? Ignore air resistance

2. Relevant equations J=p =sumF Δt=mΔv

3. The attempt at a solution mvsine(theta) = mgt v=43.07 m/s p=mv = 206.74 kg m/s but its says thats wrong i tried it a dif way but got the same answer please help...

2. Oct 11, 2012

### kushan

hello omc1 ,
If you notice momentum will remain conserved along horizontal axis . ie component of velocity along horizontal axis will remain same throughout .
So it will not cause any change in momentum .
Then what will cause a change in momentum ?
When you project the mass upward , the vertical component of mass will vary , and you might also know it will come back with the same speed with which it was thrown .
Try to find out this change

3. Oct 12, 2012

### omc1

so msin(theta)gt=p ??

4. Oct 12, 2012

### kushan

twice of that

5. Oct 15, 2012

### omc1

when i did that it is still giving me a wrong answer ....277.44

6. Oct 15, 2012

### HallsofIvy

With no air resistance, horizontal speed doesn't change and and vertical speed, once the object has hit the ground, will have the same magnitude but opposite sign.

7. Oct 16, 2012

### omc1

the neg of that answer doesnt work either.....