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Kinetic Energy Calculation Need Advice ASAP

Hello, I have been tasked with finding the kinetic energy of a body of mass 5ng travelling at 200 km/s.
So converting 5 ng to kg = 0.000000000005 kg
200000 km/s = 200m/s
So KE=1/2*m*v^2
KE=1/2*0.000000000005*200000^2
KE=0.1 J

This is spatially multiple choice, the options being;
1.100 J
2.0.001 J
3.1 J
4. 100000J
5. 17J

Where have I gone wrong? 😳

(edited 4 years ago)

Reply 1

Pangol

Original post by Lyrapettigrew

Hello, I have been tasked with finding the kinetic energy of a body of mass 5ng travelling at 200 km/s.
So converting 5 ng to kg = 0.000000000005 kg
200000 km/s = 200m/s
So KE=1/2*m*v^2
KE=1/2*0.000000000005*200^2
KE=0.1 J

This is spatially multiple choice, the options being;
1.100 J
2.0.001 J
3.1 J
4. 100000J
5. 17J

Where have I gone wrong? 😳

If something has a speed of 1 km/s, how many m/s does it cover?

(Also far better to use standard form than all those zeros!)

Reply 2

Lyrapettigrew

Original post by Pangol

If something has a speed of 1 km/s, how many m/s does it cover?

(Also far better to use standard form than all those zeros!)

1 km/s = 1000 m/s I think
So 200 km/s = 200,000 m/s?
Or am I wrong?

Normally I use standard form but I wanted to be explicit in what I was calculating in case of errors.

So 1/2 * 5 * 10^-12 * 200000^2 = 0.1 J

Reply 3

I Luen

1km/s= 1000 m/s
1ng = 1x10^-12 kg

(edited 4 years ago)

Reply 4

Pangol

Original post by Lyrapettigrew

You wrote the conversion the wrong way around in your original post, but I see that you used the right figure in your calculation. Assuming you've given us all the right information, then the answer doesn't appear to be one of the options.

Reply 5

Lyrapettigrew

Well that is what I thought, do you think it is definitely a mistake in the question?

(I had calculated using 200000^2 but wrote 200^2 on my post just rushing stupidly)

Reply 6

Pangol

Original post by Lyrapettigrew

Well that is what I thought, do you think it is definitely a mistake in the question?

(I had calculated using 200000^2 but wrote 200^2 on my post just rushing stupidly)

Looks like it.

Reply 7

Kallisto

Entertainment Forum Helper

Original post by Lyrapettigrew

If you convert 200000 km in m, it is thousand times bigger, so you have to multiply by 1000! that's it. Do the math again and you finally get your result from multiple choice.

Reply 8

Lyrapettigrew

Original post by Kallisto

If you convert 200000 km in m, it is thousand times bigger, so you have to multiply by 1000! that's it. Do the math again and you finally get your result from multiple choice.

But is is converting 200km/s to m/s not 200,000 km to m

Reply 9

Kallisto

Entertainment Forum Helper

Original post by Lyrapettigrew

But is is converting 200km/s to m/s not 200,000 km to m

Okay, to be a bit more precise:

In km/s and m/s the time units in the denominators are the same (marked in bold print!), namely s for seconds! and if they are the same, they don't need to consider for your conversion and you just need to convert the metric units in the numerators. That is why:

200 km/s = 200,000 m/s

And that is what I was talking about: when you converting km to m only because the time units are the same (indicated in seconds!), the factor to multiply is 1000!