# Warm-Up Exercise for lecture 4

## Due 8:00 am, Thurs, Sep 11

Physics 105, Fall 2009

I throw a ball at an upward angle across a flat field. Neglecting air resistance, at what part of its path does the ball have its maximum speed?
☑ right before it hits the ground
☐ halfway to the top
☐ at the top of its path
☐ right after it leaves my hand
☐ There's not enough information to say

In 2D projectile problems, usually you use equations from one of the directions to figure out the time the projectile is in the air. (Look over the book examples, see if they do this.)
☑ True
☐ False

Neglecting air resistance, at what angle should you throw a ball on a flat field in order to get the maximum range?
☐ 30°
☑ 45°
☐ 60°
☐ It depends on the initial speed

Ralph asked me a question the other day about a ball that is thrown upwards at an angle. Ralph thought that since the ball is still moving upwards for a while after it is thrown, it must have some upwards acceleration in the air after it leaves my hand that continues to propel the ball. I told him "No, that's not quite what is happening." Can you help Ralph understand what is happening?
The upwards acceleration occurs while Ralph is pushing the ball upwards. Once it leaves his hand, though, it is in freefall and immediately starts slowing down. There is still upwards *velocity* (which is probably what Ralph means about something still propelling the ball), but the acceleration is now downwards.