2 Kinematics - BC Science Physics 11

Practice Problems 2.4.1 — Free Fall
2. How high is the cliff if you toss a small rock off of it with an initial speed of 5.0 m/s and the rock takes 3.1 s
to reach the water?
3. (a) At the Pacific National Exhibition in Vancouver, one of the amusement park rides drops a person in free
fall for 1.9 s. What will be the final velocity of the rider at the end of this ride?
(b) What is the height of this ride?
4. At an air show, a jet car accelerates from rest at a rate of 3g, where g is 9.81 m/s 2 . How far does the jet car
travel down the runway in a time of 4.0 s?
Projectiles
There are different types of projectile motion. From throwing a ball up and having it land
on your hand to a golf ball being hit off a tee to throwing a rock off a cliff. The golf ball
and rock examples require analysis of motion using two dimensions and will be studied in
later courses. The example of throwing a ball up in the air and having it land back in your
hand is an example of projectile motion in one dimension. This type of projectile motion
will be studied next.
Recall that acceleration is a vector quantity with both magnitude and direction.
In situations where the acceleration is caused by the force of gravity, the direction of
acceleration is downward. This is a negative direction. In gravitational acceleration
problems, therefore, you use a = − 9.81 m/s 2 .
If a body is thrown into the air, it accelerates downward (a = − 9.81 m/s 2 ) at all times
during the trajectory, whether the velocity is upward (+), zero, or downward (−).
You throw a baseball straight up. It leaves your hand with an initial velocity of
10.0 m/s. Figure 2.4.2 is a graph showing how the velocity of the ball varies with time,
starting when you begin to throw the ball and ending when you finish catching it. At A,
the ball has just left your hand. At C, the ball has just reached the glove in which you will
catch the ball. Notice that between A and C, the velocity is changing at a uniform rate. If
you take the slope of this graph, you will obtain the acceleration of the ball due to gravity
alone.
66 Chapter 2 Kinematics © Edvantage Interactive 2012 ISBN 978-0-9864778-3-6