# Projectile Motion of a tennis ball homework

1. Jul 29, 2008

### mizulyn

1). Is the acceleration of a projectile equal to zero when it reaches the top of its trajectory?

I know that when a projectile reaches the top of its trajectory, v=0 because velocity is momentarily horizontal. However, how would I relate this to acceleration?

2). A tennis ball is hit into the air and moves along an arc. Ignoring air resistance, where along the arc is the speed of the ball at its minimum and maximum? Why?

Would it be correct to say that speed is at a minimum at the top of the arc and at a maximum as the arc goes downward?

3). A wrench is dropped from the top of the mast on a sailboat. Will the wrench hit at the same place on the deck, whether the sailboat is at rest or moving with a constant velocity?

I'm completely stuck on this one. ^^;; Any pointers in the right direction would be greatly appreciated.

Thanks in advance for the help!

2. Jul 29, 2008

### bjd40@hotmail.com

1. the accln or deccln of the projectile is always g, as long as it is moving in a constant field of gravity.
the velocity is not always be zero at the top. it is zero only when the mass is thrown vertically upward.
the velocity reduces as the initial K.E. of the body reduces and converts itself into P.E. at the top P.E. is max and velocity is min.

2. yes u r right. just go through the ans of q no. 1 (the energy related part).

3. yes it will. whenever a particle is moving in a force field(in this case gravity field) and it has a velocity not aligned with the direction of force, always cosider the components of velocity one along the direction of force and the other to adjust the total velocity to be the actual one. the velocity along the direction of force or exactly opposite to it will be affected with accln or deccln, the other component being parallel to the force will remain unaffected.
in the present case the wrench has a velocoty downward (acclertd) and a velocity along the motion of boat(treating boat as moving). the second velocity is unaffected or unchanged. has the same value with the velocity of the boat. so it will hit the same place as long as it has the same value with the velocity of the boat. the result remain unaltered if it has a value zero(i.e. the boat it at rest). because in this case both the wrench and the boat has the same horizontal velicity, which is, however zero.