Rotational Dynamics Review

When will a force not apply a torque?

• The the force is directed towards or away from the axis of rotation.

• When the force is applied perpendicular to the lever arm.

• When the force is a gravitational force.

• When the force is a normal force.

Answer explanation

Forces towards or away from the AoR make an angle of 0 degrees with the lever arm, and therefore do not exert torques on systems.

Think: can you spin a bike wheel by pushing towards the center of the wheel?

If the body is moving in a circle of radius r with a constant speed v, its angular velocity is

• v² / r

• vr

• v / r

• r / v

Klyde's angular velocity is

• the same as Bonnie's

• twice Bonnie's

• half of Bonnie's

• 1/4 Bonnie's

Answer explanation

Angular speed is the same for all points on a rotating rigid body.

What is happening to the ice skater as she brings her leg closer to her torso?

• Decreased Angular velocity, decreased rotational inertia

• Decreased Angular velocity, increased rotational inertia

• Increased Angular velocity, increased rotational inertia

• Increased Angular velocity, decreased rotational inertia

Answer explanation

Net external torque is 0Nm on the skater, so L is conserved.

Since L is Iw, as I decreases, w must increase so L remains constant.

Tags

NGSS.HS-PS2-1

The graph above shows the angular velocity of an object with respect to time.

During which time intervals is there a net torque on the object? Select all that apply.

• 0-2s

• 2-6s

• 6-8s

• 8-10s

Answer explanation

Net torques will cause angular accelerations.

Angular acceleration is the slope of a w vs. t graph.

The graph above shows the angular velocity of an object with respect to time.

During which time intervals is there a positive change in angular momentum of the object? Select all that apply.

• 0-2s

• 2-6s

• 6-8s

• 8-10s

A metal hoop is released from rest on an incline. The hoop then rolls without slipping down the incline.

Which of the following forces will cause a change in the hoops angular momentum as it rolls? Consider the axis of rotation to be the center of the hoop. Select all that apply.

• The normal force

• The gravitational force

• The static friction force

• The kinetic friction force

Answer explanation

Gravitational force and normal force are directed towards/away the axis of rotation.

Rolling without slipping means kinetic friction is not present.

Tags

NGSS.HS-PS2-1

A metal hoop is released from rest on an incline. The hoop then rolls without slipping down the incline.

Which of the following answer choices best describes the direction of the friction force on the hoop?

• Up the incline

• Down the incline

• Perpendicular away from the incline

• Perpendicular towards the incline

Tags

NGSS.HS-PS2-1

• The merry-go-round and the child.

• The child only.

• The merry-go-round only.

• You do not need to define a system to solve this problem.

Answer explanation

Since the merry-go-round and child are rotating together, it makes sense to analyze the system containing both objects.

Tags

NGSS.HS-PS2-1

NGSS.HS-PS2-2