Aim: Review all motion problems

Opposite motion problems: Remember that Distance is = to the rate or speed times the time D = RT in all of the following problems D is replace by the values of RT when known or by one of the values and a variable either R or T.


Example: Two cars start from the same point at the same time and travel in opposite directions. The slow car travels at 28 mph,and the fast car travels at 35 mph.
In how many hours will the cars be 252 miles apart?
Given is R for each car and the total distance travelled by both cars going in opposite directions.
Find T, the time, in hours travelled that each car travelled.

Solution: Start by defining a Let Statement:
Let x = the number of hours traveled by each car. Both cars are travelling for the same time T.
Let Slow car distance be D1 and fast car distance be D2

For Car 1 D1 = R1T For Car 2 D2 = R2T

D1 + D2 = Total Distance Now substituting RT for distances D1 and D2

R1T + R2T = Total Distance
28mph x X + 35mph x X = 252 miles
28X + 35X = 252 miles.
63X = 252
X= 4 hrs

Practice Exercise:
http://kutasoftware.com/FreeWorksheets/Distance%20Rate%20Time%20Word%20Problems.

Lesson Review:

Example problem: How far can a man drive out into the country at the average rate of 40 mph and return over the same road at the average rate 30 mph if he travels a total of 7 hours?
Need to find T then substitute in RT to find distance in D=RT

Tout + T back = 7 hrs and Distance out is same as the Distance back

Tout + Tback = 7hrs.
Let Tout = X
X +Tback = 7hrs
subtract X from both sides:
Tback = 7hrs - X
So: T out = X and T back = 7-X

Dout = RT = 40mph x X
Dback = RT 30mph x (7-X)
If Dout = Dback
Then 40X = 30 (7-X)
40X = 210 -30X
40X + 30X =210
70X = 210
X=3 hrs
Dout = RT = 40(3) = 120 miles
as a check Dout should = Dback
Dback = RT = 30 (7-x) = 30(7-3) = 30(4) = 120 miles

Practice Exercise:http://kutasoftware.com/FreeWorksheets/Distance%20Rate%20Time%20Word%20Problems.pdf