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Chapter 11: Forces
11.1: Forces change motion
11.2: Force and mass determine acceleration
11.3 Forces act in pairs
11.4 Forces transfer momentum
11.2 Forces and mass determine
acceleration
 Newton’s
second law relates to force,
mass, and acceleration
Slide a milk container: use more force if full
 Force = mass x acceleration
F = m x a F = ma
To give two objects with different mass (m) the
same acceleration (a), must apply a different
force (F).

Combining Forces
 In
the same direction (think tailwind)
 In
opposite directions (headwind)
Newton’s Second Law
 The
acceleration of an object increases
with increased force and decreases with
increased mass


F = ma
Also: the direction in which an object
accelerates is the same as the direction of the
force
Calculating Acceleration
If a team pulls with a combined force of 9000 N on an airplane with a
mass of 30,000 kg, what is the acceleration of the airplane?
What do you know?
What do you want to find out?
Formula?
Substitute:
Calculate:
Check units:
Mass and Acceleration
 If
the same force acts on two objects, the
object with less mass will have the greater
acceleration
F = ma
Calculating Mass: model rocket it accelerating at 2 m/s2.
Force on it is 1 N. What is the mass of the rocket?
Forces can change the direction of
motion
 Force
can either speed up or slow down
the motion of an object, and can also
make an object change direction


Recall: if an object changes direction, it is
accelerating
Can change the direction of an object without
changin its speed
Forces can change the direction of
motion

Centripetal Force





Think spinning in a circle with a friend: arms get tired
– force used to change the direction of your motion to
keep in a circle
Any force that keeps an object moving in a circle
Points toward the center if the circle
Ball on a string stays in a circle due to the force on
the string changing direction, yet always along the
string toward the center
Centrifugal Force

Points towards the outside of a circle




The centrifugal force does not act on the body in
motion; the only force acting on the body in motion is
the centripetal force
The centrifugal force acts on the source of the
centripetal force to displace it radially from the center of
the path
Thus, in twirling a mass on a string, the centripetal
force transmitted by the string pulls in on the mass to
keep it in its circular path, while the centrifugal force
transmitted by the string pulls outward on its point of
attachment at the center of the path
The centrifugal force is often mistakenly thought to
cause a body to fly out of its circular path when it is
released; rather, it is the removal of the centripetal
force that allows the body to travel in a straight line as
required by Newton's first law
Circular Motion and Newton’s
Second Law





Skater in photo p55: must apply more force for a faster
direction change
If you know the size of the centripetal force acting upon
the object, you can find its acceleration
A greater acceleration requires a greater centripetal
force
A more massive object requires a greater centripetal
force to have the same circular speed as the less
massive object
Force and acceleration are always directed towards the
center of the circle
Newton's second law can describe the exact effects of what
happens when things like the air bag and crumple zone are put to
use in an automobile. Newton's second law can be easily expressed
by one equation alone. Acceleration - Force / Mass. This law
governs acceleration and is simple to understand. You can lower the
acceleration which is produced by your body in a collision by using
safety features like crumple zones and air bags. "The acceleration a
of a mass m by an unbalanced force F is directly proportional to the
force and inversely proportional to the mass, or a- F / m." This is the
second law of motion.
"Every action has an equal and opposite re-action." This is Newton's third
law and it also has plays a role in exactly how both airbags and crumple
zones work. Newton's third law would probably be the most important of all
of them. When you have an impact with your car, not only does your car hit
another object, but the object pushes back on your car. Since the crumple
zone on your car is made soft to absorb impacts, it will take the brunt of the
collision so that the time it takes the car to come to a complete stop is
spread out over a longer period of time. Not only may this save other
people's lives it may also save money in low speed collisions.