Projectiles



A projectile is an object which only the force of gravity (Fg) acts on. There are many different examples of a projectile. One example is a pen that falls from a table (rest).
Pen.JPG

The pen is a projectile because once the pen leaves the table, the only force acting on it is gravity. If you throw a ball straight up in the air, it is also a projectile. Another example of a projectile is a ball being tossed to a friend standing next to you. When the ball is not in your hand or your friend's hand, it is a projectile. In all of the examples it is only a projectile if we are not considering air resistance or if air resistance is negligible.
Now that we know a projectile is an object which only gravity acts on, we know what the force diagram would look like. All projectiles have the same force diagram because all projectiles only have one force, gravity, acting on them. If any other force was acting on the object it would not be a projectile. The force diagram for a projectile has the single force, Fg, acting down on the object. The force of gravity always acts down on a projectile whether it is moving down, up, left, right, or with 2-D motion.
force_diagram_of_a_projectile.png
Many projectiles have 2-D motion, meaning that they move both vertically and horizontally. When an object has motion both vertically, in the y-axis, and horizontally, in the x-axis, we have to look at the two separately. We look at the two motions separately because the forces acting in y only affect the motion in the vertical direction,and the forces acting in x only affect the motion in the horizontal direction. We know from our force diagram that a projectile only has Fg acting in the y-axis. This means that there are no forces acting horizontally on a projectile. Horizontally, a projectile travels with a constant velocity (no acceleration) since there is not a net force acting on the x-axis. Since Fg is acting on a projectile, there is a net force in the y-axis causing a projectile to move with a constant acceleration of -9.8. This means that the velocity of a projectile in the y-axis is changing.
From all the information we know about projectiles, we can use our kinematics equations to solve projectile problems.

An Example of this: Tad drops his bowling ball out the car window 1.0 m above the ground while traveling down the road at 18 m/s. How far, horizontally, from the initial dropping point will the ball hit the ground?

X
Y
Vi
18
0
Vf
18

A
0
9.8
T
0.45
0.45
X
8.1
1
Kinematics: y=1/2 a(t(t))+vi(t) 1=1/2 9.8(t(t))+0(t) 1= 4.9(t(t))+0 t=0.45 x=v*t x=18*0.45 x=8.1
Tad's bowling ball will hit the ground 8.1 meters after he dropped it out of his window.








"The Monkey and the Hunter" is an experiment often used to illustrate the effect of gravity on projectile motion. We even talked about it in class but here's the problem again; a hunter with a blowgun goes out in the woods to hunt for monkeys and sees one hanging in a tree. The monkey, we suppose, releases its grip the instant the hunter fires his blowgun. Where should the hunter aim in order to hit the monkey?

According to Galileo's law, all objects fall with the same constant acceleration, 9.8 meters per second per second, regardless of the object's weight. And because horizontal motions and vertical motions are independent, gravity acts only upon an object's vertical velocity, not upon its velocity in the horizontal direction. The hunter's dart, therefore, falls with the same acceleration as the monkey, so he should aim directly at it.

Projectiles 2.0 by Bob and Fred





Throw 1:

12.JPG
12.JPG

Throw 2:

13.JPG
13.JPG