# Physics Fanatics help me out.

1. Mar 3, 2009

### euro-ignit3

1. The problem statement, all variables and given/known data

Got a couple of physics question from my class:

1)A rock is thrown vertically upward from the surface of the earth. The rock rises to its maximum height and then falls back towards the surface of the earth. Which of the following statements is true if air resistance is neglected?
A- As the rock rises, the acceleration vector points upward
B- the acceleration of the rock is zero when the rock is at the higest point
C-The speed of the rock always points
D- The velocity and acceleration of the rock always points in the same direction.
E- The acceleration vector always points downward.

2) A skydiver with a weight of 50 N falls with a constant downward velocity after her parachute is deployed. Which Statement is not true concerning the situation?
A- The skydiver is not acceleration
B- The magnitude of the air drag force is 50 N
C- The net force on the skydiver is zero
D- The direction of the air drag force in downward
E- The direction of the weight force is downward

For question 3, can sombody give me some example?

3) Give an example in which an object, at a given instant, will have zero velocity and non-zero acceleration. Give an example in which an object can have zero acceleration and non-zero velocity. In other worlds, you are to give two different examples for this problem.

2. Relevant equations

3. The attempt at a solution
Its my first physic class and im having a hard time understanding it...It is not a homework question, its just a question i want to work on.

2. Mar 3, 2009

### bakin

You have to remember something with the direction and magnitude of acceleration: something can be going 0m/s and still have acceleration. In fact, it can have the same acceleration as something going 999999m/s.

For your problem, just because the rock is slowing down when it's thrown up doesn't necessarily mean it's acceleration is changing.

Are you able to figure out all the forces acting on the rock? (Newtons second law?)

For part B) Let's think about each statement:
A) What happens to an objects speed if there's no acceleration?
B) Try setting up a force equation with newton's second law.
C) Again, newtons second law
D) This would be a sign issue. Once you figure out all the forces, think about the direction they're going.
E) Of all the forces, which one causes the skydiver to fall?

3. Mar 3, 2009

### euro-ignit3

From reading the text book, i come to Answer #1) E and 2) D....For #3, two example would be 1- an example of zero velocity and non-zero acceleration would be kind of like the question #1 with the rock....as for the second example with zero acceleration and non-zero velocity.....?

4. Mar 3, 2009

### bakin

The first two are correct, as well as part 1 of number 3, but do you know why?

Something with zero acceleration and non zero gravity; too many to list! Can you think of way something can move with constant velocity?