Physics with Relative Velocity

**In the latest Indian Jones film, Indy is supposed to throw a grenade from his car, which is going 43.8 m/s, to his enemy's car, which is going 55.8 m/s. The enemy's car is 15.2 m in front of the Indy's when he lets go of the grenade.**

A) If Indy throws the grenade so its initial velocity relative to him is at an angle of 45 above the horizontal, what should the magnitude of the initial velocity be? The cars are both traveling in the same direction on a level road. You can ignore air resistance.

My set up here is

horizontal distance = V * cos45 * Vj * t

[where V is the initial velocity and Vj is Jones' car velocity]

distance between cars = 15.2 + (Vj - Ve) * t

[where Ve is the enemy's car velocity]

I set these two equations equal to each other to find the time it takes the grenade to reach the other car.

(Vcos45)(43.9)(t) = 15.2 + (43.8 - 55.88) * (t)

Then I'm not sure if I'm head in the right direction...

**B) Find the magnitude of the velocity relative to the earth.**

I'm also not sure how to start this portion.

Re: Physics with Relative Velocity

Re: Physics with Relative Velocity

I found that the initial velocity that Indy throws the grenade is 23.33 and this is the grenade's velocity with respect to Indy's car.

Mastering Physics wants it in km/h so I converted that to 83.88. But I'm actually not sure why it's telling me that both answers are wrong..

For the second part,

I think that to find the magnitude of the grenade with respect to the earth, I'll have to set up a relative velocity equation something like:

where

is the grenade's velocity with respect to the Earth

is the grenade's velocity with respect to Indy's car

is Indy's car's velocity with respect to the Earth

I'll have to find the x-component and y-component for and , add them, and find the magnitude from there?

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Re: Physics with Relative Velocity

I get

look at the vector sketch to determine the velocity relative to the Earth ...

I get