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A truck travelling at 80 km/h has a pitching machine firing baseballs backwards off the back. The pitching machine is set to fire at 80 km/h.

a) how does a passenger in the truck see the ball move?

b) how does a person at the side of the road see the ball move?

attempt

a) since the ball is going -80km/h [forward] and the truck is going 80km/h [forward] would the ball appear "not moving"??

b) the person at the side of the road sees the ball going downwards, or south?

Jessica
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1 Answers1

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To the passenger the ball appears to be moving at 80 kph backwards. The ball is fired relative to his frame of reference, so the fact that the truck is moving is irrelevant.

To a person at the side of the road it looks like the ball is stationary -- it will fall straight down according to gravity. The forward motion of the truck and the backwards speed of the pitch exactly cancel each other out.

Jeff Snider
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  • but isn't the passenger IN the truck, meaning that he himself is moving 80 kph forward? therefore shouldn't the speeds cancel out making the ball appear stationary – Jessica Feb 27 '14 at 04:11
  • The speeds cancel for a person standing on the side of the road, but not in the truck. Imagine the truck is a big moving truck with the doors closed, with the passenger and the pitching machine inside. Does the speed of the truck affect what the passenger sees inside the truck? Imagine you're in an airplane going 1000kph, and the pitching machine shoots the ball backward at 80kph -- what do the passengers see? – Jeff Snider Feb 27 '14 at 04:16
  • okay good example thanks!! – Jessica Feb 27 '14 at 04:36
  • because even tho the ball is stationary in the air, to you it is still moving backwards as you are moving forwards..aahhh physics thanks again! – Jessica Feb 27 '14 at 04:38
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    No problem, I'm glad it helped. – Jeff Snider Feb 27 '14 at 04:39