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A 60 kg. woman skydives from an airplane. As she exits the plane, what is the net force applied to her?
F=ma
F= (60 kg.) (9.8 m/s)
magnitude
+
direction
force is measured in Newtons (N)
if an object is moving left,
the force is negative.
if an object is moving right,
the force is positive.
F= -588 N
F= 588 kg.m/s
A 2.5 kg. rifle fires a 0.0065 kg. bullet at 1200m/s. What is the rifle's acceleration? After 0.25 seconds, what is it's velocity?
Law I
F=ma
so
a=F/m
F= (0.0065 kg.) (1200m/s2)
a= -7.8 N/2.5 kg.
objects will remain in their current state of movement until acted upon by an unbalanced force
F= 7.8 N
a= -3.12 m/s2
NEGATIVE DOES NOT INDICATE A
NEGATIVE VALUE, JUST DIRECTION.
F= -7.8 N
Law II
a= vf-vi/t so vf=at=vi
v=at
v= (-3.12m/s) (0.25s)
v=-0.78 m/s
balanced force: the sum of all
the forces exerted on an object is 0
unbalanced force: the sum of all the forces being exerted on one object is either more or less than 0 N
- 10 N + 10 N = 0 N
force equals the mass times the acceleration of an object
-20 N + 10 N = -10 N
-10 N + 15 N = 5 N
unbalanced;
moving right
(positive)
unbalanced;
moving left
(negative)
if the object is still moving forward but forces are being applied, the forces are balanced.
In the airplane, all forces on the
person are balanced so she does not move.
If an object pushed down on a table with -10 N of force, the table will push back with 10 N of force. Even thought the signs are opposite (negative and positive) the forces are equal; remember the negative sign just means direction.
Air Resistance
Once she falls out of the airplane, she begins
to accelerate, but the air resistance is very low.
She increases in speed because the force of
gravity overpowers the small air resistance.
Law III
Air resistance, a type of force, is an excellent example of how net force can affect an object.
Eventually, the air resistance grows to equal the force of gravity. This is called terminal velocity, and she is now falling at a constant speed.
the force applied from object A to object B is always equal to the force applied by object B to object A
Finally, she deploys the parachute. This immediately increases the air resistance and she begins to slow.
If two objects are of different masses but are accelerating at the same rate, their forces will be different. If they are of different masses, and therefore exerting different forces on each other, their acceleration will not be the same either.
a= 4 m/s
a= 2 m/s
If two objects are of equal mass and are accelerating at the same rate, the force they apply to each other will be equal. If two objects are of equal mass and are exerting the same amount of force on each other, their acceleration will be equal.
a= 3 m/s
how do the II and III law affect each other?
This is because the forces aren't actually balanced. Though it looks like the two forces add up, only one force is acting on each object. The "yellow" force is the only one affecting the skater; the "red" force affects the wall. Because the forces exerted on him are not balanced, the skater moves.