# Homework Help: Impulse/inclined plane

1. Jul 31, 2011

### jigoku_snow

1. The problem statement, all variables and given/known data

1) a girl applies a steady force to a stationary trolley which has a mass of 18 kg for 6 s. she moves it 36 m in this time. determine the impulse which she produced.

2) a satellite of mass 240kg moving in a free space at velocity of 600 m /s is struck by an unknown object which causes it to be deflected onto a new path at right angles to its original direction of motion in 0.5 s. find the impulse which acted upon the satellite if it continued to move at 600 m/s after deflection.

3) the impulse of a force causes a stationary mass to move at 4 m/s in the direction of the force. the same impulse causes a 2nd body also at rest to move at 6 m/s in the direction of the force. what velocity would be produced if the impulse were to act on the combined masses initially at rest?

4) a block of mass 100kg is being moved steadily by pulling on a rope attached to the block making an angle of 30 degree with the horizontal.if the tension in the rope is 120 N,what are the force of friction and the coefficient of friction?

2. Relevant equations

1) 2) 3) F=ma, Ft=m(v-u)
4) T- (mg sin theta + frictional force) = ma

3. The attempt at a solution
1) v of trolley= 36/6= 6 m/s
18 x 6 = 108 Ns ( answer given = 220 Ns)

2) p b4 collision =240 x 600 = 144000 kg m/s
p after collision = 240 x 600 = 144000 kg m/s
change in momentum = impulse = 0 ( answer 2.04 x 10^6)

3) impulse on 1st body= m(v-u) = m(4-0) = 4m
impulse on 2nd body = m(6-0)= 6m [ans : 2.4 m/s]

4) a=0, 120 = 500+ friction
friction = 380
coefficient of friction = 380/ mg cos 30= 0.438 [ answer 0.113]
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 31, 2011

### Staff: Mentor

The velocity is not constant (but the acceleration is). What's the final velocity?

Momentum is a vector. To find the change in momentum, you must subtract the two momentum vectors, not just their magnitudes.

The impulse on each body is the same. What's different is their mass. Figure out the total mass (in terms of the impulse), then use it to find the speed.

What's the component of the rope tension pulling the block forward? What's the normal force on the block?

3. Jul 31, 2011

### jigoku_snow

component of the rope tension= mg sin theta = 500N . given the tension of the rope is 120 N. is that means the total pulling force = 120 + 500= 620 ?
'moved steadily' which means a=0 ? then pulling force = sliding? if yes then mg sin theta + friction force = 620. friction force = 120 N. coefficient of friction = 120/ mg cos theta.

4. Jul 31, 2011

### Staff: Mentor

The rope tension is only 120 N. So what's the horizontal component of that force?
Yes. So what's the net force in any direction?

5. Jul 31, 2011

### jigoku_snow

net force =0

6. Jul 31, 2011

### Staff: Mentor

Yes. Keep going. (Careful with quotes.)

7. Jul 31, 2011

### jigoku_snow

i don't get what 'the velocity is not constant but the acceleration is' means. isn't to find a variable (a or v) from the formula a=(v-u)/t ,we will get the same answer ?
i'm stuck here :
a=(v-u)/t, v=s/t=6.
a=1 m/s^2.
u=0 (at rest) , 1=v/6, final v= 6 m/s

is it a need to find the displacement traveled by the satellite after collision first of all? "to be deflected onto a new path at right angles to its original direction of motion" . we will have a resultant velocity. SQRT (600)^2 + (-600)^2 = 849 . is that the final velocity?

impulse acted on the first body = 4m.
impulse on the 2nd body = 6m. is that means the mass of these 2 bodies differ by 2? i have an unknown which is m. how to identify the value of m?

* hope this time i quote them correctly ;)

Last edited by a moderator: Jul 31, 2011
8. Jul 31, 2011

### Staff: Mentor

v = s/t gives the average velocity, not the final velocity. How are they related?

You are subtracting vectors, not finding displacement. You are finding the change in velocity, not the final velocity. (The final speed is given.)

Call the impulse I.
For the first mass alone: I = m14
For the second mass alone: I = m26
What's the total mass?

Not quite, but I fixed it.

9. Jul 31, 2011

### jigoku_snow

v = s/t gives the average velocity, not the final velocity. How are they related?

okay. now i know that the final velocity is twice the average velocity. i thought the average v will be the final v (OMG)

You are subtracting vectors, not finding displacement. You are finding the change in velocity, not the final velocity. (The final speed is given.)

arent to get the change in v , first we need to have final v? ( final v - initial v= change in v)

Call the impulse I.
For the first mass alone: I = m14
For the second mass alone: I = m26
What's the total mass?

m1 + m2 = total mass.
m1/m2=6/4.
is that means that m1=6?

Not quite, but I fixed it.

lol

Last edited: Jul 31, 2011
10. Jul 31, 2011

### jigoku_snow

11. Jul 31, 2011

### Staff: Mentor

Good.

You are given the final v. If initial v = 600 x, then final v is 600 y (for example).

OK
Here's what I meant:
If I = m1*4, then m1 = I/4. Now apply the same impulse to the total mass to get the velocity. (You don't have enough information to solve for the actual masses, but you don't need that anyway.)

12. Jul 31, 2011

### jigoku_snow

all right . i got it ;) thanks .

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook