Rotational Motion 2

Rotational Motion 2

Fill in the Blanks

What causes Angular Acceleration?

To make something accelerate you need a ___ _____.

Likewise, to produce an angular acceleration requires a net torque.

Fill in the Blanks

Torque is the turning effect of a force

It depends on:

The size of the _____

and the distance to the _____

Multiple Choice

The see saw is balanced so the two torques are equal and opposite. Min is 75kg. Calculate Max’s mass.

• 100 kg

• 110 kg

• 113 kg

• 97 kg

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Rotational Inertia

We know that to change an object’s linear velocity requires a force.

Likewise to change an object’s angular velocity requires a torque.

Mass, or inertia means “how hard it is to change the velocity of an object”.

The rotational equivalent of mass is rotational inertia.

It means “how hard it is to change the _______ ________ of an object”.

Fill in the Blanks

A bicycle wheel is much ______ to spin than a car wheel because the bicycle wheel has less rotational inertia.

A ferris wheel is much ______ to stop spinning than a fan because the ferris wheel has more rotational inertia.

Multiple Choice

Explain which has more rotational inertia a large 250 g metal can or a small 250g metal can.

• Large metal can, its mass is further from the centre

• Small metal can, its mass is closer to the centre

• They are the same as they have the same mass

Fill in the Blanks

Explain in terms of energy why a larger wheel has more rotational inertia than a smaller wheel of the same mass.

If the wheel has a larger radius, the rim has a greater _____ for the same angular velocity and therefore more kinetic energy. More ____ is needed to give this extra kinetic energy. Hence more work is done against a larger rotational inertia in the larger wheel.

Fill in the Blanks

In summary we can say that rotational inertia depends on two things:

1) mass

2)

of mass from centre

Multiple Select

Explain why a ring is harder to spin than a solid disc (of same mass and radius)

• the ring has its mass further from the center (on average)

• the solid disc has its mass further from the center (on average)

• The ring has a smaller rotational inertia

• The ring has a larger rotational inertia

Fill in the Blanks

The equation that relates mass and acceleration for objects that are moving in straight lines is:

 $F_{net}=ma$  
The equivalent equation for rotational motion is:  $\tau=I\alpha$  
where I is ___________  _______

Multiple Select

Hannah has a bike wheel with a mass of 250 g and a radius of 74 cm.

Estimate the rotational inertia. What assumptions are you making?

• I = 0.14 kgm²

• assuming all mass is at the rim

• I = 0.19 kgm²

• assuming mass is evenly distributed

Multiple Choice

A bike wheel with a rotational inertia of 0.23 kgm² is initially moving with ω = 34 rads^-1. A force of 5.0 N is applied at 0.55 m from the centre of rotation until the wheel is brought to rest. What is the size of the angular acceleration and how long does it take to stop?

• α = 12 rads^-2 and t = 2.8 s

• α = 2.9 rads^-2 and t = 2.9 s

• α = 16 rads^-2 and t = 5.0 s