If I were to throw a rock of a cliff at 12 m/s as a horizontal velocity, what will the final velocity of the rock become when it hits the ground? I am tempted to say 12 m/s because the horizontal portion is independent of vertical but it doesn't make sense that way.

For the horizontal portion, there is no acceleration, initial velocity is 12 m/s and final, I have no idea. For the vertical portion, acceleration is -9.81 m/s/s (gravity), velocity is 0 m/s?

Please help me, can someone explain this concept?


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  • $\begingroup$ It's going to depend on the height of the cliff. You are right though, the horizontal component of the velocity will remain 12 m/s - assuming no effect from air resistance. $\endgroup$ – M. Enns Jul 25 '16 at 1:12
  • $\begingroup$ @M.Enns Can you explain why the horizontal velocity won't change? Gravity makes it go down faster, so doesn't that mean that it will slow down horizontally? $\endgroup$ – Imagine Dragons Jul 25 '16 at 1:22
  • $\begingroup$ There's no force in the horizontal direction. $\endgroup$ – M. Enns Jul 25 '16 at 1:23
  • $\begingroup$ @M.Enns If the cliff was 20 kilometres deep and I threw it at 12 m/s, it would continue at the same rate until it reaches the bottom? $\endgroup$ – Imagine Dragons Jul 25 '16 at 1:31
  • $\begingroup$ That's right. Why wouldn't it? $\endgroup$ – M. Enns Jul 25 '16 at 2:10

For the horizontal portion, there is no acceleration, initial velocity is 12 m/s and final, I have no idea.

See Newton's first law

an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Your rock moving at 12 m/s horizontally has momentum in that direction. That momentum is conserved.

These are observations about how matter behaves. In classical physics they are so well supported by evidence that these statements are described as laws.

Unless your cliff is on a planet with no atmosphere, the rock will be acted on by an unbalanced force - namely friction with the gases comprising the atmosphere. These frictional forces, called air-resistance on Earth, will act to slow down the rock.


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