Physics final conceptual questions
What two conditions must hold to reach mechanical equilibrium?
1) net force must equal zero 2) torque on object must equal zero
A projectile is fired with initial speed V0 at an angle of 45 degrees about the horizontal. Assume no air resistance. True or false: during the flight, the x-component of the projectile's momentum remains constant.
true
At what angle should force be applied to a lever arm to achieve minimum torque?
0 degrees
At what angle should force be applies to a lever arm to achieve maximum torque?
90 degrees
A jello spinning around on a plate with an angular speed (w) can be modeled as a rigid body?
Depends on w; depends on jello
What is the equation of Hooke's Law?
F= -kx
Normal reaction is the force that opposes the force of gravity and acts in the direction of the force of gravity
False
rue or False: A woman in a store buys a 100-W lightbulb. This means that the bulb will convert 100-W of electrical power to visible light.
False
Which form of Newton's second law would you use to solve a problem in which mass varies?
Fnet = deltaP/delta t
Consider a rocket fuel cell used to launch a shuttle into space. What is the initial form of energy in the cell? What forms of energy is it transformed into at the point of liftoff?
Initial- Chemical energy Conversion- Thermal, radiant, sound, and mechanical energies
Mechanical energy is defined to be
KE+PE
If you wanted to maximize the velocity of ball 2 after impact, how would you change the settings for the masses of the balls, the initial speed of ball 1, and the elasticity setting? Why?
Maximize the mass of ball 1 and initial speed of ball 1; minimize the mass of ball 2; and set elasticity to 100%
A friend slides a box along a flat floor to you, which you slow by applying a force at a 30° angle. Is the work you do to slow the box positive or negative? If you instead placed a book directly on top of the box as it passed you, what can you say about the work done by the book?
Negative work; No work done
In which of these scenarios is useful mechanical work being done? 1) Dave is climbing a ladder 2) Dave's stomach begins to digest his lunch 3) Dave washes his car
Scenarios 1 and 3
Which of the following statements best descries a non-rotating object in static equilibrium? 1) it has zero velocity and zero acceleration 2) it has nonzero velocity and zero acceleration 3) it has zero velocity and nonzero acceleration 4) it has nonzero velocity and nonzero acceleration
Statement 1
Which of the following systems is in stable equilibrium? 1) a skateboard at rest at the bottom of a U-shaped ramp 2) a ball at rest on the edge of a table 3) a quarter, at rest, balanced on its edge
System 1
law of conservation of energy
total energy is constant in any process. It may change in form or be transferred from one system to another, but the total remains the same
What is the statement for Newtons second law in terms of momentum?
The net external force equals the change in momentum of a system divided by the time during which it changes
True or False: Newton's Second Law can be interreted based on Newton's First Law.
True
While carrying a heavy box, a boy walks horizontally across a room at a constant speed. True of False: the boy is doing no work on the box.
True
A box weighing 500N is at rest on the floor. A person pushes against it and it starts moving when 100N force is applied to it. What can be said about the coefficient of kinetic friction between the box and the floor?
Uk < 0.2
Mechanical equilibrium
an object is motionless- two conditions must hold
Rigid body
an object or a system of particles in which the distances between particles are fixed (remain constant)
What direction does torque have and does it produce angular acceleration?
direction is along the axis of rotation; produces an angular acceleration
Through what means is kinetic energy lost in collisions between everyday objects?
due to friction experienced by the objects
Select all of the following real life situations that approximate an elastic collision: 1)car runs into a brick wall 2)cue ball strikes another billiard ball 3)two oxygen molecules bump into each other, form O2, and travel in the direction the faster moving molecule was headed
example 2 only
A passenger on a moving train tosses a ball straight up into the air and catches it. True or False: In relation to a person standing still on a platform outside the train, the ball only has vertical velocity. a
false
How would the final velocity of the car-plus-truck system change if the truck had some initial velocity moving in the same direction as the car? What if the truck were moving in the opposite direction of the car initially? Why?
if the truck was initially moving in the same direction as the car, the final velocity would be greater. If the truck was initially moving in the opposite direction of the car, the final velocity would be smaller.
A force is applied 1m away from the pivot point on a lever arm and it produces torque. How much force would have to be applied to produce the same amount of torque if it were applied 4m from the pivot point? Assume that both forces are applied perpendicularly to the lever arm.
one fourth of the initial force
tourque (t)
product of force and the lever arm
Suppose a particle of known mass and velocity collides with a second particle of known mass that is at rest. You try to solve for the final velocities of both particles in the plane of collision, but when you apply the equations of conservation of momentum along the x-axis and y-axis, you realize that there are too many unknowns. How many pieces of information do you need to solve for each particle's final velocity?
two
Total momentum equals
vector sum of each object
lever arm
the perpendicular distance from the line of force to the axis of rotation
Define work for one-dimensional motion
the product of the force and the distance
When only conservative forces act on/withing a system:
the total mechanical energy is constant
An object that has a small mass and an object that has a large mass have the same momentum. Which object has the largest speed?
the small mass
Describe the changes in KE and PE of a person jumping up and down on a trampoline.
While going up, the person's KE would change to PE. While coming down, the person's PE would change to KE.
A ball is at rest at the highest point on a hill. Is the ball in equilibrium, and if so, what kind of equilibrium?
Yes, the ball is in unstable equilibrium
At the beginning of a roller coaster ride, the roller coaster car has an initial energy mostly in the form of PE. Which statement explains why the fastest speeds of the car will be at the lowest points in the ride?
because at the bottom of the slope kinetic energy is at its maximum value and potential energy is at its minimum value
Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on?
no
The starting line of a cross country foot race is at the bottom of a hill. Which form(s) of mechanical energy of the runners will change when the starting gun is fired?
both kinetic and potential energy
How does a car airbag reduce the impact of the person driving?
by increasing the time over which the momentum of the driver changes
Inelastic collision
collision in which the internal kinetic energy changes (conserved)
Perfectly inelastic
collision in which the objects stick together; reduces internal kinetic energy more than does any other type of inelastic collision
What does it mean when we say that force is a vector quantity?
force has both magnitude and direction
Two objects having equal masses and speeds, but with velocities oppositely oriented, collide head-on with each other and come to a rest. What type of a collision is this and why?
inelastic collision because internal kinetic energy is not conserved
If a linear force is applied to a lever arm farther away from the pivot point, what happens to the resultant torque?
it increases
If a linear force applied to a lever arm increases, what happens to the torque?
it increases, because torque is directly proportional to the applied linear force
Tennis racquets have "sweet spots". If the ball hits a sweet spot then the player's arm is not jarred as much as it would be otherwise. How does the "sweet spot" change the impact time of the ball?
it inreases impact time
What is SI of momentum?
kg x m/s
Two objects collide with each other and come to a rest. How can you use the equation of conservation of momentum to describe this situation?
m1v1 + m2v2 = 0
State the law of conservation of momentum.
momentum is conserved for an isolated system with any number of objects in it
What two conditions must be met to achieve equilibrium?
motion without linear or rotational acceleration
According to the impulse-momentum theorem, what does impulse depend on?
on the magnitude of force as well as the duration over which it acts
Nonconservative force
one for which work depends on the path taken
Conservative force
one for which work depends on the starting and ending points of a motion, not the path taken
What is an elastic collision?
one in which the objects after impact dont lose any of their internal kinetic energy
When a car is coasting downhill, how are its potential and kinetic energies changing?
potential energy decreasing, kinetic energy increasing
Linear momentum
product of mass and velocity
A mass is hanging from the end of a horizontal bar which pivots about an axis through its center. Initially the bar is held stationary. The bar is released and begins to rotate. As the bar rotates down from horizontal to vertical, the magnitude of the torque on the bar.
remains constant
Torque
takes a force to start an object rotating; more effective the further from axis of rotation (closer to being perpendicular)
What is the direction of acceleration of an object in uniform circular motion?
the acceleration is directed toward the center of rotation
Which of the following statements explains why a racecar going around a curve is accelerating, even if the speed is constant?
the car is accelerating because the direction of velocity is changing
How can object rotating counterclockwise have a positive angular acceleration?
the counterclockwise rotation of an object produces positive angular velocity. Therefore, when the object is speeding up and rotating counterclockwise, the angular acceleration is positive
What does conservation of kinetic energy and momentum together allow?
the final velocities to be calculated in terms of initial velocities and masses in one dimensional two-body collisions
How can you maximize the torque applied to a given lever arm without applying more force?
the force should be applied perpendicularly to the lever arm as far as possible from the pivot point
What is the first condition necessary to achieve equilibrium?
the net external force on the system must be zero
Three pendulums all have the same length and start from the same height. The first pendulum is very light and has a mass of 67 g. The second pendulum has a mass of 1.5 kg. The final pendulum has a mass of 3.3 kg. Assume that the effects of air drag are negligible. Which pendulum attains the highest velocity?
they will all attain the same maximum velocity
True of false: if no external force acts on a system, then the sum of all momentum vectors for the system must not change.
true
True or False: A car runs out of gas and coasts down one hill and up another until it comes to a stop. The point at which the car stops is lower than the point at which it started coasting because part of the original potential energy has been converted to a quantity of heat, which makes the tires feel warm.
true
True or False: A rotating body can be in one of the types of equilibrium.
true
True or False: The energy increase of an object acted on only by a gravitational force is equal to the product of the object's weight and the distance the object falls.
true
true or false: the total energy of a system is always conserved whenever its momentum is conserved
true
True or False: To calculate the speed of the pebble dropped from the cliff as it hits the ground requires you to only know the height of the cliff and acceleration due to gravity. a
true (v=(2gh)^1/2)
Identify two properties that are expressed in units of joules
work and energy
A basketball is dropped and each time it bounces, the maximum height it reaches becomes smaller and smaller. Is this an example of a system in which nonconservative forces are acting?
yes
Carlos is deciding if he will climb a mountain in a zigzag path or straight up the side, starting from rest and ending at rest. Neglecting friction and disregarding other factors, will his potential and kinetic energy consumption be the same for both paths?
yes
Imagine a roller coaster, with an initial speed of 5m/s downhill, descends 20m in height. Now suppose the roller coaster, with an initial speed of 5m/s uphill, coasts uphill, stops, and then rolls back down to a final point 20m below the starting point. Would the final speed be the same in these two cases? Assume friction is negligible.
yes
Is momentum a vector?
yes
Is torque a vector?
yes
Two objects having equal masses and velocities collide with each other and come to a rest. Is momentum conserved in this case?
yes, net force on the system is zero
