Posted by AAKa on .
A merrygoround (pictured) is sitting in a playground. It is free to rotate, but is currently stationary. You can model it as a uniform disk of mass 190 kg and radius 100 cm (consider the metal poles to have a negligible mass compared to the merrygoround). The poles near the edge are 90 cm from the center.
Someone hits one of the poles with a 8 kg sledgehammer moving at 18 m/s in a direction tangent to the edge of the merrygoround. The hammer is not moving after it hits the merrygoround.
How much energy (modulus of (deltaE)) is lost in this collision? (enter a positive number for the absolute value in Joules)

PHYSICS 
enjoy,
example of inelastic collision, where momentum before = momentum after, and simply calculate the difference in energy then!

PHYSICS 
AAKa,
hi enjoy, what is the use of radius here?

PHYSICS 
Anonymous,
1/2 *[(m_1*m_2)/m_1+m_2)]*v_1^2

PHYSICS 
Daoine,
So Anonymous you got 964.29 N??

PHYSICS 
Anonymous,
First, get all the variables:
m_1: The mass of the merrygoround
m_2: The mass of the sledgehammer
v_1: The velocity of the merrygoround before the collision
v_2: The velocity of the sledgehammer before the collision
v_1': The velocity of the merrygoround after the collision
v_2': The velocity of the sledgehammer after the collision
Apply the conservation of momentum:
(m_1*v_1)+(m_2*v_2)=(m_1*v_1')+(m_2*v_2')
You know everything except for v_1', so you can solve the above equation for v_1'. Then look at the difference in kinetic energy before and after the collision:
((0.5*m_1*v_1^2)+(0.5*m_2*v_2^2))((0.5*m_1*v_1'^2)+(0.5*m_2*v_2'^2))
The above is your answer. 
PHYSICS 
Lewin,
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PHYSICS 
fima,
so the radius is not important???
and we'll have to consider v_1=0 and v_2'=0 
PHYSICS 
tiklam,
First Calculate the KE of the sledge hammer.
2: Calculate the speed of the Merrygoround (MGR) from m1.v1+m2.v2=m1.v1'+m2.v2'. Note: v2 and v1' are zero
3: Calculate angular KE of the MGR
4: Lost energy =13
So if you can't solve this then you should go to elementary school