# Homework Help: How to find a percentages in kinetic energy

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1. May 20, 2016

### Kofyebs

• Misposted so no template
Stefan throws a baseball with the speed of 140 km/ℎ. The ball goes obviously in goal, but it bounces back out of the net with a speed of 50 km/ℎ. What percentage of
the kinetic energy remaining after contact with the net?

2. May 20, 2016

### Replusz

For ’small’ velocities E=0,5m*v*v.
percentage=100*E2/E1

3. May 20, 2016

### Kofyebs

Please check this if I am right.
BOLL = △KE = ½ = mv²
BOLL= ½(2kg)(140km/h)² - ½(2kg)(50km/h)²
BOLL= 17,100 J
%△ = 19600 J – 2500 J = 17100 J
= 17100 J / 19600 J = 0.87
= 0.80 x 100 = 87
%△ = 87%

4. May 20, 2016

### Replusz

That’s how much was wasted…

5. May 20, 2016

### Kofyebs

Thanks very Replusz.

6. May 20, 2016

### Replusz

Okay :)

7. May 20, 2016

### nasu

To get energy in Joules the speeds have to be in m/s.
Your KE values are all wrong. Or at least in the wrong units.

The percentage does not depend on the units though.
Still your value does not look right. If the speed is halved the KE is 1/4 of the initial value.

8. May 20, 2016

### Replusz

Yes, I know that, but we only wanted to know the percentage! Thats why I didn’t care about the conversion…
And the speed is not halfed…

9. May 20, 2016

### Kofyebs

Can you please show me the right equation then.
Thanks

10. May 20, 2016

### Replusz

The answer is correct! (I think)

11. May 20, 2016

### nasu

However I think is important to point the wrong use of units, even if the final answer is ok.
It is already too often that students think that throw in some numbers without any justification is enough to call a problem "solved". :)