Download CHAPTER THREE NOTES - NEWTON`S SECOND LAW OF

CHAPTER THREE NOTES - NEWTON'S SECOND LAW OF MOTION
CONCEPTUAL PHYSICAL SCIENCE (EXPLORATIONS HEWITT)
Force and acceleration can be represented by vectors which are lines with arrows. The longer a
vector, the greater the force or rate of acceleration. Arrow indicates direction.
Note: Students need to draw and interpret velocity-time graphs, acceleration - time graphs and
force vectors.
VECTOR QUANTITY -can be represented by a vector (line with arrow at end) and shows both
magnitude and direction. Examples of vector quantities are velocity and force.
SCALAR QUANTITY - shows the magnitude only (no direction) so cannot be drawn and isn't
as valuable as information as a vector quantity. Examples of scalar quantities are speed, mass,
volume.
ACCELERATION
ACCELERATION - Change in velocity divided by the time interval in which the change took
place. Any change in velocity is called an acceleration.
CHANGE IN ACCELERATION HAPPENS- when the speed changes (speeding up or
slowing down) or the direction changes.
The formula for average acceleration is change in velocity divided by the change in time.
CONSTANT ACCELERATION happens when an object changes velocity at a predictable, even
rate. This would happen with a ball rolling down an incline track or a falling object in free fall.
Two formulas can be used for constant acceleration.
v=at for finding the speed of a constantly accelerating object and
d=1/2at2 for finding the distance traveled by a constantly accelerating object
Free fall - objects falling under the influence of gravity alone.
Free fall acceleration near earth is 9.8 m/s2
A constantly accelerating object can be graphed as a horizontal line on an acceleration-time
graph.
When air resistance is acting on a falling object, the object will not maintain a constant
acceleration. As speed increases, air resistance increases. Eventually the force of gravity pulling
the object toward earth will be equal to the air resistance and terminal velocity will be reached.
Terminal velocity is when the falling object stops accelerating and travels at a constant speed
due to the balanced forces of gravity and air resistance.
Weight - the force of gravity acting on an object. Weight is equal to mass times acceleration due
to gravity. w=mg where weight is in Newtons, mass is in kilograms and g is in m/s/s.
Mass - the amount of matter in an object.
Inertia - the tendency of an object to resist a change in motion. The more mass an object has, the
more inertia it has. They are directly proportional.
Volume - the amount of space an object occupies.
Friction - a force that opposes motion caused when two or more surfaces move over each other.
The amount of friction depends upon the type of surface and how hard the surfaces are pressing
on each other.
Static friction - the frictional force exerted when an object is not moving.
Kinetic friction - the frictional force exerted while an object is moving.
Static friction is generally greater than kinetic friction.
Friction opposes motion so it can cause problems or create more work. Reducing friction means
you can reduce the force necessary to move an object. Wheels (rolling friction) are a very
effective means of reducing friction. Fluid friction is also useful. Smoothing the surfaces can also
reduce friction. Although friction can be a problem, it is essential for motion. You couldn't even
walk if there were no friction. Tires have tread to increase friction so the car stays on the road
even when it rains (unless the car is going too fast).
Newton's Second Law F = ma Acceleration of an object is directly proportional to the force and
inversely proportional to the mass.
Object in free fall
Object falling with
some air resistance
Object at terminal velocity