Momentum/Impulse Question

  • Thread starter Draco
  • Start date
In summary, momentum is a physical quantity that describes an object's motion, while impulse is a change in momentum. These two concepts are closely related as impulse is the cause of a change in momentum. Both momentum and impulse can be negative, indicating opposite directions of motion or force. The conservation of momentum is a fundamental law in physics stating that the total momentum of a closed system remains constant unless acted upon by an external force.
  • #1
Draco
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Homework Statement


A firecracker in a coconut blows the coconut into three pieces. Two pieces of equal mass fly off south and west, perpendicular to each other, at 20m/s. The third piece has twice the mass as the other two.

Homework Equations


p=mv
pi=pf

The Attempt at a Solution


My attempt involved giving the variable m to the two equal pieces and 2m to the third piece. I gave a value to m. Also the momentum before and after is equal to zero so I worked out the momentum and re arranged the equation to solve for v. I got 20m/s but i was wrong.

Homework Statement


Far in space, where gravity is negligible, a 495 kg rocket traveling at 95.0 m/s fires its engines. The figure View Figure shows the thrust force as a function of time. The mass lost by the rocket during these 30 s is negligible.

http://img212.imageshack.us/img212/8301/knightfigure0926xe8.th.jpg [Broken]
1)What impulse does the engine impart to the rocket?
2)At what time does the rocket reach its maximum speed?
3)What is the maximum speed of the rocket?

Homework Equations


Impulse= (Force)(Change in Time)

The Attempt at a Solution


Um I tried finding the area under the graph to find the impulse but that didn't work. I'm really having a hard time trying to read the graph.

Any tips for a struggling student? :S
 
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  • #2
1. You don't have to ascribe a value to m, thta'll just make it more messy. The m will cancel out. Equate the x and y components of the momentum. Try again, you must have made some error.

2. Area under graph = impulse. Remember something else which impulse is? It's equal to change in...
 
Last edited:
  • #3
1) I'm not sure exactly what you mean. Can you be a bit clearer?

2) isn't it change in momentum? but like I'm not sure what to do with that.
 
  • #4
1. The total momentum is zero, initially and finally. If v is the velo of the bigger piece of mass 2m, then its momentum is 2mv. Choose +ve x-axis in the direction east, and +ve y-axis towards north.
Then, if ‘b’ is the angle of the direction in which the mass 2m is going with the +ve x-axis, its x component of the momentum is 2mv*(cos b). The x component of the other two pieces are: 0 and (–m*20). So, 2mv*(cos b) –20m+0=0. Similarly, for the y component. Then, you have two equations and you can solve for v. (Here, by symmetry, the bigger piece must be going in the NE direction, and so b=45 degree.)

2. If you know the mass of the rocket, and the change in momentum, then you should be able to find the change in velo. The area under the graph gives you the total impulse, which is the change in momentum.

When do you think the speed of the rocket is max? When is the acceleration max?
 
  • #5
>"I'm really having a hard time trying to read the graph."

The graph is simple. Just find the area as you would do in a triangle. Take the numbers on the axes as given. So, you'll be multiplying some seconds by some Newtons to get the value of the impulse.
 
  • #6
ok so i got the maximum speed, but I am still not sure how to find the time. Shouldnt the rocket be at maximum speed when it reaches the highest force?
 
  • #7
Tell me when it reaches the max speed.

Suppose you smack a ball and just let it move in space without friction and no other force is acting on it, do you think the ball slows down?

Also, tell me when is the accelerarion max.
 
  • #8
In case you are still not very sure, the max speed is reached at the end of 30 s, when the engine has been switched off. As long as there is a thrust, the speed will continue to increase.

It is the accn which is max when the force is max, i.e., at 10 s.
 

What is momentum?

Momentum is a physical quantity that describes the amount of motion an object has. It is calculated as the product of an object's mass and its velocity.

What is impulse?

Impulse is a change in momentum. It is calculated as the product of force and the time over which the force acts.

How are momentum and impulse related?

Momentum and impulse are closely related because impulse is the cause of a change in momentum. In other words, the impulse acting on an object is equal to the change in its momentum.

Can momentum and impulse be negative?

Yes, both momentum and impulse can be negative. A negative momentum indicates that an object is moving in the opposite direction of its positive momentum, while a negative impulse indicates that the force is acting in the opposite direction of the object's motion.

What is the conservation of momentum?

The conservation of momentum is a fundamental law of physics that states that the total momentum of a system remains constant, unless acted upon by an external force. This means that in a closed system, the total momentum before an event is equal to the total momentum after the event.

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