11/8 Rotational Energy

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# 11/8 Rotational Energy - PowerPoint PPT Presentation

11/8 Rotational Energy. Today: Rotational Energy Examples HW: 11/8 HW Handout “Skateboard” due Thursday 11/14 Exam 3: Thursday, 11/14 5-7 in 116, 6-8 in 114, Email me if other time needed Monday holiday-no lab next week!. Systems of objects.

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11/8 Rotational Energy
• Today: Rotational Energy Examples
• HW: 11/8 HW Handout “Skateboard” due Thursday 11/14
• Exam 3: Thursday, 11/145-7 in 116, 6-8 in 114, Email me if other time needed
• Monday holiday-no lab next week!
Systems of objects

A Skateboard made of 4 wheels (disks, 1kg each) and a board (2kg). Rolls without slipping at 1m/s

So 1/4 in rotation and 3/4 in translation

Total Energy = 4J

Sort the total energy into buckets:

KET = Board and Wheels

= 3J

KER = Just Wheels

KER of disk = 1/2 KET of disk

KET of disk = 0.5J so KER disk = 0.25J

KER = 1J for all 4 wheels

R

R

T

T

g

g

S

S

Systems of objects

A Skateboard made of 4 wheels (disks, 1kg each) and a board (2kg). Rolls without slipping at 1m/s

So 1/4 in rotation and 3/4 in translation

How fast would the skateboard be moving if it rolled from rest without slipping down a ramp, losing 4m of altitude?

Energy still split 1/4 : 3/4

0

0

0

0

0

KER = 60J

Top of ramp

Bottom of ramp

KET = 180J

PE = 240J

1/2mv2 = 180J

v = 7.75m/s

?

?

?

0

R

R

T

T

g

g

S

S

Pulley and Block

We have a block with mass 3m hanging from a ring (pulley) of mass m and radius r, which rides on a frictionless axle. The system is released from rest.

Axle

Energy of the system

m

top

0

0

0

PE is for the block only.

Where is the energy?How is it split?

bottom

0

1/4 : 3/4 3 parts T and 1 part R

3m

v = 0

How do the velocities of the block and ring compare?

How do the velocities of the block and string compare?

h

vB = vString = vR not slipping!

v = ?

How do the masses of the block and ring compare?

How does the KER of the ring compare to the KET of the block?

?

?

?

6gh

v =

R

R

T

T

g

g

S

S

4

0

0

0

Pulley and Block

We have a block with mass 3m hanging from a ring (pulley) of mass m and radius r, which rides on a frictionless axle. The system is released from rest.

Axle

m

PE is for the block only.

Where is the energy?How is it split?

0

0

1/4 : 3/4

3/4mgh = 1/2mv2

3m

v = 0

h

v = ?