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# Chapter 5 - Newton's Laws of Motion (Study this one first!)

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What does Newton's first law ask?
-what is the natural state of an object?
-if you leave an object alone, what will it do?
What does Newton's first law state about an object at rest?
-an object at rest remains at rest as long as no net force acts on it
What does Newton's first law state about an object moving with constant velocity?
-an object with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it
What two things can the term "no net force" refer to?
1. No force acts on the object; or
2. Forces act on the object, but they sum to zero
What is Newton's first law also known as?
-"law of inertia"
According to Newton's first law, what is the relationship between being at rest and moving with constant velocity?
-the two are actually equivalent
Why does an eraser at rest on a table stay at rest?
-because there is no net force acting on it
Are there any forces acting on the eraser? If so, which forces?
-yes
-force of gravity and the support force from the table
Why does an eraser in deep space stay at rest?
-because there is no net force acting on it
Are there any forces acting on the eraser? If so, which forces?
-no forces act on the eraser in deep space
Restate Newton's first law, beginning with the phrase "the natural state of an object..."
-remains so long as there is no net force acting on it
-(being in motion is just as natural a state as being at rest)
If the net force on an object is zero, what is the state of its velocity?
-the velocity of the object is constant
-(motion will be constant if no net force is imposed)
What is Newton's second law?
Net Force = ma
How is the first law contained within the second law?
-because the second law addresses what will happen when an object is "left alone" (when there is no net force on an object)
-(when there is no net force on an object, the object moves with constant velocity-->no acceleration)
If the same new force is applied to two masses, one large and one small, how will the two masses accelerate?
-the smaller mass will accelerate more quickly than the larger mass
What is the relationship between the direction of net force and acceleration?
-net force and acceleration will always move in the same direction
What is inertia?
-the resistance of any physical object to a change in its state of motion or rest
What is the relationship between mass and inertia?
-an object's inertia is directly proportional to its mass
-the heavier an object is, the more inertia it has
-hence, an object's mass measures its inertia
What then is the relationship between mass and acceleration?
-if an object has more mass, it will be more resistant to changes in acceleration
What are the units of force derived from?
Net force = ma
Thus, what are the units of force (not in Newtons)?
kg * m / s^2
If we state that a force has a quantity of one newton, what do we imply about its abilities to move mass?
-a force of one newton is able to take 1 kg and accelerate it by 1 m/s^2
If net force and mass are constant, what is acceleration?
-also constant
What do forces always come in?
-pairs
What do forces in a pair act on?
-different objects
What is the relative size of the two forces that comprise a pair?
-the forces are equal in magnitude and opposite in direction
What do these statements form the basis of?
-newton's third law
What is Newton's third law?
-for every force that acts on an object, there is a reaction force acting on a different object that is equal in magnitude and opposite in direction
Do the forces that form an action-reaction pair cancel each other out?
-no they do not
Why not?
-because the action-reaction forces always act on different objects
What type of accelerations/motions do action-reaction forces general produce?
-different accelerations/motions
Why?
-because the action-reaction forces act on different objects
-different objects will have different masses
Can action-reaction pairs exist independently?
-no, they cannot
Describe the baseball-Earth action-reaction pair: which forces are exerted on what objects?
-the Earth exerts a force on the ball (force of gravity)
-the ball exerts a force on the Earth (force of gravity)
What are the relative sizes of these two forces?
-they are the same
Why can we only see the movement caused by the force of gravity exerted on the ball by Earth?
-because of the large relative mass of the Earth
What is one of the action-reaction pairs that exists when a brick is supported by a hand?
-force on hand from brick
-force on brick from hand
What is the second?
-force on brick from Earth
-force on Earth from brick
Should both partners of an action-reaction pair be used when performing calculations with F = ma? Why or why not?
-no, both partners should not be used
-you should only focus on one half of an action reaction pair, because the only important component is the half that effects the object