How to find a percentages in kinetic energy

In summary, 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?
  • #1
Kofyebs
12
2
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?
 
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  • #2
For ’small’ velocities E=0,5m*v*v.
percentage=100*E2/E1
 
  • #3
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
That’s how much was wasted…
remaining(%)=50*50/140*140*100=12,755% (some simplifications made)
So the answer: 12,8%
 
  • #5
Thanks very Replusz.
 
  • #6
Okay :)
 
  • #7
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
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
nasu said:
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.
Can you please show me the right equation then.
Thanks
 
  • #10
The answer is correct! (I think)
 
  • #11
Sorry, my bad. I read initial speed as 100 km/h.
Yes, the answer is correct.

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". :)
 
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Related to How to find a percentages in kinetic energy

What is kinetic energy?

Kinetic energy is the energy possessed by a moving object due to its motion.

How do you calculate kinetic energy?

Kinetic energy can be calculated using the formula KE = 1/2 * m * v^2, where m is the mass of the object and v is its velocity.

What is the relationship between kinetic energy and mass?

The kinetic energy of an object is directly proportional to its mass. This means that as the mass of an object increases, its kinetic energy also increases.

What is the relationship between kinetic energy and velocity?

The kinetic energy of an object is directly proportional to the square of its velocity. This means that as the velocity of an object increases, its kinetic energy increases at a faster rate.

How can percentages be used to represent kinetic energy?

Percentages can be used to represent the relative amount of kinetic energy in a system. For example, if two objects have different kinetic energies, the higher energy object would have a higher percentage of kinetic energy compared to the lower energy object.

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