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Ch. 11: Forces Study Guide
Newton’s 3 laws of motion
The law that states that every object maintains a constant velocity unless acted on by an
unbalanced force is Newton’s first law of motion.
The law that states that the unbalanced force acting on an object equals the object’s mass
times its acceleration is Newton’s second law of motion.
The law that states that for every action force there is an equal and opposite reaction force is
Newton’s third law.
Section 1: Laws of Motion
The SI unit of force, named for the scientist who described the relationship between motion
and force, is called the newton.
One newton is the force that can give an object with a mass of 1kg an acceleration of 1 m/s2.
1N = 1kg X 1m/s2
The unit used to measure acceleration in free fall is m/s2.
The unbalanced force that slows down a ball rolling across the floor is the force of friction.
The tendency of an object at rest to remain at rest, or if moving, to continue moving at a
constant velocity is inertia.
An object with a small mass has less inertia than an object with a large mass.
Section 2: Gravity
Weight is best described as the downward force exerted on objects due to gravity.
When objects are moved further apart from each other, the force of gravity decreases.
Of the following, the greatest gravitational force would occur between the moon and Earth.
 a marble and a baseball 5 meters apart.
 a loaded freighter on the high seas and Earth.
 the moon and an astronaut standing on the moon.
The others are less
 the moon and Earth.
massive or further apart.
The equation for weight is w = mg.
Weight and mass are proportional but not equal. Weight is the gravitational force an object
experiences due to its mass. The weight of an object on Earth is greater than the weight of the
same object on the surface of the moon, but the object’s mass stays the same.
When air resistance balances the weight of an object that is falling, the velocity remains
constant.
When gravitational forces and air resistance equalize on an object that is falling toward Earth
and the object stops accelerating, the final velocity is called the terminal velocity.
In the absence of air resistance, for a 1.5kg book and the acceleration of a 15kg rock dropped
from the same height, the accelerations would not differ; they would be the same.
Immediately after a skydiver opens the parachute, the air resistance is greatly increased.
The motion of an object resulting from only the force of gravity is free fall.
According to the universal gravitation equation, the gravitational force between two objects
decreases as the distance between the objects increases.
Section 3: Newton’s Third Law
Action-reaction forces always occur in twos, are always equal, and always occur at the same
time.
The reaction force when you place a cup on a table is the force of the table on the cup.
Mass, direction, and speed are all factors in calculating momentum. Acceleration is not.
If you divide momentum by velocity, the result is the value of the object’s mass.
Whenever an object is standing still, the speed, velocity, and momentum are all zero.
A 10.0kg dog chasing a rabbit north at 6.0m/s has a momentum of 60.0 kg  m/s north.
Earth pulls on the moon and holds the moon in its orbit. The moon pulls on Earth with an
equal and opposite force. This is an example of Newton’s third law.
A rock by the side of the road does not have momentum, because it is not moving. If two
objects with different masses and traveling with different velocities collide, the law of
conservation of momentum allows you to predict the motion of the objects after the collision.
If you are given the mass of an object in pounds, the time in seconds, and the distance in feet,
you must convert the mass to kilograms and the distance to meters before you can calculate
the momentum in SI units.
Mass times velocity equals momentum.