Problem regarding concept of weightlessness 
A simple pendulum having a bob of mass m is suspended from the ceiling of a car used in a stunt film shooting. The car moves up along an inclined cliff at a speed v and makes a jump to leave the cliff and lands at some distance. Let R be the maximum height of the car from the top of the cliff. Tension in the pendulum's string when the car is in the air is
A) mg
B) mg-mv²/R
C) mg + mv²/R
D) zero
The answer given is D) zero

I tried to analyze the bob with respect to the frame of the car which is obviously non inertial.At the max height, we have to apply a pseudo force (centrifugal) mg upwards on the bob which balances gravitational force (mg downwards) on the bob. But i dont know what that says about the tension in the string. Is this the right approach, can someone help me out.These similar type of problems where a pseudo force balances gravity in a non inertial frame confuse me many times. Can someone give a clear solution as to why tension will be zero?
 A: Why is there tension in the first place, i.e. when the car is standing on the ground? The gravitational force of the bob of mass m is responsible for this. The bob is hanging from the ceiling without movement because the gravitational force is compensated by the tension in the string. There is a net force of 0 N.
If you now have a different force which completely compensates the gravitational force acting on the bob, the string will not be tense. It does not move either if there is an exact balance of the forces but no more tension is required to hold the bob in place. 
Now for your understanding, consider this every-day example: If you drive in your car and you pass a curve. You will feel a pull towards one side. This is because inertia wants to keep you moving in the direction you moved before whereas your car (and the seat belt holding you) is steering in a different direction. However, it feels like something is pulling you to the side. This is the so-called pseudo force.
Those forces are real but depending on the frame from which you look at them, they appear to be different. Same with the pendulum, it's called a pseudo-force but there is a very real force acting on your pendulum. I strongly suggest reading the wiki article on weightlessness ;)
A: While the bob and the car are in free fall, there is no force between them, and the string's tension is necessarily zero...
