CheckPoint Questions Flip it Physics 213

A wheel which is initially at rest starts to turn with a constant angular acceleration. After 4 seconds it has made 4 complete revolutions. 1)How many revolutions has it made after 8 seconds?

16 w0 is 0 and we know theta is 4 and theta i is 0 so we can use the the first constant angular acceleration equations to determine acceleration then plug the new time back in to find the new # of revolutions

A girl stands on a moving sidewalk (conveyor belt) that is moving to the right at a speed of 2 m/s relative to the ground.A dog runs on the belt toward the girl at a speed of 8 m/s relative to the belt. 1)What is the speed of the dog relative to the ground?

6 m/s The conveyor belt is moving the opposite direction so when you add them up the dog is only going 6m/s

In case 1, one end of a horizontal plank of mass M and length L is attached to a wall by a hinge and the other end is held up by a wire attached to the wall. In case 2 the plank is half the length but has the same mass as in case 1, and the wire makes the same angle with the plank. 1)In which case is the tension in the wire biggest?

Same Tension depends on weight and angle and those are the same

In case 1, one end of a horizontal massless rod of length L is attached to a vertical wall by a hinge, and the other end holds a ball of mass M. In case 2 the massless rod is twice as long and makes an angle of 30o with the wall as shown. 1)In which case is the total torque about an axis through the hinge biggest?

Same case 1:moment about hinge M1 = MgL case 2:length is doubled and inclined to wall at 30 degree angle moment about hinge M2 = Mgsin(30)2L = Mg2L/2 = MgL

Two guys who weight the same are holding onto a massless pole while standing on horizontal frictionless ice. 1)If the guy on the left starts to pull on the pole, where do they meet?

At zero There is no friction so he should be moving in too (I think)

A ball of mass 3M at x=0 is connected to a ball of mass M at x=L by a massless rod. Consider the three rotation axes A, B and C as shown, all parallel to the y axis. 1)For which rotation axis is the moment of inertia of the object smallest? (It may help you to figure out where the center of mass of the object is.)

B I found the center of mass and if it is at the center of mass (b) then the inertia is the smallest

A block and a ball have the same mass and move with the same initial velocity across a floor and then encounter identical ramps. The block slides without friction and the ball rolls without slipping. 1)Which one makes it furthest up the ramp?

Ball It has rotational energy too

A physics demo launches a ball horizontally while dropping a second ball vertically at exactly the same time.

Both hit at the same time They both have the same vertical distance to the ground and are heading to the ground at the same time at the same initial vertical velocity and experiencing the same acceleration.

A destroyer simultaneously fires two shells with different initial speeds at two different enemy ships. The shells follow the parabolic trajectories shown. Which ship is hit first?

Both ships are hit at about the same time. If the speeds are correctly accounted for, it should make up for the angle they are shot at and it should strike both ships at the same time.

Two balls of equal mass are thrown horizontally with the same initial velocity. They hit identical stationary boxes resting on a frictionless horizontal surface. The ball hitting box 1 bounces back, while the ball hitting box 2 gets stuck. 1)Which box ends up moving faster?

Box 1 It doesn't have the added mass of the ball

A girl twirls a rock on the end of a string in a horizontal circle above her head.The diagram illustrates how this looks from above. 1)If the string breaks at the instant shown, which arrow best represents the path the rock will follow?

C It should continue in a tangent motion

Suppose you hang one end of a beam from the ceiling by a rope and the bottom of the beam rests on a frictionless sheet of ice. The center of mass of the beam is marked with a black spot. 1)Which of the above configurations best represents the equilibrium condition of this setup?

C The rope is completely straight

In the two cases shown above identical ladders are leaning against frictionless walls and are not sliding. 1)In which case is the force of friction between the ladder and the ground the biggest?

Case 1 the farther out it moves the greater the force of friction is needed to keep it in place

In both cases the beam has the same mass and length and is attached to the wall by a hinge. 1)In which of the static cases shown is the tension in the supporting wire bigger?

Case 2 have to solve for balancing each out also holding onto something in the center vs at the edge requires more strength

A cylinder and a hoop have the same mass and radius. They are released at the same time and roll down a ramp without slipping. 1)Which one reaches the bottom first?

Cylinder It has the larger moment of inertia

A triangular shape is made from identical balls and identical rigid, massless rods as shown. The moment of inertia about the a, b, and c axes is Ia, Ib, and Ic respectively. 1)Which of the following orderings is correct?

Ia>Ic>Ib a is far away from two of the masses and c is far away from one mass and b is in the middle so it goes a c then b

Which plot best represents the acceleration curve associated with the displacement and velocity curves shown above?

It is the graph where the straight line goes through the (0,0) point at a steady slope

At t=0 a ball, initially at rest, starts to roll down a ramp with constant acceleration. You notice it moves 1 foot between t=0 seconds and t = 1 second.

It moves three feet (solve for acceleration) then subtract off what it has already moved

A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from sliding as the truck accelerates to the left as shown. 1)Which of the following diagrams best describes the static frictional force acting on the box?

It should be to the left as static friction will move in the same direction as acceleration

A block slides to the right with speed V on a frictionless floor and collides with a bigger block which is initially at rest. After the collision the speed of both blocks is V/3 in opposite directions. 1)Was the collision elastic?

No, they don't equal the original velocity

A destroyer simultaneously fires two shells with the same initial speed at two different enemy ships. The shells follow the parabolic trajectories shown. Which ship is hit first?

Ship 2 is hit first The time should depend on velocity as well as angle. If they have the same velocity, the angle is the only thing that should affect the timing. Thus the smaller angle should be the one to reach the other boat first, and the other will spend more time in the air before hitting ship 1

A small light cylinder and a large heavy cylinder are released at the same time and roll down a ramp without slipping. 1)Which one reaches the bottom first?

The same It is independent of mass and radius

n Case 1, a force F is pushing perpendicular on an object a distance L/2 from the rotation axis. In Case 2 the same force is pushing at an angle of 30 degrees a distance L from the axis. 1)In which case is the torque due to the force about the rotation axis biggest?

They are the same Solving for torque using t=Frsinθ you get .25f for each

An object is made by hanging a ball of mass M from one end of a plank having the same mass and length L. The object is then pivoted at a point a distance L/4 from the end of the plank supporting the ball, as shown below. 1)Is the object balanced?

Yes T=Force x distance; finding it on each side you get MGL/4

Suppose you are on a cart, initially at rest, which rides on a frictionless horizontal track. If you throw a ball off the cart towards the left, will the cart be put into motion?

Yes it moves to the right momentum should be conserved thus something left means movement right

A yummy glazed doughnut is shown above. 1)Where is the center of mass of this fantastic culinary delight? a) In the center of the hole. b) Somewhere inside the solid part of the doughnut. c) The center of mass is not defined in cases where mass is missing.

a) In the center of the hole. do the center of mass equation for a ring

A block slides on a table pulled by a string attached to a hanging weight. In case 1 the block slides without friction and in case 2 there is kinetic friction between the sliding block and the table. 1)In which case is the tension in the string the biggest? a) Case 1 b) Case 2 c) Same

b) Case 2 It not only has the pull, but it has a pull back from static friction making it resist movement and increasing tension

The net force on a box is in the positive x direction. Which of the following statements best describes the motion of the box? a) Its velocity is parallel to the x axis. b) Its acceleration is parallel to the x axis. c) Both its velocity and its acceleration are parallel to the x axis. d) Neither its velocity nor its acceleration need to be parallel to the x axis.

b) Its acceleration is parallel to the x axis. force and acceleration are directly related

A box of mass m hangs by a string from the ceiling of an elevator that is accelerating upward. 1)Which of the following best describes the tension T in the string? a) T < mg b) T = mg c) T > mg

c) T > mg It is accelerating upward thus it has not only mg acting on it but also the tension from pulling it up

The angular momentum of a freely rotating disk around its center is Ldisk. You toss a heavy block horizontally onto the disk along the direction shown. Friction acts between the disk and the block so that eventually the block is at rest on the disk and rotates with it. We will choose the initial angular momentum of the disk to be positive. 1)What is the Ltotal, the magnitude of the angular momentum of the disk-block system?

ltotal=ldisk Since it is pushing with or against wouldn't it negate and be equal

A box sliding on a horizontal frictionless surface runs into a fixed spring, compressing it a distance x1 from its relaxed position while momentarily coming to rest. 1)If the initial speed of the box were doubled, how far x2 would the spring compress?

x2=2*x1 Compressed length and velocity should be proportional thus making the ratio double

A block of mass m, initially held at rest on a frictionless ramp a vertical distance H above the floor, slides down the ramp and onto a floor where friction causes it to stop a distance D from the bottom of the ramp. The coefficient of kinetic friction between the box and the floor is μk. 1)What is the macroscopic work done on the block by friction during this process? 2)What is the macroscopic work done on the block by all forces during this process?

1) -mgh only one force working opposite the block 2)0 starts and ends at rest

Two blocks on a horizontal frictionless track head toward each other as shown. One block has twice the mass and half the velocity of the other. 1)The velocity of the center of mass of this system before the collision is... 2)Suppose the blocks collide elastically. Picking the positive direction to the right, what is the velocity of the bigger block after the collision takes place?

1)zero ptotal=mv-mv so 1(2)-(2)1 gives zero 2)V they are both conserved

A block sits at rest on a horizontal frictionless surface. Which of the follwoing sketches most closely resembles the correct free body diagram for all forces acting on the block. Each red arrow represents a force.

A It has the weight force and the normal force

In both cases shown below a hula hoop with mass M and radius R is spun with the same angular velocity about a vertical axis through its center. In Case 1 the plane of the hoop is parallel to the floor and in Case 2 it is perpendicular. 1)In which case does the spinning hoop have the most kinetic energy?

Case 1 Inertia of a hoop is .5mr^2 and the radius is .5 in the other and KE= .5IW so the second case yields a smaller number

Identical balls are dropped from the same initial height and bounce back to half the initial height. In Case 1 the ball bounces off a cement floor and in case 2 the ball bounces off a piece of stretchy rubber. 1)In which case is the average force acting on the ball during the collision the biggest?

Case 1 The contact time with the floor is less thus the average force is more

Consider the two collisions shown above. In both cases a solid disk of mass M, radius R, and initial angular velocity ω0 is dropped onto an initially stationary second disk having the same radius. In Case 2 the mass of the bottom disk is twice as big as in Case 1. 1)If there are no external torques acting on either system, in which case is the final kinetic energy of the system biggest?

Case 1 ke=.5IW^2 so if w/2 or w/3 multiplied by w/2 gives w/4 or w/6 making w/4 bigger making case 1 bigger

Two objects, one having twice the mass of the other, are initially at rest. Two forces, one twice as big as the other, act on the objects in opposite directions as shown above. 1)Which of the following statements about the acceleration of the center of mass of the system is true?

F/3M to the left F=ma But you add 3M and the force is split in 3's

Two wheels can rotate freely about fixed axles through their centers. The wheels have the same mass, but one has twice the radius of the other. Forces F1 and F2 are applied as shown such that the angular acceleration of the two wheels is the same. 1)Compare the magnitudes of the two forces

F2= 2*F1 it is double since it's FR and the radius is double thus 2F

A constant force acts for a time Δt on a block that is initially at rest on a frictionless surface, resulting in a final velocity V. 1)Suppose the experiment is repeated on a block with twice the mass using a force that's half as big. For how long would the force have to act to result in the same final velocity?

Four times as long △t=mv/F so if 2mv/(F/2) the two would flip up thus 4mv/F

Consider two identical objects released from rest high above the surface of the earth (neglect air resistance for this question).In Case 1 we release an object from a height above the surface of the earth equal to 1 earth radius, and we measure its kinetic energy just before it hits the earth to be K1.In Case 2 we release an object from a height above the surface of the earth equal to 2 earth radii, and we measure its kinetic energy just before it hits the earth to be K2.

K2 = (4/3) K1

A box sliding on a frictionless surface collides and sticks to a second identical box which is initially at rest. 1)Compare the initial and final kinetic energies of the system of two boxes.

Kinitial>Kfinal You use the conservation of momentum to get velocity then do kinetic energy broken into two parts

The angular momentum of a freely rotating disk around its center is Ldisk. You toss a heavy block horizontally onto the disk along the direction shown. Friction acts between the disk and the block so that eventually the block is at rest on the disk and rotates with it. We will choose the initial angular momentum of the disk to be positive. 1)What is the magnitude of the final angular momentum of the disk-block system?

Ltotal<Ldisk It's at an angle so it slows versus the other entered perpendicular so it should remain the same as it doesn't push back in the opposite direction of rotation

A green block of mass m slides to the right on a frictionless floor and collides elastically with a red block of mass M which is initially at rest. After the collision the green block is at rest and the red block is moving to the right. 1)How does M compare to m?

M=m doesn't it just switch because momentum is conserved so if v1 was 5 and v2 was 0 then if v1 goes to zero v2 goes to 5

1) The net force acting on your car a) Points radially inward, toward the center of the track. b) Points radially outward, away from the center of the track. c) Points forward, in the same direction you are moving. d) Points backward, opposite to the direction you are moving. e) Is zero. 2) The momentum of your car a) Points radially inward, toward the center of the track. b) Points radially outward, away from the center of the track. c) Points forward, in the same direction you are moving. d) Points backward, opposite to the direction you are moving. e) Is zero.

a) Points radially inward, toward the center of the track. c) Points forward, in the same direction you are moving. Net force is related to acceleration and acceleration points in. Velocity goes tangent and momentum relates to velocity, thus the momentum moves tangent.

The work done on the box by the static frictional force as the accelerating truck moves a distance D to the left is? a) Positive b) Zero c) Negative d) Depends on the speed of the truck.

a) Positive it moves in the same direction as the truck and there is no angle. It should rely on friction and distance

The potential energy of an object U as a function of x looks like the plot shown above. 1)Where is the force the biggest in the negative x direction?

c It has the greatest slope

A box of mass m is hung by a spring from the ceiling of an elevator. When the elevator is at rest, the length of the spring is L = 1m. 1)As the elevator accelerates upward, the length of the spring will be? a) L < 1 m b) L = 1 m c) L > 1 m

c) L > 1 m The string stretches even more with acceleration upward forcing it to stretch out more

A box sits on a horizontal table. A string with tension T pulls to the right, but static friction between the box and the table prevents the box from moving. 1)What is the magnitude of the static frictional force acting on the box? a) Mg b) μMg c) T d) 0

c0 T It is not moving therefore static friction needs to equal the force so the net force is zero

Three objects having the same mass begin at the same height, and all move down the same vertical distance H. One falls straight down, one slides down a frictionless inclined plane, and one swings on the end of of a string. 1)In which case does the object have the biggest total work done on it by all forces during its motion? a) Free Fall b) Incline c) String d) Same

d) Same Work doesn't depend on path

Three balls of equal mass are fired simultaneously with equal speeds from the same height h above the ground. Ball 1 is fired straight up, ball 2 is fired straight down, and ball 3 is fired horizontally. 1)Rank in order from largest to smallest the speeds of the balls, v1, v2, and v3, just before each ball hits the ground.

v1=v2=v3 They should all have the same initial energy potential since it depends on height velocity and mass which are all the same and because there is a conservation of energy they would end with the same energy and have the same velocity

A car drives up a hill with constant speed. 1)Which statement best describes the total work done on the car by all forces as it moves up the hill with a constant speed? a) Positive b) Zero c) Negative

work is the force times displacement thus if the force is zero work is then zero