What's Incorrect About Calculating Impulse in This Volleyball Spike Example?

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In summary, a ball with a mass of 0.24 kg and initial speed of 6.4 m/s is spiked, causing its speed to increase to 21 m/s. To find the magnitude of the impulse from the spike, we must subtract the initial and final momentum as vectors, taking into account direction. The result is a magnitude of 3.5 kg-m/s. However, this calculation is incorrect as it only considers the magnitude of the vectors and not their direction. To correctly find the magnitude of the impulse, we must find the components of the momentum vectors and subtract them, resulting in a magnitude of 5.04 kg-m/s.
  • #1
Ion1776
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What wrong with this problem?

A ball is traveling horizontally over a volleyball net when a player "spikes" it, driving it straight down to the ground. The ball's mass is 0.24 kg, its speed before being hit is 6.4 m/s and its speed immediately after the spike is 21 m/s. What is the magnitude of the impulse from the spike?

J=(.24kg)(21m/s)-(.24kg)(6.4m/s)=3.5

this is wrong, what's wrong
 
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  • #2


Still need some input on this please?
 
  • #3


Ion1776 said:
J=(.24kg)(21m/s)-(.24kg)(6.4m/s)=3.5

this is wrong, what's wrong
Impulse and momentum are vectors--you must take direction into account. Subtract them as vectors, not just numbers.
 
  • #4


This probably sounds stupid but what do u mean by subtract them as vectors.

I know know what vectors are but...
 
  • #5


Ion1776 said:
This probably sounds stupid but what do u mean by subtract them as vectors.

I know know what vectors are but...
You just subtracted them as numbers--which means you treated them as if they pointed in the same direction.

To subtract vectors, draw yourself a diagram. The two vectors are perpendicular to each other. Or you can find their components and subtract them.

Hint: Subtracting is the inverse of adding. How would you add two perpendicular vectors?
 
  • #6


again this sounds dumb, but would add these as vectors give you

(.48kg, 27.4m/s)
 
  • #7


Ion1776 said:
again this sounds dumb, but would add these as vectors give you

(.48kg, 27.4m/s)
No, that doesn't quite make sense.

I'll start you off. Let's call horizontal and vertical the x and y axes (like usual).

The initial momentum is:
mv = (.24 kg)*(6.4 m/s) = 1.54 kg-m/s in the + x direction
Expressed as a vector using components, it would be:
(1.54, 0)

The final momentum is
mv = (.24 kg)*(21 m/s) = 5.04 kg-m/s in the - y direction
Expressed as a vector using components, it would be:
(0, -5.04)

Now subtract those two momentum vectors and find the magnitude of the impulse.
 
  • #8


Alright got it
 

Related to What's Incorrect About Calculating Impulse in This Volleyball Spike Example?

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