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Romashka [77]
3 years ago
9

A 25 kg child stands 2.5 m from the center of a frictionless merry‐go‐round, which has a 200 kg*m^2 moment of inertia and is spi

nning with one revolution every two seconds. The child then moves inward to a radius of 1.5 m. a) What is the initial angular velocity of the merry‐go‐round?
b) What is the new angular velocity of the merry‐go‐round, after the child moves?
c) By how much did the kinetic energy of the merry‐go‐round increase? Where did this energy come from?
Physics
1 answer:
vodka [1.7K]3 years ago
7 0

Answer:

Explanation:

a ) Time period  T = 2 s

Angular velocity ω = 2π / T

=  2π / 2 = 3.14 rad /s

Initial moment of inertia I₁ = 200 + mr²

= 200 + 25 x 2.5²

=356.25

Final moment of inertia

I₂ = 200 + 25 X 1.5 X 1.5

= 256.25

b ) We apply law of conservation of momentum

I₁ X ω₁ =  I₂ X ω₂

ω₂ = I₁ X ω₁ / I₂

Putting the values

w_2=\frac{356.25\times3.14}{256.25}

ω₂ = 4.365 rad s⁻¹

c ) Increase in rotational kinetic energy

=1/2 I₂ X ω₂² -  1/2 I₁ X ω₁²

.5 X 256.25 X 4.365² - .5 X 356.25 X 3.14²

= 684.95 J

This energy comes from work done against the centripetal pseudo -force.

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A 1500 kg sedan goes through a wide intersection traveling from north to south when it is hit by a 2500 kg SUV traveling from ea
kolezko [41]

Answer:

v = 19.33 m / s   South

Explanation:

To solve this exercise we must use the conservation of momentum, for which we must define a system formed by the two cars, therefore the forces during the collision are internal and therefore the moment is conserved.

Since it is a vector quantity, we are going to work on each axis, the x axis is in the East-West direction

initial instant. Before the crash

        p₀ = m 0 + M v₂ₓ

final instant. Right after the crash

        p_f = (m + M) vₓ

        p₀ = 0_pf

        M v₂ₓ = (m + M) vₓ

In this case m is the mass of the car and M the mass of the SUV

        vₓ = \frac{M}{m+My}  v₂ₓ            (1)

in the Y axis (North - South direction)

initial instant

       p₀ = m v_{1y} + M 0

final moment

       p_f = (m + M) v_y

       p₀ = p_f

       m v_{1y} + M 0 = (m + M) v_y

       v_y = \frac{m}{m+M}  \  v_{1y}       (2)

With these speeds we can use the relationship between work and the variation of kinetic energy, in this part the two cars are already united.

         W = ΔK

friction force work is

         W = - fr d

the friction force is described by the equation

         fr = μ N

Newton's second law

         N-W = 0

         N = W

         

we substitute

         W = - μ (m + M) g d

as the car stops the final kinetic energy is zero and

the initial kinetic energy is

         K₀ = ½ (m + M) v²

we substitute

         - μ (m + M) g d = 0 - ½ (m + M) v²

            μ g d = ½ v²

            v² = 2 μ g d

the distance traveled can be found with the Pythagorean theorem

        d = \sqrt{x^2+y^2}

        d = \sqrt{5.48^2 + 6.37^2}

        d = 8.40 m

let's calculate the speed

         v² = 2 0.75 9.8 8.40

         v = √123.48

         v = 11.11 m / s

this velocity is in the direction of motion so we can use trigonometry to find the angles

          tan θ = y / x

          θ = tan⁻¹ y / x

          θ = tan⁻¹ (-5.48 / -6.37)

          θ = 40.7º

Since the two magnitudes are negative, this angle is in the third quadrant, measured from the positive side of the x-axis in a counterclockwise direction.

          θ'= 180 + 40.7

          θ’= 220.7º

In the exercise they indicate the the sedan moves in the y-axis, therefore

          sin θ'= v_y / v

          v_y = v sin 220.7

          v_y = 11.11 sin 220.7

          v_y = -7.25 m / s

the negative sign indicates that it is moving south

To find the speed we substitute in equation 2

          v_y = \frac{m}{m+M}  \ v_{1y}

          v_{1y} = v_ y   \frac{m+M}{m}

           

let's calculate

         v_{1y} = -7.25    \frac{1500+2500}{1500}

         v_{1y} = - 19.33 m/s

therefore the speed of the sedan is v = 19.33 m / s with a direction towards the South

4 0
3 years ago
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