So, I learned about projectile motion in class a fe days ago, and am having trouble understanding. I main problem is seeing how velocity works in projectile motion. For example, why is the velocity in the x direction always the same?

It would also be really helpful if you can do the example below:

In 1993,Wayne Brian threw a spear a record distance of 201.24 m. Suppose Brian threw the spear at a 35.0° angle with respect to the horizontal. What was the initial speed of the spear?

please, and thank you


closed as off-topic by ACuriousMind, user36790, Gert, garyp, DilithiumMatrix Dec 30 '15 at 19:23

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  • $\begingroup$ Please do a little research before asking a basic question: en.wikipedia.org/wiki/… $\endgroup$ – Gert Dec 30 '15 at 18:11
  • $\begingroup$ You could watch a youtube video on projectiles instead of asking what it is on SE. I suppose SE is not the place to get answers for problems which you can solve on your own by putting in a little effort. $\endgroup$ – Quark Dec 30 '15 at 20:43

Velocity in the horizontal direction is constant because gravity only acts downward, not sideways. Velocity in the vertical direction is accelerated at 9.81 m/s^2. The kinematic equations are normally used for projectile motion calculations, with velocity and distance in the horizontal direction being totally independent from velocity and distance in the vertical direction.

For the spear problem, you need to use the range equation to calculate initial velocity. Your text book should have a derivation of the range equation. If not, Google it.


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