implying that its
velocity will decrease;
this means it will start
to slow down
simply stated, it will not
go higher than this
position; we call this is
Maximum Height; If it
did not accelerate, and
it had zero velocity,
would it ever fall back
down? 😊
you could say that its
velocity is becoming
more negative as it
continues to accelerate
GENERAL PHYSICS 1
M O D U L E 3
The following are the lessons contained in this module:
1. FREE-FALL MOTION
2. RELATIVE MOTION
At the end of this module, you are expected to:
Solve for unknown quantities in equations involving one-dimensional uniformly
accelerated motion, including free fall motion
Solve problems involving one-dimensional motion with constant acceleration in contexts
such as, but not limited to, the “tail-gating phenomenon”, pursuit, rocket launch, and
freefall problems
Describe motion using the concept of relative velocities in 1D and 2D
Lesson 4
FREE-FALL
“Free-fall” motion is a Uniformly Accelerated Motion that takes place in a vertical
direction. Anytime an object moves vertically, either going upwards, or going downwards, we say
it is in Free-fall. There are two key, critical concepts that we must understand when discussing
objects in Free-fall motion:
1. The constant acceleration of the object is ALWAYS directed downwards due to the
influence of Earth’s gravity; the acceleration due to gravity on Earth is referred to as “ g”,
and has a value of 9.81 m/s2. Because of sign conventions, where we choose up to be the
positive direction and down to be the negative, we will say that, in Free-fall, the vertical
acceleration of an object is -g = -9.81 m/s2.
[Recall: the negative sign indicates that the direction of the acceleration is downwards.]
2. When an object is in Free-fall, we will always ignore any effects that air resistance may
have on the object’s motion. To demonstrate, take a sheet of paper, and allow it to fall to
the floor. Due to air resistance, the paper will twist and turn, gliding back and forth, slowly
downwards to the ground. Now, drop a pen to the floor. Notice that it falls quickly, straight
down to the ground. We will assume that objects fall more like pens than pieces of paper.
[For a demonstration of a feather and steel ball being dropped in air, and then without air
resistance, view this YouTube video: http://www.youtube.com/watch?v=_XJcZ-KoL9o]
Again, understand that the acceleration will ALWAYS be
directed downwards. For instance, when a tennis ball is tossed
upwards into the air, its initial velocity is directed upwards, and
would be considered to be positive. As soon as it leaves your hand,
it will begin to accelerate downwards, at a rate of -9.81 m/s2.
Eventually, the ball will stop moving; at the instant it has
stopped moving, its velocity is zero, and it has reached its greatest
vertical position. However, it is still accelerating downwards, at a
rate of -9.81 m/s2!
So, it begins to fall back down; this implies its velocity will
now be negative, because down is considered the negative
direction. Because the acceleration is also negative, this tells us
that the velocity is decreasing; perhaps confusingly, we will find in
this case that the tennis ball is going to speed up... although the
velocity is still negative.
KEEP IN MIND that negative sign does not mean a slowing motion. It means that the direction
of the motion in on downwards (South) or Westward direction based on the Cartesian plane.
Notation: In order to reinforce that Free-fall is a vertical motion (as opposed to the Horizontal
motions we’ve discussed so far), we make a few small notational changes:
ox for position will be replaced by y (because the vertical axis is the y-axis)
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