Physics Ch 4
dynamic equilibrium
an object that moves in a straight line at a constant speed and has an acceleration of zero
static equilibrium
an object that remains at rest and has an acceleration of zero
Frequency tells us?
how often it circulates Symbol: f Unit: "per second" or Hertz
When we split a force into its separate components what do we get?
(Sigma)Fx=max (Sigma)Fy=may
Friction forces are always pointing _______ the motion
against
max range of a projected object
45* angle
Force of gravity is equal to
9.8 m/s^2 x mass
unbalanced force
A force that is not canceled out Example: Falling rock air resistance and gravity.
Newton's 3rd law can be summarized as follows:
A physical interaction (e.g., gravity) operates between two interacting objects and generates a pair of opposite forces, one on each object. It offers you a way to test for real forces (i.e., those that belong on the force side of ∑F⃗ =ma⃗ )—there should be a 3rd law pair force operating on some other object for each real force that acts on the object whose acceleration is under consideration.
Two forces, of magnitude 4 N and 10 N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________. Check all that apply. (a) Cannot have a magnitude equal to 5 N (b) Cannot have a magnitude equal to 10 N (c) Cannot have the same direction as the force with magnitude 10 N (d) Must have a magnitude greater than 10 N
Cannot have a magnitude equal to 5 N Since force is a vector quantity you can deduce the smallest possible force using different forces from exactly opposite directions. E.g. subtracting 4 from 10. The result is 6, therefore you know 6 is the smallest net force possible and #1 must be a correct answer.
T/F: An unbalanced force on a body will always impact the object's speed
False, unbalance force impacts velocity NOT speed. No direction in speed therefore, no impact.
T/F: An unbalanced force on an object will always change the object's direction
False, you can apply an unbalanced force in the same direction, or in oppose to it but it will only slow it down. Not necessarily change the direction.
Formula for kinetic friction:
Fk = ukn
Newton's 2nd Law of Motion
Force = mass * acceleration Force and acceleration are vector quantities.
Formula for static friction:
Fs = usn
A block of mass is acted upon by two forces: (directed to the left) and (directed to the right). What can you say about the block's motion? (a) It must be moving to the left. (b) It must be moving to the right. (c) It must be at rest. (d) It could be moving to the left, moving to the right, or be instantaneously at rest.
It could be moving to the left, moving to the right, or be instantaneously at rest. If the block were already moving, the addition of the two forces wouldn't necessarily move it in either direction or cause it to stop.
A block of mass 2kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's motion?
It could be moving to the left, moving to the right, or be instantaneously at rest. The acceleration of an object tells you nothing about its velocity--the direction and speed at which it is moving. In this case, the net force on (and therefore the acceleration of) the block is to the right, but the block could be moving left, right, or in any other direction.
Centrifugal Force
It only appears for the circulating object but doesn't exist as a real force seen from a inertial system. A technical application
Formula for rolling friction:
Kr = urn
A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________. (a) Continuously changing direction (b) Moving at constant velocity (c) Moving with a constant nonzero acceleration (d) Moving with continuously increasing acceleration
Moving with a constant nonzero acceleration Remember that F = ma. If there is a force, there must be acceleration. Since the force is constant, the acceleration must also be constant, therefore #3 is the correct answer.
What centripetal force do I need to apply to spin an object around?
Must have m, f, and r. Steps: find v, find a, find the centripetal force
Normal Force
Opposing force that cancels out gravity
The acceleration that an object experiences is
Proportional to the net force, inversely proportional to the mass.
Compare the force of static friction when there is no applied force to when there is an applied force of 100 N (pushing up the ramp). How do the two forces of static friction compare?
The force of static friction when there is no applied force is greater than the case when there is an applied force. In order for the crate to be stationary, the sum of the applied force and the force of static friction must have the same magnitude as the component of gravity parallel to the ramp (so that the net force is zero). Thus, the force of friction decreases by 100 N when the applied force goes from zero to 100 N.
How does the maximum angle for which the crate can remain at rest on the ramp depend on the mass of the crate?
The maximum angle does not depend on the mass. Although the normal force and thus the maximum force of static friction increases with increasing mass, the component of the force of gravity parallel to the ramp increases at the same rate. The maximum angle is therefore independent of the mass.
Slowly adjust the applied force (pushing both up and down the ramp) until the crate begins to move. Determine the minimum strength of the pushing force needed to accelerate the crate up the ramp and the minimum strength of the pushing force needed to accelerate the crate down the ramp. How do these two minimum strengths compare to each other?
The minimum push needed to get the crate to slide up the ramp is greater than that to get the crate to slide down the ramp.
An object cannot remain at rest unless?
The net force acting on it is zero.
An object cannot remain at rest unless which of the following holds? (a) The net force acting on it is zero. (b) The net force acting on it is constant and nonzero. (c) There are no forces at all acting on it. (d) There is only one force acting on it.
The net force acting on it is zero. If all forces cancel out, the object can remain at rest.
If a block is moving to the left at a constant velocity, what can one conclude? (a) There is exactly one force applied to the block. (b) The net force applied to the block is directed to the left. (c) The net force applied to the block is zero. (d) There must be no forces at all applied to the block.
The net force applied to the block is zero. F = ma. Since the velocity is constant, acceleration must be zero, and therefore force must also be zero.
If a block is moving to the left at a constant velocity, what can one conclude?
The net force applied to the block is zero. If there is a net force acting on a body, regardless of whether the body is already moving, the body accelerates. If a body is moving with constant velocity, then it is not accelerating and the net force acting on it is zero. The net force could be zero either because there are no forces acting on the body at all or because several forces are acting on the body but they all cancel out.
Since the maximum force of static friction decreases due to the smaller coefficient of friction, a smaller component of the force of gravity along the ramp is required to make the crate ________.
accelerate
A skydiver has reached terminal velocity—she now falls at a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction?
There is no net force
T/F: If the net force on a body is zero, its velocity will not change.
True
T/F: The reason why initially moving objects tend to come to rest in our "everyday life" is because they are being acted on by unbalanced forces
True, because we have the unbalanced force of friction.
For a stationary crate (with a coefficient of friction of 0.7) on the 15∘ ramp, can the force of static friction ever be zero?
Yes, but only for a specific applied force directed up the ramp.
Formula for centripetal acceleration
a = v^2/r
balanced force
a force that is canceled out and has a net force of zero. Example: gravity and normal force or friction and push force
"action" refers to?
both the force due to an interaction and the changes in momentum that it imparts to the two interacting objects
Two forces, of magnitude 4N and 10N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________.
cannot have a magnitude equal to 5N
Swirling an object around on a string Roller coaster driving through a loop Running a curve These are examples of what kind of force?
centripetal force
Range
how far an object moves horizontally in projectile motion (xf-xi)
Period tells us?
how long it takes to complete one revolution Symbol = T Unit: seconds (s)
According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is
equal in magnitude to, and in the opposite direction from, the force on the earth due to the moon.
As the astronaut throws the tool away from the shuttle, she exerts a force in the direction away from the shuttle. Then, by Newton's 3rd law, the tool will
exert an opposite force on her. Thus, as she throws the tool, a force directed toward the shuttle will act on the astronaut. Newton's 2nd law stipulates that the astronaut would acquire an acceleration toward the shuttle
If two boxes are both pushed against the wall by the same force, the should experience the same ______.
force, the push will be cancelled out since the wall pushes back with equal force.
As the child rides in the seat,their head and back rest against the padded back of the seat. If the car is brought to a rapid stop the child will continue to move _______ at the before-crash speed until they hit something.
forward
As the ramp angle increases, the force of static friction
increases
The object hit is the back of the seat which is padded and as a result the force ________ to the maximum value over a time interval.
increases
Newton's 1st law is also known as the law of?
inertia
lower launch angle
larger horizontal speed, less flight time
High launch angle
long flight time, small horizontal speed, little range
Contact Force
macro level of normal force, electron repulsion
Newton's Law of gravity
masses attract each other with F = G(m1 x m2)/r^2 where G = 6.67 X 10^-11 Nm^2/kg^2 r = distance between the two masses
A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________.
moving with a constant nonzero acceleration Since there is a net force acting, the body does not move at a constant velocity, but it accelerates instead. However, the force acting on the body is constant. Hence, according to Newton's 2nd law of motion, the acceleration of the body is also constant.
The normal force is ____ related to the mass since the boxes are at rest, Newton's 2nd law dictates that the horizontal forces on each box must add up to ______
not, zero
terminal velocity
object now falls at a constant speed
Normal forces are always ________ to the surface
perpendicular
granted this time interval may be ______ but that is considerably better than instantaneous. Also since the head is supported there will be no _______
small, no whiplash
For the block sliding on the table in the video, the force of kinetic friction was __________ the maximum value of the static friction force; this is generally true.
smaller than
In newton's second law the acceleration force is proportional to ________
the force
Assuming that the astronaut can throw any tool with the same force, what tool should be thrown to get back to the shuttle as quickly as possible? You should consider how much mass is left behind as the object is thrown as well as the mass of the object itself.
the tool with the largest mass If she throws the tool with the largest mass, the remaining mass (the astronaut plus her remaining tools) would be smallest—and the acceleration the greatest!
With newton's second law what is the relationship between acceleration and gravity?
they are equal
Newton's third law of motion
to every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions
in centripetal force, how do we calculate velocity?
v = 2(pi)rf
The gravity force (weight) depends on the location, because g depends on it. An example of this is?
why humans can jump higher/further on the moon than on earth
With the crate stationary on a horizontal ramp, the force of static friction is
zero
