Question 1

A metal bar is hanging from a hook in the ceiling when it is suddenly struck by a ball that is moving horizontally (see figure).The ball is covered with glue,so it sticks to the bar.During this collision

Accepted Answer

A)  the angular momentum of the system (ball and bar) is conserved about the hook because only gravity is acting on the system. 
B)  the angular momentum of the system (ball and bar) is not conserved because the hook exerts a force on the bar. 
C)  the angular momentum of the system (ball and bar) is conserved about the hook because neither the hook nor gravity exerts any torque on this system about the hook. 
D)  both the angular momentum of the system (ball and bar) and its kinetic energy are conserved. 
E)  both the linear momentum and the angular momentum of the system (ball and bar) are conserved.

Question 2

If  = -4 
- 2 
- 3 
,what is 
× 
?

Accepted Answer

A)  +3  - 4
 
B)  +3  + 4
 
C)  -3  + 4
 
D)  +3  + 2
 - 4
 
E)  -3  - 2
 + 4

Question 3

If the torque on an object adds up to zero

Accepted Answer

A)  the forces on it also add up to zero. 
B)  the object is at rest. 
C)  the object cannot be turning. 
D)  the object could be accelerating linearly but it could not be turning. 
E)  the object could be both turning and accelerating linearly.

Question 4

A very thin horizontal,2.00-m long,5.00-kg uniform beam that lies along the east-west direction is acted on by two forces. At the east end of the beam,a 200-N force pushes downward. At the west end of the beam,a 200-N force pushes upward. What is the angular acceleration of the beam?

Accepted Answer

A)  240 rad/s2 north 
B)  1.33 × 102 rad/s2 north 
C)  zero 
D)  240 rad/s2 south 
E)  1.33 × 102 rad/s2 south

Question 5

A 72.0-kg person pushes on a small doorknob with a force of 5.00 N perpendicular to the surface of the door. The doorknob is located 0.800 m from axis of the frictionless hinges of the door. The door begins to rotate with an angular acceleration of 2.00 rad/s2. What is the moment of inertia of the door about the hinges?

Accepted Answer

A)  4.28 kg • m2 
B)  7.52 kg • m2 
C)  1.88 kg • m2 
D)  0.684 kg • m2 
E)  2.74 kg • m2

Question 6

A dump truck has a large cubical concrete block in its bed.The coefficients of friction between this block and the floor of the bed are µk = 0.450 and µs = 0.650.As the bed is slowly tilted above the horizontal,will the brick first begin to slide or will it first tip over?

Accepted Answer

A)  It will first tip over. 
B)  It will first begin to slide. 
C)  It will tip over just as it begins to slide. 
D)  It is impossible to answer without knowing the mass of the block. 
E)  It is impossible to answer without knowing the dimensions of the block.

Question 7

A light board,10 m long,is supported by two sawhorses,one at one edge of the board and a second at the midpoint.A small 40-N weight is placed between the two sawhorses,3.0 m from the edge and 2.0 m from the center.What forces are exerted by the sawhorses on the board?

Accepted Answer

The answer of A light board,10 m long,is supported by...

Question 8

A 500-g particle is located at the point  = 4m 
+ 3m 
- 2m 
And is moving with a velocity 
= 5 m/s 
-2 ms 
+ 4 m/s 
)What is the angular momentum of this particle about the origin?

Accepted Answer

A)  (24  - 6
 - 8
 ) kg∙m2/s 
B)  (12  - 3
 - 4
 ) kg∙m2/s 
C)  (8  + 14
 - 13
 ) kg∙m2/s 
D)  (10  -
 + 2
 ) kg∙m2/s 
E)  (4  - 13
 - 11.5
 ) kg∙m2/s

Question 9

A uniform solid disk of radius 1.60 m and mass 2.30 kg rolls without slipping to the bottom of an inclined plane.If the angular velocity of the disk is 5.35 rad/s at the bottom,what is the height of the inclined plane?

Accepted Answer

A)  5.61 m 
B)  4.21 m 
C)  4.94 m 
D)  6.73 m

Question 10

What is the torque about the origin on a particle located at

= 3 m 
+ 4 m 
- 2 m 
If a force 
= 5 N 
- 2 N 
+ 3 N 
Acts on the particle?

Accepted Answer

A)  (8  +
 - 26
 ) N • m 
B)  (8  - 19
 - 26
 ) N • m 
C)  (8  + 2
 +
 ) N • m 
D)  (16  - 19
 - 26
 ) N • m 
E)  (8  - 2
 +
 ) N • m

Question 11

A child is trying to stack two uniform wooden blocks,12 cm in length,so they will protrude as much as possible over the edge of a table,without tipping over,as shown in the figure.What is the maximum possible overhang distance d?

Accepted Answer

A)  5 cm 
B)  6 cm 
C)  7 cm 
D)  8 cm 
E)  9 cm

Question 12

What is the vector product of  = 2.00 
+ 3)00 
+ 1)00 
And 
= 1)00 
- 3)00 
- 2)00 
?

Accepted Answer

A)  -3.00  + 5.00
 - 9.00
 
B)  -5.00  + 2.00
 - 6.00
 
C)  -9.00  - 3.00
 - 3.00
 
D)  -4.00  + 3.00
 - 1.00
 
E)  2.00  -9.00
 - 2.00

Question 13

A light triangular plate OAB is in a horizontal plane.Three forces,F1 = 6.0 N,F2 = 9.0 N,and
F3 = 7.0 N,act on the plate,which is pivoted about a vertical axes through point O.In the figure, 
2 is perpendicular to OB.Consider the counterclockwise sense as positive.The sum of the torques about the vertical axis through point O,acting on the plate due to forces F1,F2,and F3,is closest to

Accepted Answer

A)  4.1 N • m 
B)  5.4 N • m 
C)  -4.1 N • m 
D)  -5.4 N • m 
E)  zero

Question 14

A 95 N force exerted at the end of a 0.50 m long torque wrench gives rise to a torque of 15 N • m. What is the angle (assumed to be less than 90°)between the wrench handle and the direction of the applied force?

Accepted Answer

A)  18° 
B)  14° 
C)  22° 
D)  25°

Question 15

A string is wrapped around a pulley with a radius of 2.0 cm and no appreciable friction in its axle.The pulley is initially not turning.A constant force of 50 N is applied to the string,which does not slip,causing the pulley to rotate and the string to unwind.If the string unwinds 1.2 m in 4.9 s,what is the moment of inertia of the pulley?

Accepted Answer

A)  0.17 kg • m2 
B)  17 kg • m2 
C)  14 kg • m2 
D)  0.20 kg • m2 
E)  0.017 kg • m2

Question 16

A 30.0-kg child sits on one end of a long uniform beam having a mass of 20.0 kg,and a 40.0-kg child sits on the other end. The beam balances when a fulcrum is placed below the beam a distance 1.10 m from the 30.0-kg child. How long is the beam?

Accepted Answer

A)  2.12 m 
B)  1.98 m 
C)  1.93 m 
D)  2.07 m 
E)  2.20 m

Question 17

In the figure,a 10.0-m long bar is attached by a frictionless hinge to a wall and held horizontal by a rope that makes an angle θ of 53° with the bar.The bar is uniform and weighs 39.9 N.How far from the hinge should a 10.0-kg mass be suspended for the tension T in the rope to be 125 N?

Accepted Answer

The answer of In the figure,a 10.0-m long bar is...

Question 18

The two rotating systems shown in the figure differ only in that the two identical movable masses are positioned at different distances from the axis of rotation.If you release the hanging blocks simultaneously from rest,and if the ropes do not slip,which block lands first?

Accepted Answer

A)  The block at the left lands first. 
B)  The block at the right lands first. 
C)  Both blocks land at the same time.

Question 19

A uniform solid 5.25-kg cylinder is released from rest and rolls without slipping down an inclined plane inclined at 18° to the horizontal.How fast is it moving after it has rolled 2.2 m down the plane?

Accepted Answer

A)  4.3 m/s 
B)  5.2 m/s 
C)  3.0 m/s 
D)  3.7 m/s 
E)  2.6 m/s

Question 20

What is the magnitude of the cross product of a vector of magnitude 2.00 m pointing east and a vector of magnitude 4.00 m pointing 30.0° west of north?

Accepted Answer

A)  6.93 
B)  -6.93 
C)  4.00 
D)  -4.00 
E)  8.00

Exam 12: Torque

A metal bar is hanging from a hook in the ceiling when it is suddenly struck by a ball that is moving horizontally (see figure) .The ball is covered with glue,so it sticks to the bar.During this collision

If = -4 - 2 - 3 ,what is × ?

If the torque on an object adds up to zero

A very thin horizontal,2.00-m long,5.00-kg uniform beam that lies along the east-west direction is acted on by two forces. At the east end of the beam,a 200-N force pushes downward. At the west end of the beam,a 200-N force pushes upward. What is the angular acceleration of the beam?

A dump truck has a large cubical concrete block in its bed.The coefficients of friction between this block and the floor of the bed are µ_k = 0.450 and µ_s = 0.650.As the bed is slowly tilted above the horizontal,will the brick first begin to slide or will it first tip over?

A light board,10 m long,is supported by two sawhorses,one at one edge of the board and a second at the midpoint.A small 40-N weight is placed between the two sawhorses,3.0 m from the edge and 2.0 m from the center.What forces are exerted by the sawhorses on the board?

A 500-g particle is located at the point = 4m + 3m - 2m And is moving with a velocity = 5 m/s -2 ms + 4 m/s ) What is the angular momentum of this particle about the origin?