Homework Help: Find the speed of each car

Phys student · Sep 1, 2012

1:two cars are moving with a constant speed toward each other and the distance between them is decreasing at a rate of 3 m/s if the two cars are moving with the same speed but this time in the same direction the distance between them is decreasing at a rate 1 m/s. what is the speed of each car

2. Relevant equations : D=VT V=vo+at ?

3. The attempt at a solution: if they have the same speed then when they are moving toward each other each car's speed must be 1.5 m/s but when i read the second part that idea must be invalid

Can you please give me a hint on how to start guys?

azizlwl · Sep 1, 2012

Let car A at left and B on the right.

Please sketch the diagram for better understanding.
Let car A at origin and car B at a distance x from origin
y=at
z=-bt + x, car B is going opposite direction of car A

Distance between them is -(z-y)

s=-t(b+a) +x
Velocity as time derivative of position,
v=-(a+b)
v =-3
b+a=3 .... (1)

For same speed and direction
y=at
z=bt+x

b-a=1 ......(2)

We have 2 equations and 2 unknowns.

Phys student · Sep 2, 2012

azizlwl thanks for the reply but can you please explain what do you mean by y,z and s , and does the b and a in the two equations mean the velocity of each car ?

azizlwl · Sep 2, 2012

Phys student said: ↑

azizlwl thanks for the reply but can you please explain what do you mean by y,z and s , and does the b and a in the two equations mean the velocity of each car ?

Sorry for not defining the variables.
a and b are the velocities for car A and car B respectively.
y and z is displacement of for car A and car B respectively with respect of time.
x is the intial position of Car B whilst car A at origin at t=0.

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Homework Help: Find the speed of each car

Phys student

azizlwl

Phys student

azizlwl