Download what happens when an object changes direction

WHAT HAPPENS WHEN AN OBJECT CHANGES
DIRECTION?
We must address the question of whether a change in direction is a change in motion. We
have just learned that a change in direction in one dimension, i.e a reversal, is associated with
changing motion. Many of our experiences with motion involve a change in direction away
from a straight line. If a change in direction is a change in motion, i.e. non-uniform, then a
force (or unbalanced or net force) is required because we know that net or unbalanced forces
cause non-uniform motion. (If only one force is acting on an object, the forces must be
unbalanced and there is a net force because there is no other force to subtract with or balance
the one force.)
Gather the following equipment

basketball or ball used on playgrounds we have the soccer balls which will be great

small object like rubber cork with hole that can be whirled around the head on a string
and released in a safe manner These need to be made, see below
a)Let’s turn the question around and ask if we apply a force can we cause a change in
direction. Do an experiment with a ball such as a basketball or ball that is tightly
blown up used on playgrounds. Roll the ball down a hall, an open space in the
classroom or outside on a smooth, flat area. If you let it roll, what kind of motion is it –
straight line, circular, erratic? How does the ball change its motion if you hit it on the
side with your hand or a club? Draw a conclusion about whether a force must be
applied to the object while it moves to get it to change direction.
b)Using such a ball is also a good approach for experimenting with looking a several
different kinds of changes in motion being caused by a force. To change the motion of
the ball from resting to moving do you have to apply a force? If you apply a force to the
rolling ball can you make it change its motion from moving to resting? If the ball is
rolling by and you give it a push or kick in the direction it is moving does it change its
motion from a certain velocity to a greater velocity? Similarly, what about for slowing
down? For you and your students, these are all good examples of a force (of course a
single and hence unbalanced force) causing a change in motion. Your students and most
people in the world believe that force causes motion which is not accurate; we have
learned that force (of course, unbalanced if only one force) causes a changing motion,
i.e. non-uniform motion. Non-uniform motion can lead to motion (as an object starts
moving, there must be a change in motion), but whenever an unbalanced force is applied
to an object there is a changing motion.
c)If an object is going in a circle and moving with constant speed, it might seem that it is
uniform motion since the speed is constant. However, we just learned that circular
motion is non-uniform motion because the direction is changing and we need a net force
to make it happen. Uniform motion is motion in a straight line with constant speed.
Discuss with your group mates the fact that the moon goes in a nearly circular orbit
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around the earth and the earth goes in a nearly circular orbit around the sun. Think
about and describe the source of the forces that cause them to move in circles. What
would happen if this force were turned off? Also think about an experiment where an
object on the end of a string is being whirled about your head with constant speed. Do
you need to keep pulling on the string to make it go in a circle? What happens if you let
go of the string? Does it continue to move in a circle or does it go off in a straight line
that is tangent to the circle, i.e. a continuation in a straight line of the direction it had
when the string was released the net force in the horizontal direction made zero?
d)Do an experiment with an object like a rubber cork with a hole, tied to the end of a
string and whirled around your head. Get the object spinning around your head in a
manner that children often do. What kind of motion is it – straight line, circular, erratic?
Is your hand applying an inward force on the string and the string applying an inward
force on the moving object? What happens if you let go of the string; what kind of
motion does the object have now? Draw a conclusion about whether a force must be
applied to an object to make it move in a circle. Is a force necessary for the object to
change its direction as it must do if it is moving in a circle? The center of the moon is
attracted to the center of the earth by a gravitational force. What kind of motion does
the moon have?
e)Think about games and amusement park rides that involve circular motion or other
motion involving a change in direction. List a few and describe any forces that you
know about that are applied to the object (sometimes you) to get it to change direction.
CHECK #4: Write a summary of what you learned in this section about objects that are
changing direction and forces. Observe Martin Brock’s video from the movie “The Road
to Bali”: how is the situation shown in this video not consistent with the physics of force
and motion?
Whole class discussion of ∆v for changing direction
What is the ∆v associated with changing direction? We know that we must have a ∆v,
and vf = vi + ∆v. When we change direction, the direction of the velocity changes and we
velocity representations that look like this:
vi
vf
∆v
a = acceleration = ∆v/∆t, the rate of change of velocity. Note that
acceleration and ∆v are in the same direction. Whenever you want to
know the direction of the acceleration, determine the ∆v and you have
it.
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So we can determine the direction of acceleration for changing direction.
Net force = (constant) acceleration or net force = (Constant)a.
Since we have acceleration there must be a net force. If the net force is removed, the
object then experiences uniform motion, a = 0, ∆v = 0, straight line motion with constant
speed.
For object going in circle with constant speed, one can show that the ∆v and a are always
directed toward the center. Hence, the net force must be directed toward the center to
cause the ∆v and a that are observed. This force can be the pull of the string on the cork,
and the pull of gravity on the moon.
vi
vf
∆v
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