Greg has just started a new job at a grocery store down the road. ..

eddy2017

eddy2017

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Hi,
Greg has just started a new job at a grocery store down the road. He lives within walking distance of it. It takes him 20 minutes to walk there. He lives 2/3 of a mile from the store. Sometimes he rides his bike there instead, and it takes him only half as much time to get there.
Greg wants to know his speed in MPH when he is walking and his speed in MPH when he is riding his bike.
Calculate the speed in each case. Write your answers with walking speed first, followed by the bike riding speed.
WIK ( What I know)
it takes Greg 20 minutes to get to his job when he walks over.
it takes him half as much time when he bikes over=10 minutes.
speed in mph when walking?
speed in mph when riding his bike.

when he walks
S=d/t
S=(2/3)of a mile / 20 mts
S=92/3)/(20/1)
s=(2/3)(1/20)
S=(2)(1)/(3)(20)
S=2/60 reducing
S=1/30
S=0.033 mph
His speed when walking is 0.033 mph

when riding a bike,
S=d/t
s=(2/3)of a mile / 10 mts
s=(2/3)/(10/1)
=(2/3) (1/10)
=2/30
=1/5
=0.066
His speed when riding his back to work is 0.066 mph.


Is this ok?

Thanks in advance for corrections and tips.
 
eddy2017 said:
Hi,
Greg has just started a new job at a grocery store down the road. He lives within walking distance of it. It takes him 20 minutes to walk there. He lives 2/3 of a mile from the store. Sometimes he rides his bike there instead, and it takes him only half as much time to get there.
Greg wants to know his speed in MPH when he is walking and his speed in MPH when he is riding his bike.
Calculate the speed in each case. Write your answers with walking speed first, followed by the bike riding speed.
WIK ( What I know)
it takes Greg 20 minutes to get to his job when he walks over.
it takes him half as much time when he bikes over=10 minutes.
speed in mph when walking?
speed in mph when riding his bike.

when he walks
S=d/t
S=(2/3)of a mile / 20 mts
S=92/3)/(20/1)
s=(2/3)(1/20)
S=(2)(1)/(3)(20)
S=2/60 reducing
S=1/30
S=0.033 mph
His speed when walking is 0.033 mph

when riding a bike,
S=d/t
s=(2/3)of a mile / 10 mts
s=(2/3)/(10/1)
=(2/3) (1/10)
=2/30
=1/5
=0.066
His speed when riding his back to work is 0.066 mph.


Is this ok?

Thanks in advance for corrections and tips.
Click to expand...
No ........................... check corrections below.

S=(2/3) / 20 ........... mile/mts
S=(2/3)/(20/60) ..... mph
S=(2/3)/(1/3)
S= 2/3 * 3
S=2 mph
His speed when walking is 2 mph

when riding a bike,
S=d/t
s=(2/3)/ 10 ................................ mile /mts
s=(2/3)/(10/60) ....................... mph
=2/3 * 6 = 4
His speed when riding his back to work is 4 mph.

Eddy these are silly yet serious mistakes!!

I am afraid that you are just paying "lip-service" to paying attention to units. (... may be it is finger service)

pay attention.....
 
Last edited by a moderator:
Subhotosh Khan said:
No ........................... check corrections below.

S=(2/3) / 20 ........... mile/mts
S=(2/3)/(20/60) ..... mph
S=(2/3)/(1/3)
S= 2/3 * 3
S=2 mph
His speed when walking is 2 mph

when riding a bike,
S=d/t
s=(2/3)/ 10 ................................ mile /mts
s=(2/3)/(10/60) ....................... mph
=2/3 * 6 = 4
His speed when riding his back to work is 4 mph.

Eddy these are silly yet serious mistakes!!

I am afraid that you are just paying "lip-service" to paying attention to units. (... may be it is finger service)

pay attention.....
Click to expand...
Thank you, Doc. Yes, the blessed units. Oh my oh my!. Thanks for the corrections
 
I'll give you a little more complicated yet similar problem:

The men's 100 meters world record holder is Usain Bolt, with a time he set in 2009. The record stands at 9.58 seconds. Calculate Mr. Bolt's record making (average) speed in mph (i.e. miles/hour) with 4 significant digits (assume 1 meter = 39.37 inches). ................................... edited
 
Last edited by a moderator:
Subhotosh Khan said:
I'll give you a little more complicated yet similar problem:

The men's 100m world record holder is Usain Bolt, with a time he set in 2009. The record stands at 9.58 seconds. Calculate Mr. Bolt's record making speed in mph with 4 significant digits.
Click to expand...
I think the units should be stated explicitly to avoid ambiguity.
100m = 100 meters
mph = miles per hour (presumably this is what you meant)
 
eddy2017 said:
when he walks
S=d/t
S=(2/3)of a mile / 20 mts
S=92/3)/(20/1)
s=(2/3)(1/20)
S=(2)(1)/(3)(20)
S=2/60 reducing
S=1/30
S=0.033 mph
Click to expand...
This is very hard to follow.
I would do unit conversions before plugging values into formulas.
s = d/t.
d = 2/3 miles
t = 20 min = 1/3 hour.
Now plug them in:
(2/3)/(1/3) = (2/3)*3 = 2 mph

You missed an important step at the end - reality check. I understand that problems sometimes present very unrealistic scenarios, but if the givens are "normal", then the result should also be normal. In this case you got a bike speed of 0.066 mph from 2/3 miles in 10 min. Realistic givens, right? What about 0.066 mph? Is a bike that much slower than a car driving at 25-35 mph?
 
I missed the 60 in the denominator. I see it now.
Mph so 20/60
Great.
 
lev888 said:
This is very hard to follow.
I would do unit conversions before plugging values into formulas.
s = d/t.
d = 2/3 miles
t = 20 min = 1/3 hour.
Now plug them in:
(2/3)/(1/3) = (2/3)*3 = 2 mph

You missed an important step at the end - reality check. I understand that problems sometimes present very unrealistic scenarios, but if the givens are "normal", then the result should also be normal. In this case you got a bike speed of 0.066 mph from 2/3 miles in 10 min. Realistic givens, right? What about 0.066 mph? Is a bike that much slower than a car driving at 25-35 mph?
Click to expand...
Yes, you're right lev. Far enough from reality as to be able to realize the answer didn't make sense.
And I think conversion first would be the way to go.
 
BigBeachBanana said:
I think the units should be stated explicitly to avoid ambiguity.
100m = 100 meters
mph = miles per hour (presumably this is what you meant)
Click to expand...
You are absolutely positively correct. I have edited my problem statement. Thanks....
 
Another unit problem:

The men's official marathon world record holder is Eliud Kipchoge.

The record time stands at 2:01:39 (hrs : mts : sec). The official distance is 42.195 kilometers.

Calculate Mr. Kipchoge's record making speed in mph (→ miles/hour ) with 4 significant digits.
 
Subhotosh Khan said:
I'll give you a little more complicated yet similar problem:

The men's 100 meters world record holder is Usain Bolt, with a time he set in 2009. The record stands at 9.58 seconds. Calculate Mr. Bolt's record making (average) speed in mph (i.e. miles/hour) with 4 significant digits (assume 1 meter = 39.37 inches). ................................... edited
Click to expand...
well, dimension analysis is not so easy. It is a question of practice, like mot things in life. let's see how I fared
i converted from meters to feet to miles and from seconds to hours.
knowing that ,
1ft=0.3048m
1mi=5280 ft
1h =3,600 s
[math]100/9.58=10.4 m/s[/math][math]10.4 m/s *( 1ft/0.3048m) *(1mi/5280 ft)*(3600/1h)[/math]I cancel like units and I am left with m/h
Let's perform the operations now, and after doing all the multiplying and diving I get that
Usain Bolt's record speed in mph is ,
23.26 mi/h
 
Subhotosh Khan said:
Another unit problem:

The men's official marathon world record holder is Eliud Kipchoge.

The record time stands at 2:01:39 (hrs : mts : sec). The official distance is 42.195 kilometers.

Calculate Mr. Kipchoge's record making speed in mph (→ miles/hour ) with 4 significant digits.
Click to expand...
well, let me try my D.A hand at this.
we have distance at 42.195 km
time=2 hours 1 minute 39 seconds = total in seconds =7299s
1 mi is approximately 1.61 km

[math]42195 km / 7299 * (1mi / 1.61km )* (60s / 1mi)*(60mi / 1h)= [B]12924 mi/h[/math][/B]

after canceling like units, Mr. Kipchoge's record making speed in mph (→ miles/hour ) is 12924 mi/h
 
eddy2017 said:
well, dimension analysis is not so easy. It is a question of practice, like mot things in life. let's see how I fared
i converted from meters to feet to miles and from seconds to hours.
knowing that ,
1ft=0.3048m
1mi=5280 ft
1h =3,600 s
[math]100/9.58=10.4 m/s[/math][math]10.4 m/s *( 1ft/0.3048m) *(1mi/5280 ft)*(3600/1h)[/math]I cancel like units and I am left with m/h
Let's perform the operations now, and after doing all the multiplying and diving I get that
Usain Bolt's record speed in mph is ,
23.26 mi/h
Click to expand...
yey!!
 
eddy2017 said:
well, let me try my D.A hand at this.
we have distance at 42.195 km
time=2 hours 1 minute 39 seconds = total in seconds =7299s
1 mi is approximately 1.61 km

[math]42195 km / 7299 * (1mi / 1.61km )* (60s / 1mi)*(60mi / 1h)= [B]12924 mi/h[/math][/B]

after canceling like units, Mr. Kipchoge's record making speed in mph (→ miles/hour ) is 12924 mi/h
Click to expand...
Is that possible? Think about it. Speed of sound through air is < 770 mph !!!!
 
after recalculating and re-canceling like units,
Mr. Kipchoge's record-making speed in mph (→ miles/hour ) is 12.92 mph.
 
eddy2017 said:
after recalculating and re-canceling like units,
Mr. Kipchoge's record-making speed in mph (→ miles/hour ) is 12.92 mph.
Click to expand...
According to the conversion factors you have posted:

eddy2017 said:
1ft=0.3048m
1mi=5280 ft
1h =3,600 s
Click to expand...
I get speed = 26.21876/2.0275 = 12.93157 → 12.93 mph (does not match your answer to 4 significant digits)

However, the disappointing part was that your first posted answer was 1000 times more than that - and you agreed to that!!!!
​
​
 
Subhotosh Khan said:
According to the conversion factors you have posted:


I get speed = 26.21876/2.0275 = 12.93157 → 12.93 mph (does not match your answer to 4 significant digits)

However, the disappointing part was that your first posted answer was 1000 times more than that - and you agreed to that!!!!
​
​
Click to expand...
any hint or pointing out where the mistake lies?
 
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