Physics Help Forum horizontal motion

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 Jan 29th 2011, 11:42 AM #1 Junior Member   Join Date: Jan 2011 Posts: 9 horizontal motion Why is horizontal motion accelerated in y direction? When the object is moving up(half of trajectory), the gravity is showing down not up? What force cause that acceleration?
Jan 29th 2011, 11:51 AM   #2

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 Originally Posted by kapital Why is horizontal motion accelerated in y direction? When the object is moving up(half of trajectory), the gravity is showing down not up? What force cause that acceleration?
If I understand where your question comes from we are talking about 2-D (projectile) motion on the Earth, yes?

There are two things you need to keep in mind
1) Velocity and acceleration are vectors and you are working with the components of them.

2) Motion in the horizontal and vertical directions are independent.

So if we have a particle flying off the edge of a table (for example) the initial velocity will be horizontal, but as soon as it leaves the table there is now a gravitational force acting in the vertical direction (downward.) This acceleration is constant.

For the particle that has been thrown upward, the acceleration due to gravity is always downward...it adds a "downward component" to the velocity, which first slows the particle down until it reaches its highest point, then increases the speed of the particle downward. As it rises the velocity of the particle is upward and that gravitational acceleration is opposite to this. As it falls the velocity is downward and the gravitational acceleration is also downward.

-Dan
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 Jan 29th 2011, 12:16 PM #3 Junior Member   Join Date: Jan 2011 Posts: 9 Hm. When you trow a ball like this: At first half of time the acceleration in y direction is 10 m/ss. Why?
Jan 29th 2011, 01:31 PM   #4

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 Originally Posted by kapital Hm. When you trow a ball like this: At first half of time the acceleration in y direction is 10 m/ss. Why?
Not for just the first half of the motion, but all of it. Ignoring air drag if you release, or throw, an object it has an acceleration of g downward at all times it is in the air. That's due to the fact that any mass is attracted to the Earth. Near the Earth's surface this acceleration is a constant 9.8 m/s^2 (or 10 m/s^2 depending on how nice your professor is in giving you Math problems.)

-Dan
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 Jan 29th 2011, 02:25 PM #5 Junior Member   Join Date: Jan 2011 Posts: 9 yes, but why is than object accelerating up?
Jan 29th 2011, 04:31 PM   #6

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 Originally Posted by kapital yes, but why is than object accelerating up?
It is not accelerating up. It has an initial component of velocity that is upward, so the object travels up, but gravity acts on this upward velocity, making it smaller.

Look at the diagram. In red we have the vertical component of the initial velocity. As time goes on the acceleration of the object causes a larger value of gt (in blue), which is caused by gravity. But the final vertical velocity (in purple) is still upward, so the object continues to rise.

At some point gt is the same size as v0y. At that point the vertical component of the velocity is zero. At this time the object has reached its maximum height.

After this point, by looking at the vector addition the vector gt is going to be longer than v0y. This makes the final velocity (v) point downward. Thus the object starts to fall back to the ground.

-Dan
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 Jan 29th 2011, 11:24 PM #7 Junior Member   Join Date: Jan 2011 Posts: 9 OK.I understand now. Thank you.

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