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Two uniform motion problems

  • Thread starter DB
  • Start date
  • #1
DB
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hey guys, in order to get the rest of problems on this sheet, i gotta know how to do the first 2, and unfortunatly im stuck, could u guys gimme a push in the right direction?

1. two cars are traveling in opposite directions at the speeds of 18 mph and 22 mph respectively. if they started from the same place and same time then in how many hours will they be 200 miles apart?

so far i got:
[tex]d_1+d_2=200[/tex]

[tex]\Delta t=\frac{\Delta d}{v}[/tex]
so

[tex]\Delta t=\frac{200-d_2}{v}[/tex]

and

[tex]\Delta t=\frac{200-d_1}{v}[/tex]

can i do this?:

[tex]\frac{200-d_2}{v}=\frac{200-d_1}{v}[/tex]
if i can, then the algebra's got me stuck

------------
2. two cars started towards eachother at the same time from points which are 300 km apart, and met in 5 hours. if one traveled twice the speed of the other, then wat were their speeds?

so far i know i have use:

[tex]v=\frac{\Delta d}{\Delta t}[/tex]

and

[tex]v_1=x [/tex]
[tex]v_2=2x[/tex]

but then im stumped, i know i need another equation with the 300 km but i cant see it :(

thanks in advance
 

Answers and Replies

  • #2
Doc Al
Mentor
44,865
1,112
The same approach will work for both problems.

For the first problem, realize that:
[tex]d_1 = v_1 t[/tex]
[tex]d_2 = v_2 t[/tex]

Just combine with your equation:
[tex]d_1+d_2=200[/tex]
and solve for t.

For the second problem, just like the first problem you have:
[tex]d_1+d_2=300[/tex]

And you have:
[tex]d_1 = v_1 t[/tex]
[tex]d_2 = v_2 t = 2 v_1 t[/tex]

This time t is known. Combine these and solve for [itex]v_1[/itex], and then [itex]v_2[/itex].
 
  • #3
DB
501
0
wow i feel stupid its so easy! lol anyway i get 5 hours for the first n 20 km/h n 40 km/h for the second, thanks doc al.
 
  • #4
Doc Al
Mentor
44,865
1,112
DB said:
anyway i get 5 hours for the first n 20 km/h n 40 km/h for the second
Right. Don't forget that in the second problem you need to find the speeds of both cars.
 

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