Calculating the mass of an object

  • Thread starter Ross duffy
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Homework Statement



A dog runs 480m in 28 seconds , we’ve to work out the mass of the dog

Homework Equations


F=ma
[/B]
M=f/a

The Attempt at a Solution


Really at a cross roads as every angle I work at this I need the mass of the dog to calculate the other components , a simple yes or no answer
Can you calculate the mass knowing distance travel time velocity acceleration etc
 

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  • #2
kuruman
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Hi Ross duffy and welcome to PF.

Is this all that's given to you? If it is, then you can only calculate the average speed of the dog, nothing else unless you make some assumptions.
 
  • #3
scottdave
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Do you have any other data? Are there any dogs which can run that fast? Maybe thatst the clue to what size dog it could be. Just throwing out some ideas.
 
  • #4
kuruman
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Even with complete kinematic information, the mass cannot be determined without some sort of connection to dynamic information, like the change in the dog's momentum over the given time interval. A dog, a motorcycle or a truck can have identical kinematic data, yet their masses are distinctly different.
 
  • #5
Hi Kuraman , the only information other than the above is a split time, so the dog ran 480 metres in 29.88 seconds then a split time was given of 2.53 seconds which was the time of the first 28 metres in the same race only other information is the dog had a standing start
 
  • #6
kuruman
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Hi Kuraman , the only information other than the above is a split time, so the dog ran 480 metres in 29.88 seconds then a split time was given of 2.53 seconds which was the time of the first 28 metres in the same race only other information is the dog had a standing start
With the additional information, you can assume that the acceleration is constant for the first leg of the run 0 < t < 2.53 s and use the kinematic equations to calculate the dog's acceleration and final speed when it starts from rest. Then you can assume that the acceleration is also constant (but different from the first leg) for the second leg of the run 2.53 s < t < 29.88 s. Use the kinematic equations again to find the new acceleration and final speed at the end of the run. I do not believe you can find the mass of the dog from this information alone.
 
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PeroK
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Homework Statement



A dog runs 480m in 28 seconds , we’ve to work the mass of the dog
That's not physics. That's a riddle!
 
  • #8
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I've seen this kind of problems before, like find the color of the bear after you've found g at a place on earth.

All you can do is find the speed of the dog like 38 mph in your example, then find which breed can run that fast and find its medium weight. The only problem is that the given speed is too slow for a dog and there more than 2 breeds which can attain that speed.

If the problem said a man runs 45 km/h it would have been Usain Bolt...
 
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  • #9
scottdave
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I've seen this kind of problems before, like find the color of the bear after you've found g at a place on earth.

All you can do is find the speed of the dog like 38 mph in your example, then find which breed can run that fast and find its medium weight. The only problem is that the given speed is too slow for a dog and there more than 2 breeds which can attain that speed.

If the problem said a man runs 45 km/h it would have been Usain Bolt...
That was the same line of thinking that I had.
 

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