Workshop 2 Web Highlights
"Balanced Forces and Net Force"
If an object is not moving, we can be sure that the forces acting
on it are balanced. There is no net force. As we will see in a later
workshop, it is also true that there are no net forces acting on
an object that moves at a constant speed in a straight line. No
net force means no acceleration, and in both of these cases, the
acceleration is zero. So, while we have seen that an object "at
rest" has no force acting on it, it is also possible that a moving
object has no net force acting on it either.
"Unbalanced and Balanced Forces"
If an object is speeding up, slowing down, or changing direction,
a net force is needed to cause that change in motion. We use the
term "net force" since there could be many forces acting on an object
and we will only see the change if the forces are unbalanced or
do not add up to zero. The leftover force is the net force that
causes the object to accelerate.
In the simplest cases where only two forces act on an object, like
a ball resting on your hand or a weight hanging from a spring scale,
the two forces pushing upward and downward are equal and opposite.
They add up to zero.
Friction is a force that opposes motion. It occurs when the surfaces
of objects roll on or slide past each other. In the cars used in
Barbara's class, the wheels fit in axles that in turn fit in holes
on blocks mounted on either side of the car. The students adjusted
the axles, wheel, and frame of their cars to minimize the amount
of friction. If the wheels rub and do not turn freely, the car will
be harder to move and will not roll as far. If the frame is bent
rather than square, the front and rear axles will not be parallel,
causing the wheels to slide rather than roll. We will be looking
more closely at friction in later workshops.