this one is tough about speed and acceleration?

Question

Johnny is driving and frankie is the passenger they are going 50 mph so what is frankie's speed according to johnny's viewpoint? Bobby is on the corner as they pass what is franie's speed according to bobby's viewpoint and last a helicopter is above what is frankie's speed according to the copter's viewpoint?

Answer 1

This problem does not have anything to do with acceleration actually; it only deals with relative velocities between different references frames (observers / people).

Remember that velocity is a vector and has both a speed and a direction. If you subtract the velocity vectors of two different objects, you will get the relative velocities between the two objects. To get the relative speeds (just the magnitude of the velocity, no direction), take the sum of the square of each component of the velocity vector and then take the square root. For example, 50 mph North is a velocity that has a magnitude [speed] of 50 mph.

If, for example, one object is traveling 50 mph with respect to the ground in the Northern direction and another object is traveling 50 mph with respect to the ground in the Southern direction, we can write out these two velocities as,
V1 = +50 mph
V2 = -50 mph
Here I took the Northern direction to mean positive and the Southern direction to be negative.
Subtracting V2 from V1 gives us the relative velocity of object 1 with respect to object 2.
V1 – V2 = (+50 mph) – (-50 mph) = +100 mph, and since the answer is positive, the relative velocity is North…object 1 is traveling 100 mph in the Northern direction with respect to object 2. The relative speed would then be 100 mph.
Similarly for the relative velocity of object 2 with respect to object 1,
V2 – V1 = (-50 mph) – (+50 mph) = -100 mph, and since it is negative it indicates the Southern direction. So, with respect to object 1, object 2 is traveling 100 mph South.

First off, it is always important to note in what reference frame a particular measurement was taken since there is no ‘universal’ frame of reference for things.
The problem states that the car Johnny and Frankie are in is traveling at 50 mph…..but 50 mph with respect to what? Since (unfortunately, since the whole point of this problem is dealing with relative velocities) it is unstated, we will have to assume it is taken with respect the ground / street they are driving on.

If both Johnny and Frankie are in the car, and they are staying in the car (i.e. they are not falling out the back or some such thing), their speed relative to the car is zero, which means that their speed relative to the ground is the same as the car’s speed relative to the ground. So Johnny and Frankie’s speed relative to the ground is 50 mph (same as the car) and relative to each other is 0 mph, since they are both at rest in the car and with respect to each other (we can only assume from the lax wording of the question).

As the car passes Bobby who (again we have to assume) is a rest on the street corner with respect to the ground (i.e. he is not walking/running/….), the car (and thus Johnny and Frankie, as well) will have a relative speed to Bobby of 50 mph.

On the last part, with the Helicopter, it is very hard to say what the question intended by “is above”. Does this mean that it is hovering above a fixed point on the ground or flying constantly over the car, or what? The question is very vague in this respect and keeps up from making any type of statement with regards to the relative speed between the car and the helicopter.


My guess is that what you were typing the question into Yahoo! Answers from your homework problem, you omitted some words (possibly accidentally). However, in problems like this, it is important to known exactly how things are phrased in order to get the correct relative speed. For example, without knowing further details about the helicopter’s motion, we can’t say what the relative speed is between the car and the helicopter.

Answer 2

Well, when Johnny looks at Frankie, are they moving apart?
No! So Frankie is going 0 mph according to Johnny.

(Since Johnny and Frankie are both in a car going 50 mph, they each see the other as stationary).


Since Bobby is staitionary, he would see Frankie pass by at 50 mph.


If the helicopter is hovering (i.e. - stationary), he would also see Frankie traveling at 50 mph.

The moral of the question? All is relative!

Answer 3

If two bodies move with speeds x and y in the same direction then their relative speed is x-y.and if they are moving in opposite direction then their relative speed is x+y.
therefore from johnys view point frankie is still ie speed is zero since both are moving with same speed.If bobby is outside the car and is not moving then from bobbys view frankies speed is the speed of the car itself.If bobby is inside the car then frankies speed is nil.As said above if copter is moving in same diretion then frankies speed is copters speed minus speed of car if copter is moving in opposite diretion then plus sign instead of minus