A ball is hanging from a long string that is tied to the ceiling of a train car traveling eastward on horizontal tracks. An observer inside the train car sees the ball hang motionless. Draw a clearly labeled free-body diagram for the ball if (b) the train is speeding up uniformly. Is the net force on the ball zero in either case? Explain.

Video transcript

Hey everyone today, we're being told that we have a rescue operation going on and a soldier is hanging from a long rope to a helicopter and this helicopter is traveling eastward horizontally to lift people so eastward horizontally to lift people and rescue them from a recent flood affected area. Now, if the helicopter is speeding up uniformly, then we're being asked to draw a clearly labeled free body diagram for the soldier. So following the scheme given in the answer choices here, let's draw out our free body diagram with our soldier being represented by a red dot. Now the soldier has mass and this means that the weight of the soldier, which is nothing but mass times gravity. Oops! Because weight is equal to mass times the force of acceleration due to gravity. So we would have a weight force pointing directly down to the center of the earth, right? Because that is the origin of the gravity. So we can already eliminate answer choice D simply due to the fact that the weight is in the wrong direction. No, we have a helicopter with a constant velocity, which means that the or if I should say if the helicopter had a horizontal velocity and let's just put a helicopter right here, whoa, It's a very poor helicopter and then we have a little rotor right there. If we have a helicopter and if it is moving with a constant velocity, the attention of the rope would be vertical upwards because there'd be no acceleration in that case. However, the helicopter is speeding up uniformly and it's traveling horizontally eastward, which means it's heading east and it's speeding up. So A. Is changing which means they're very velocity is variable, it is not constant, which means the force due to tension therefore would point up like this. It wouldn't be answer Choice C because the force of direction is in the opposite way where the tension is in the opposite direction and it wouldn't be A because that represents the tension. If velocity was constant, this leaves us with answer choice B. I hope this helps and look forward to seeing you all in the next one.