 Bob sees the ball flying to the left at 100 meters per second, but Alice is also moving to the left at 90 meters per second. The speed that Bob sees the ball hit Alice's hands at will be the difference between these two, or 10 meters per second. So was Alice moving or not? See, if you asked Bob, Bob would have said I'm standing on the platform and I feel stationary, I know I'm stationary, and I can see Alice whizzing past at 90 meters per second. But if you asked Alice, Alice would have said I am stationary, I feel stationary. And it's true, if you get onto an aeroplane or a train and you shut your eyes, if the ride is smooth enough, it feels like you are stationary. See, the reason for that is because there's no such thing as absolute velocity. The only thing that you can talk about, the only thing that actually makes sense are relative velocities. I am stationary with regards to the platform, or I am stationary with regards to the train, or the ball is moving at 10 meters per second with regards to Alice. Now you may think this sounds a bit contrived, I mean surely we can all agree that the platform isn't moving and the train is, but the platform is on the earth, which is going around the sun at 30 kilometers per second. Claiming that Alice on the train is stationary is just as valid as claiming that Bob on the platform is stationary.