17 March 2011
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Throwing and
catching a ball
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When you apply a certain force, whether pulling it towards you or
pushing it away from you, it produces different results.
The force you apply
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When we throw a ball up in the air, we are immediately applying a
force to it. But the ball is pushed downwards by another force, which is
gravity. After the ball goes up in the air it gradually slows down, comes
to a stop, and then falls back downwards.
Gravity
Gravity
However, if the ball is pushed up in a very straight line in an upward
direction, it will be completely cancelled by downward gravity. The ball
will fall back along the original path to the original position in your hand.
If your hand is in the original position, it will catch the ball.
If the initial force is applied in a slanted direction upwards, gravity can
only cancel the force that is directly opposite it. There is a horizontal
force that cannot be cancelled. As a result, the ball will slope horizontally
and will not fall back to the original position in your hand. It is hard to
guess the exact position that the ball will fall. You will likely miss it.
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force (n) 力
gravity (n) 地心吸力
upward (adj) 向上的
slanted (adj) 斜的
horizontal (adj) 水平的
slope (v) 傾斜