However, I don't understand why the Force of Friction is going in the same direction as the displacement by doing positive e work and also why is not considered in the calculations. Here is the problem: A hover board and rider of total mass 80 kg is moving at 1 m/s and encounters a slope of angle 2o. After moving a distance of 10 m up the slope the linear velocity of the hover board is 2 m/s. An electric motor, which is connected to the axle between the two wheels, produces a driving torque acting on the axle. The total moment of inertia of the two wheels, the axle and the motor is 0.02 kgm2. The wheels have a radius of 100 mm. If the wheels, axle and motor rotate through an angle of 100 radians during the motion find the magnitude of driving torque acting on the axle.

Elements Of Electromagnetics
7th Edition
ISBN:9780190698614
Author:Sadiku, Matthew N. O.
Publisher:Sadiku, Matthew N. O.
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Section: Chapter Questions
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Hi,

I have the solution for this problem given. However, I don't understand why the Force of Friction is going in the same direction as the displacement by doing positive e work and also why is not considered in the calculations. Here is the problem:

A hover board and rider of total mass 80 kg is moving at 1 m/s and encounters a slope of angle 2o. After moving a distance of 10 m up the slope the linear velocity of the hover board is 2 m/s. An electric motor, which is connected to the axle between the two wheels, produces a driving torque acting on the axle. The total moment of inertia of the two wheels, the axle and the motor is 0.02 kgm2. The wheels have a radius of 100 mm. If the wheels, axle and motor rotate through an angle of 100 radians during the motion find the magnitude of driving torque acting on the axle.

Worked Example 3: Rotational Energy and Work
you
WI
A hover board and rider of total mass 80 kg is moving at 1 m/s
and encounters a slope of angle 2°. After moving a distance of
10 m up the slope the linear velocity of the hover board is 2 m/s.
An electric motor, which is connected to the axle between the
two wheels, produces a driving torque acting on the axle. The
total moment of inertia of the two wheels, the axle and the motor
is 0.02 kgm². If the wheels, axle and motor rotate through an
angle of 100 radians during the motion find the magnitude of
driving torque acting on the axle. Radius of wheels = 100mm
Pictorial
tue
2₂
10
motion
V₁
motion
5=100m
TT
angle of slope
knowns
rider
radius of
wheels,
total mass of rider
m
2
and hoverboard > M = 80kg
accel of gravity; g = 9.81 m/5
weight of rider + hoverboard, mg
initial velocity of hoverboard, V₁ = 1 m/s
final velocity of hoverboard, V₂=200/5
moment of inertia of motor,
axle and wheels
slope
KL
b
W 2
'I=0.02 ky
V₂
hover
board.
wheels
}
up the slope, 5=10m
distance moved up the slope
Radius of wheels; M = 100mm
angular displacement of wheels, f = 100 radions
angle of slope) 0 = 2°
height, h
Unknowns
driving torque from motor, Tm=?
initial angular velocity of wheels
axle and motor
= ?
>
final angular velocity, W₂ = ?
final height above initial position h=2
Normal contact force acting on wheels, N=?
2 Friction force from slope acting on whells; FR=?
height h = 9 sinf
datom for
Potential Energy
الله
= 100 sin (20)
= 0.349m
J
Transcribed Image Text:Worked Example 3: Rotational Energy and Work you WI A hover board and rider of total mass 80 kg is moving at 1 m/s and encounters a slope of angle 2°. After moving a distance of 10 m up the slope the linear velocity of the hover board is 2 m/s. An electric motor, which is connected to the axle between the two wheels, produces a driving torque acting on the axle. The total moment of inertia of the two wheels, the axle and the motor is 0.02 kgm². If the wheels, axle and motor rotate through an angle of 100 radians during the motion find the magnitude of driving torque acting on the axle. Radius of wheels = 100mm Pictorial tue 2₂ 10 motion V₁ motion 5=100m TT angle of slope knowns rider radius of wheels, total mass of rider m 2 and hoverboard > M = 80kg accel of gravity; g = 9.81 m/5 weight of rider + hoverboard, mg initial velocity of hoverboard, V₁ = 1 m/s final velocity of hoverboard, V₂=200/5 moment of inertia of motor, axle and wheels slope KL b W 2 'I=0.02 ky V₂ hover board. wheels } up the slope, 5=10m distance moved up the slope Radius of wheels; M = 100mm angular displacement of wheels, f = 100 radions angle of slope) 0 = 2° height, h Unknowns driving torque from motor, Tm=? initial angular velocity of wheels axle and motor = ? > final angular velocity, W₂ = ? final height above initial position h=2 Normal contact force acting on wheels, N=? 2 Friction force from slope acting on whells; FR=? height h = 9 sinf datom for Potential Energy الله = 100 sin (20) = 0.349m J
Gonceptual
G
mg
A
FR
Im
N
tve
x
Freebody Diagram
The weight of the rider and hoverboard mg does negative work and is
conservative (PE: mgh)
The normal contact force, Nidoes zero work since perpendicular to motion (unconservative)
The static friction for FR does zero work since there is no slip and no motion
between the wheels and the ground (unconservative)
The driving torque, Tm does positive work and is unconservative.
We assume the wheels of the hoverboard roll without slipping therefore
the velocity Y = Wr. This is the constraint to the motion.
Since there are unconservative forces doing work we will
use the Work - Energy Equation.
Transcribed Image Text:Gonceptual G mg A FR Im N tve x Freebody Diagram The weight of the rider and hoverboard mg does negative work and is conservative (PE: mgh) The normal contact force, Nidoes zero work since perpendicular to motion (unconservative) The static friction for FR does zero work since there is no slip and no motion between the wheels and the ground (unconservative) The driving torque, Tm does positive work and is unconservative. We assume the wheels of the hoverboard roll without slipping therefore the velocity Y = Wr. This is the constraint to the motion. Since there are unconservative forces doing work we will use the Work - Energy Equation.
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Hi,

I am still not sure I am understanding this correctly. The hoverboard is moving in the same direction as the friction force, but technically the friction force should be opposite to the direction of motion of the object. I am sorry to bother you, could you please try to explain this further?

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