General Physics I Quiz Samples for Chapter 6 Force and Motion - II March 30, 2020 Name: Department: Student ID #: Notice +2 (-1) points per correct (incorrect) answer. No penalty for an unanswered question. Fill the blank ( ) with (✘) if the statement is correct (incorrect). Textbook: Walker, Halliday, Resnick, Principles of Physics, Tenth Edition, John Wiley & Sons (2014). 6-1 Friction 1. Consider the case if there is no friction. (a) () Walking is possible because of friction. When you are walking, you step on the ground and push the ground back. Against your action, the reaction of the ground pushes the bottom of your shoe forward. If there is no friction, then the contact force between the bottom of your shoe and the ground vanishes. Thus you cannot walk. (b) () Bicycles and automobiles are useless because they go forward only if there is friction between the tire and the ground. (c) () It is impossible to fasten nails and screws into the wall. (d) () However, you can move backward if you exhale or if you throw something away from your body forward. Newton’s third law can be employed to explain this phenomenon. 2. () The coefficient of kinetic friction is dimensionless quantity. 3. () The kinetic friction is independent of the area of contact. 4. () The coefficient of the static friction μ s is slightly greater than the coefficient of the kinetic friction μ k . However, it does not mean that the static frictional force is always greater than the kinetic frictional force. 5. () The coefficient of the static friction μ s is dimensionless quantity. 6. A boy pulls a wooden box along a rough horizontal floor at the constant speed by means of a force P as shown. In the diagram f is the magnitude of the force of friction, N is the magnitude of the normal force, and F g is the magnitude of the force of gravity. (a) () P = f . (b) () N = F g . 7. A boy pulls a wooden box of a mass m along a rough horizontal floor at the constant acceleration a to the right by means of a force P as shown. In the diagram f is the magnitude of the force of friction, N is the magnitude of the normal force, and F g is the magnitude of the force of gravity. (a) () a = P cos θ - f m . (b) () N = F g - P sin θ. (c) () f = μ k (F g - P sin θ) if the coefficient of kinetic friction is μ k . 8. A horizontal force F h pushes a block of mass m against a vertical wall. The block is initially at 2020 KPOPE All rights reserved. Korea University Page 1 of 5
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General Physics IQuiz Samples for Chapter 6
Force and Motion - IIMarch 30, 2020
Name: Department: Student ID #:
Notice
� +2 (−1) points per correct (incorrect) answer.
� No penalty for an unanswered question.
� Fill the blank ( ) with � (8) if the statement iscorrect (incorrect).
(a) (�) Walking is possible because of friction.When you are walking, you step on theground and push the ground back. Againstyour action, the reaction of the ground pushesthe bottom of your shoe forward. If there isno friction, then the contact force between thebottom of your shoe and the ground vanishes.Thus you cannot walk.
(b) (�) Bicycles and automobiles are uselessbecause they go forward only if there isfriction between the tire and the ground.
(c) (�) It is impossible to fasten nails and screwsinto the wall.
(d) (�) However, you can move backward if youexhale or if you throw something away fromyour body forward. Newton’s third law can beemployed to explain this phenomenon.
2. (�) The coefficient of kinetic friction isdimensionless quantity.
3. (�) The kinetic friction is independent of the areaof contact.
4. (�) The coefficient of the static friction µs isslightly greater than the coefficient of the kineticfriction µk. However, it does not mean that thestatic frictional force is always greater than thekinetic frictional force.
5. (�) The coefficient of the static friction µs isdimensionless quantity.
6. A boy pulls a wooden box along a rough horizontalfloor at the constant speed by means of a force Pas shown. In the diagram f is the magnitude ofthe force of friction, N is the magnitude of thenormal force, and Fg is the magnitude of the forceof gravity.
(a) (�) P = f .
(b) (�) N = Fg.
7. A boy pulls a wooden box of a mass m along arough horizontal floor at the constant accelerationa to the right by means of a force P as shown. Inthe diagram f is the magnitude of the force offriction, N is the magnitude of the normal force,and Fg is the magnitude of the force of gravity.
(a) (�) a =P cos θ − f
m.
(b) (�) N = Fg − P sin θ.
(c) (�) f = µk(Fg − P sin θ) if the coefficient ofkinetic friction is µk.
8. A horizontal force Fh pushes a block of mass magainst a vertical wall. The block is initially at
rest. The coefficients of static and kinetic frictionare given by µs and µk, respectively.
(a) (�) The block does not move if
Fh ≥mg
µs.
(b) (�) The block starts to move if
Fh <mg
µs.
(c) (�) If the block moves, then the accelerationis along the gravitational acceleration and itsmagnitude is
a = g − µkFh
m.
9. Block A, with mass mA, rests on a horizontal tabletop. The coefficient of static friction is µs. Ahorizontal string is attached to A and passes over amassless, frictionless pulley as shown. We find thesmallest mass mB of block B, attached to thedangling end, that will start A moving when it isattached to the other end of the string.
(a) (�) The block does not move if
µsmAg ≥ mBg.
(b) (�) The block starts to move if
µsmAg < mBg.
Thus the minimum value of mB is larger thanµsmA.
10. A car is traveling at v0 on a horizontal road. Thebrakes are applied at t = t0 and the car skids to astop at t = t0 + T . The coefficient of kineticfriction between the tires and road is µk.
R RR R R R
(a) (�) The average acceleration during the timeinterval [t0, t0 + T ] is
a = −v20
2s.
(b) (�) The average acceleration during the timeinterval [t0, t0 + T ] is
a = − 2s
T 2.
(c) (�) The average acceleration during the timeinterval [t0, t0 + T ] is
a = −v0T.
(d) (�) The coefficient of kinetic friction is
µk = −ag.
11. Block A, with a mass mA, is climbing up a θincline. The coefficient of kinetic friction is µk. Anattached string is parallel to the incline and passesover a massless, frictionless pulley at the top.Block B of mass mB (> mA) is attached to thedangling end. We denote the displacement of Bfrom the initial position by x(> 0). In addition,v = x, a = x, and T is the tension.
1. A fluid is anything that can flow—generally eithera gas or a liquid. The drag force is the forceapplied to a body moving in a fluid that resists themotion.
(a) (�) The drag force depends on the density ρof the fluid.
(b) (�) The drag force depends on the relativevelocity v of the body and the fluid.
(c) (�) The drag force depends on the effectivecross section A perpendicular to the relativevelocity.
(d) (�) The phenomenological (not fundamental)formula for a magnitude of the drag force is
D =1
2CρAv2,
where C is the drag coefficient that is to bedetermined experimentally.
2. Consider a falling object of gravitational forceFg = −Fg j under a drag force.
(a) (�) If a falling body slows down due to thedrag force D = D j, then the acceleration isgiven by a = a j, where
a =D − Fg
m.
(b) (�) If a falling body slows down to reach theterminal speed, then the drag force and thegravity cancel:
D − Fg = 0.
This constraint can be used to determine theterminal velocity
vterminal =
√2Fg
CρA.
3. (�) Raindrops fall with a constant speed duringthe later stages of their descent. The reason is thatair resistance just balances the force of gravity.
4. (�) A ball of mass m is thrown downward from theedge of a cliff with an initial speed that is threetimes the terminal speed. Initially the drag forceon it is upward and greater than mg.
5. (�) In air, ρ( ≈ 1.2 kg/m3), a baseball has aterminal speed of vterminal. We vary the fluiddensity from ρ→ ρ′, keeping every other parameterinvariant. Then the terminal velocity varies as
vterminal → v′terminal = vterminal
√ρ
ρ′.
6-3 Uniform Circular Motion
1. We check the elementary properties of a uniformcircular motion.
(a) (�) The acceleration and the velocity arealways perpendicular.
(b) (�) The velocity is always tangent to thepath.
(c) (�) The acceleration is always perpendicularto the path.
(d) (�) The centripetal force of constantmagnitude is always directed toward the samefixed point.
2. An object of mass m and another object of mass2m are each forced to move along a circle of radiusr at a constant speed of v.
(a) (�) The magnitudes of the accelerations areequal.
(b) (�) The centripetal forces are proportional tothe mass.
3. (�) The driver of a car of mass M tries to turnthrough a circle of radius R on an unbanked curveat a speed of v. The maximum staticl frictionalforce between the tires and a slippery road has amagnitude of f . The car slides off to the outside ofthe curve if
Mv2
R> f.
4. (�) An object moves around a circle. If the radiusis doubled keeping the speed the same then themagnitude of the centripetal force must be half asgreat.
5. As is shown in the figure, a stuntman drives a car(without negative lift) over the top of a hill ofwhich can be approximated by a circle of radius R.
R
R
(a) (�) The greatest speed at which he can drivewithout the car leaving the road at the top ofthe hill is
v =√gR.
(b) (�) At v =√gR, the normal force of the road
to sustain the car is vanishing.
6. A puck of mass m slides in a circle of radius r on africtionless table while attached to a hangingcylinder of mass M by means of a cord thatextends through a hole in the table as shown in thefigure. We compute the speed v that keeps thecylinder at rest.
(a) (�) If the radius is stationary, then thetension must be the same as the centripetalforce:
T =mv2
r.
(b) (�) If the radius is stationary, then thetension T must be the same as thegravitational force on the cylinder:
T = Mg.
(c) (�) The speed v is given by
v =
√Mrg
m.
If the radius is stationary, then the tension Tmust be the same as the gravitational force.
7. The iron ball of a mass m shown is being swung ina vertical circle at the end of a string of length `.We want to find the minimum speed at the topposition to make the ball undergoes circularmotion without having the string go slack.
(b) (�) The gravity must be the only externalforce on the ball at the top.
(c) (�) If the speed at the top is v, then thecentripetal acceleration is
v2
`.
(d) (�) The minimum value of v is
vmin =√g`.
8. The iron ball of mass m shown is being swung in avertical circle at the end of a string of length `.The ball passes the bottom point at speed v(6= 0).
(a) (�) The tension T must be greater than thegravitational force Fg = mg.
(b) (�) The centripetal force is T −mg.
(c) (�) The tension is
T = mg +mv2
`.
9. Circular freeway (with radius r) entrance and exitramps are commonly banked to handle a carmoving at speed v0. We want to design a similarramp for 2v0.
(a) (�) The most important criterion is to controlthe acceleration. Thus the maximum limit ofthe acceleration should be kept.
(b) (�) The centripetal acceleration is
v20r.
Thus the radius must be increased by a factorof 4.
