
Concept explainers
(a)
The angular speed of the ball just after the blow.
(a)

Answer to Problem 106P
ω0=−5v06R
Explanation of Solution
Given: The horizontal force applied on a billiard ball at a distance h=2R3 , below the centerline
Speed of the ball just after the blow is v0
The coefficient of kinetic friction between the ball and billiard table is μk
Formula Used:
Newton’s second law of motion in rotational form
∑τ=r×F=Icmα
Calculation:
FIGURE: 1
Applying Newton’s second law in rotational form to ball,
∑τ0=Fav(h−R)=Icmα=Icmω0Δt
⇒Fav(h−R)=Icmω0Δt
ω0=Fav(h−R)ΔtIcm (1)
Where,
τ0 is the torque about the center of the ball
Fav is the average force on the ball
h is the below the centerline at which the force is applied on the ball
R is radius of the ball
Icm is the moment of inertia with respect to an axis through the center of mass of the ball
α is angular acceleration of the ball
ω0 is angular speed of the ball after impact
Δt is elapsed time
Moment of inertia with respect to an axis through the center of mass of the ball is
Icm=25mr2
Substituting this in equation (1) ,
ω0=Fav(h−r)Δt25mr2 (2)
Applying impulse-momentum theorem to the ball,
I=FavΔt=Δp=mv0
⇒mv0=FavΔt
v0=FavΔtm (3)
Where, m is mass of the ball
Δp is momentum, and
v0 is speed of the ball just after the impact
From equation (3) , v0=FavΔtm⇒Δt=mv0Fav
Substituting the expression for Δt in equation (3) ,
ω0=Fav(h−R)mv0Fav25mR2=5v0(h−R)2R2
Substituting h=2R3 in the above equation,
ω0=5v0(2R3−R)2R2=5v0(2R−3R3)2R2=5v0(−R3)2R2=−5v06R
Conclusion:
The angular speed of the ball just after the blow is −5v06R .
(b)
The speed of the ball once it begins to roll without slipping.
(b)

Answer to Problem 106P
v=521v0
Explanation of Solution
Given: Speed of the ball just after the blow is v0
The coefficient of kinetic friction between the ball and billiard table is μk
Formula Used:
FIGURE: 2
Referring to the force diagram shown in figure 2, applying Newton’s second law to the ball when it is rolling without slipping,
Στ0=fkR=Icmα (4)
ΣFy=Fn−mg=0 (5)
And
ΣFx=−fk=ma (6)
Where,
Fx is force in the x-direction
fk is force of friction in x-direction
Fy is force in the y-direction
fn is force of friction in y-direction
m is mass of the ball
a is the acceleration of the ball
g is acceleration due to gravity
R is radius of the ball
τ0 is torque about the center of the ball
Icm is the moment of inertia with respect to an axis through the center of mass of the ball
α is angular acceleration of the ball
g is acceleration due to gravity, g=9.8 m/s2
But, fk=μkFn (7)
Where, μk is coefficient of kinetic friction
Calculations:
From equation (5) , Fn=mg
Substituting this in equation (7) ,
fk=μkmg
From equation (4) angular acceleration,
α=fkRIcm
Moment of inertia with respect to an axis through the center of mass of the ball is
Icm=25mR2
Substituting for fk and Icm ,
α=μkmgR25mR2=5μkg2R
Now let us write constant-acceleration equation that connects angular speed of the ball to the angular acceleration and time,
ω=ω0+αΔt=ω0+5μkg2RΔt (8)
Now,substituting the expression for fk in equation (6) ,
−μkmg=ma⇒a=−μkg (9)
Constant acceleration equation that relates the speed of the ball to the acceleration and time,
v=v0+aΔt (10)
Where,
v is speed of the ball
v0 is speed of the ball just after impact
a is the acceleration ball
Δt is elapsed time
Substituting for a from equation (9) in equation (10) ,
v=v0−μkgΔt (11)
Imposing the condition for rolling the ball without slipping,
R(ω0+5μkg2RΔt)=v0−μkgΔt⇒Δt=1621v0μkg
Substituting this Δt in equation (11) ,
v=v0−μkg(1621v0μkg)=521v0
Conclusion:
The speed of the ball once it begins to roll without slipping is 521v0 .
(c)
The kinetic energy of the ball just after the hit.
(c)

Answer to Problem 106P
Ki=23mv2036
Explanation of Solution
Given: The horizontal force applied on a billiard ball at a distance h=2R3 , below the centerline.
Speed of the ball just after the blow is v0 .
The coefficient of kinetic friction between the ball and billiard table is μk .
Formula Used:
Initial kinetic energy of the ball can be written as,
Ki=Ktrans+Krot (12)
Where, Ktrans is kinetic energy due to translational motion of the ball, Ktrans=12mv20
Krot is kinetic energy due to rotational energy of the ball, Krot=12Iω20
Substituting the expressions for Ktrans and Krot in equation (12) ,
Ki=12mv20+12Iω20 (13)
Where, m is mass of the ball,
v0 is speed of the ball just after impact
I is moment of inertia of the ball
ω0 is angular speed of the ball
Moment of inertia, I=25mR2 , and Angular speed, ω0=−5v06R
Substituting these in equation (13) ,
Ki=12mv20+12(25mR2)(−5v06R)2
=12mv20+15mR2×2536v20R2
=12mv20+536mv20
=18mv20+5mv2036=23mv2036
Conclusion:
The kinetic energy of the ball just after the hit is 23mv2036 .
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