Chapter 4 Flashcards
acceleration
the rate of change in velocity; the change in velocity over change in time; the SI unit for acceleration are m/s2, meters per second squared or meters per second per second; how fast the velocity is changing; vf - vo / t. E.g. Truck goes 0-60mph in 30 seconds; 60/30= 2 miles per hour per second
average velocity
displacement / time
(speed is associated with distance; displacement is associated with velocity)
Newton’s First Law
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon. I.e. object in motion stays in motion unless a force is acted upon it.
Newton’s Second Law
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed;
Acceleration is directly proportional to…
Net external force
Acceleration is exactly inversely proportional to
Mass
Acceleration of an object depends ONLY on
Net external force and mass of the object
In equation form, Newton’s Second Law:
Net Force = ma
F = ma is used to define the force in terms of…
Three basic units for mass, length and time
SI unit of force is called the
Newton (N)
1N =
1 kg x m/s 2 (note: mass, length and time)
Mass and acceleration are…
Inversely proportional to each other
Newton’s Third Law
Forces are equal and opposite
Combining Second and Third
Because forces are equal and opposite (third law), if an object with a larger mass pushes against an object with a smaller mass, the acceleration of the object with the smaller mass has to increase to keep the equation balanced. The equation for the Second Law is Net Force = mass x acceleration. (In car collision example, the smaller car’s acceleration increases with the collision to keep the equation balanced, thereby making the collision worse for the smaller car.)
Newton’s Laws of Motion describe how…
forces cause changes in motion
A force is a…
Push or pull on an object
A force is a [blank] quantity meaning…
Vector quantity. It has both magnitude and direction
Force is measured in _________
Newtons
Net Force is
vector sum of all the forces acting on an object
If two forces act in opposite directions…
One can be considered negative and the other positive
Free body diagram
Has a central point, or square, representing the object experiencing the force(s), as well as arrows representing each force acting on the object.
If something is stationary, we can assume
Net forces add to zero
“Net force is zero” For this to be true, normal force (N or FN, so as not to confuse with Newton) must…
must act equally and opposite the gravitational force
Tension force
Objects experience a tension force when pulled
If the forces acting on a body are not parallel or perpendicular to each other, it is necessary to…
resolve the forces into their components (associated with the chosen coordinate system) before summing them. In a Cartesian coordinate system, this requires the use of trigonometric functions, i.e., sine, cosine, and tangent.
Because velocity is a vector, a change in velocity implies…
either a change in speed, or a change in direction, or a change in both. Any of these changes means that an external, unbalanced force must be acting on an object.
an “unbalanced” force simply means…
a non-zero net force. If an object moves at constant velocity, there may be many forces acting on it, but the vector sum of all of the forces must be zero. (Newton’s First Law)
definition of magnitude
“magnitude” refers to the size or quantity of a physical property without considering its direction. magnitude is a scalar quantity, meaning it is fully described by a single number and a unit of measurement, while vector quantities include both magnitude and direction.
Mass can be thought of as
the amount an object “resists” a change in motion, given the same applied force.
mass and weight, as we’ll later see, are not the same. Weight is…
a measure of the gravitational force on an object, which is proportional to mass. The weight of an object on Earth is less than its weight on, for example, the Moon, since the Moon exerts less gravitational force. But the mass of the object is the same no matter where it is—mass is an intrinsic property of an object.
Force is a ______ quantity
Force is a vector quantity. It has both magnitude and direction.
Vector sum gives the ______ of an object.
Net force
If we double the force we get ____ the acceleration.
2x the acceleration
If we double the mass, we get ____ the acceleration.
Half the acceleration
Equation for Newton’s Second Law
Fnet = ma; also, a = Fnet/m
An object’s acceleration is always in the ____ direction as the net force on it
Same
Acceleration is…
rate at which an object’s velocity changes. Acceleration changes an object’s speed, direction of motion, or both.
As Newton’s first law states, a net force is not required for motion—an object moves at a constant speed in a straight line if there is no net force on it. But a net force is required to…
change an object’s velocity.
acceleration and velocity are vectors, and are not necessarily…
in the same direction (chatgpt: When the acceleration is in the opposite direction to the velocity, it can slow the object down. For example, if the car is moving east but experiences acceleration to the west (like when braking), the acceleration vector opposes the direction of the velocity.)
A net force (and therefore, acceleration) that is at an angle with respect to the velocity (other than zero or 180 degrees) will change the velocity’s…
direction as well as magnitude.
A puck on an air hockey table moves to the left at a constant speed in a straight line. What can you conclude about the net force on the puck?
Net force is zero
A car on a highway rounds a circular bend while moving at a constant speed. What can you conclude about the net force on the car?
The net force is perpendicular to the car’s velocity, and point inward toward the center of the circle.
In physics, weight is defined as
the gravitational force acting on an object.
w =
mg
Weight depends on
the gravitational force, which, as we will later see, depends on the mass of the planet or body exerting the force. On the Moon, for example, the acceleration due to gravity is about 1⁄6th of the value on Earth, or about 1.63 m/s2. An object would weigh 1⁄6th as much on the Moon as it does on Earth, even though it has the same mass.
Newton’s third law says that forces always come in
pairs—if one object acts on a second, the second must also act on the first, with the same force, but in the opposite direction.
If the acceleration is zero, by Newton’s second law, the net force
must be zero. The only way for the net force to be zero is for the upward force to have the same magnitude as the downward gravitational force.
Note that if the elevator begins to accelerate upward, these two forces are
not the same! In this case, the force due to the elevator must be greater than the gravitational force on the person, so that there is a net force upward. There is no rule that says two forces on the same object must be equal.
If the acceleration is zero, by Newton’s second law, the net force on the person must be
zero. The only way for the net force to be zero is for the upward force to have the same magnitude as the downward gravitational force.
Newton’s third law says that forces always come in
pairs, called action-reaction pairs.
In an action-reaction pair, when one object acts on a second, the second must
also act on the first, with the same force, but in the opposite direction.
Newton’s third law can be written as
F1 on 2 = −F2 on 1
