# Rotational Motion Flashcards Preview

## Harris Academy AH Physics > Rotational Motion > Flashcards

Flashcards in Rotational Motion Deck (39)
1
Q

What is meant by Torque (sometimes called the moment of a force)?

A

Torque is the turning effect of a force on a rotating object.

2
Q

What relationships would you use to convert between:

1. Linear displacement and angular displacement
2. Linear velocity and angular velocity
3. Linear acceleration and angular acceleration
A

For any of these relationships, you would multiply the angular quantity by the radius of circular motion to get the linear quantity:

3
Q

What does the prefix f stand for?

A

f = femto = x 10-15

4
Q

How would you find the instantaneous velocity of an object from a displacement-time graph such as this?

A

Take the gradient of the line.

If the graph is a curve, take the gradient of the tangent to the curve at a point.

5
Q

Which two things does the moment of inertia of an object depend on?

A
1. The mass of the object
2. The distribution of mass around a given axis of rotation (which can be quantified with the radius, r)
6
Q

What does the prefix M stand for?

A

M = mega = x 106

7
Q

What is the unit of angular displacement?

A

8
Q

What does the prefix G stand for?

A

G = giga = x 109

9
Q

What does the prefix m stand for?

A

m = milli = x 10-3

10
Q

How is angular acceleration defined in terms of angular velocity?

A

Angular acceleration is defined as the rate of change of angular velocity.

11
Q

What is the unit of moment of inertia?

A

kg m2

12
Q

What does the prefix p stand for?

A

p = pico = x 10-12

13
Q

How is angular velocity defined in terms of angular displacement?

A

Angular velocity is defined as the rate of change of angular displacement:

14
Q

What is the unit of Torque?

A

newton-metres (Nm)

15
Q

What is the unit of angular acceleration?

A

16
Q

What does the prefix k stand for?

A

k = kilo = x 103

17
Q

What does the prefix P stand for?

A

P = peta = x 1015

18
Q

What is the unit of centripetal acceleration?

A

ms-2

19
Q

What is the unit of angular velocity?

A

20
Q

A

21
Q

The following experimental setup can be used to determine the moment of inertia of a disc which is allowed to rotate on a frictionless air bearing turntable.

Explain the measurements which will need to be taken, and how to use these to determine the moment of inertia of the disc.

A

Find the torque applied to the disc by using the relationship T = Fr.

Use the light gate to measure the angular acceleration of the disc.

Vary the hanging mass to vary the torque, and repeat a few times.

Plot T against α, and take the gradient of the straight line which will be equal to the moment of inertia of the disc.

22
Q

What is the definition of moment of inertia of an object?

A

The moment of inertia of an object is a measure of its resistance to angular acceleration about a given axis.

23
Q

How would you find the instantaneous acceleration of an object from a velocity-time graph such as this?

A

Take the gradient of the line.

If the graph is a curve, take the gradient of the tangent to the curve at a point.

24
Q

Which two quantities does the rotational kinetic energy of an object depend on?

A

Moment of Inertia and angular velocity

25
Q

What does the prefix T stand for?

A

T= tera = x 1012

26
Q

What is the unit of angular momentum?

A

kilogram metres squared per second

(kg m2 s-1)

(kg m2 rad s-1 also acceptable)

27
Q

Explain the difference between angular, tangential and centripetal acceleration.

A
• Angular acceleration is the rate of change of angular velocity
• Tangential acceleration is the rate of change of tangential speed
• Centripetal acceleration is the rate of change of tangential velocity- this causes the direction of the object to change and results in circular motion.
28
Q

What does the prefix µ stand for?

A

µ = micro = x 10-6

29
Q

A ‘conical pendulum’ is shown below.

In terms of the components of tension in the string determine expressions for the:

1. Centripetal force
2. Weight of the ‘bob’.
A
1. Centripetal force = T sinθ
2. Weight = T cosθ
30
Q

In a ‘loop the loop’ ride at a funfair, how would you determine the minimum speed required to complete a full loop?

A

The minimum speed required is when there is no reaction force acting on the ‘car’, so.

Centripetal force = Weight + Reaction force

simplifies to

Centripetal force = Weight

mv2/r = mg

v = √gr

31
Q

An ice skater is spinning with his arms outward as shown.

He then pulls his arms inward.

Explain what happens to his angular momentum and rotational kinetic energy.

A

Angular momentum constant because angular momentum is always conserved

Rotational kinetic energy increases because angular velocity increases.

32
Q

What is the unit of tangential acceleration?

A

ms-2

33
Q

A ball of weight W attached to a string is ‘whirling’ in a horizontal circle as shown below.

What provides the centripetal force acting on the ball?

A

The centripetal force is suppled by the tension, T only.

T is the only force acting into the centre.

34
Q

A ball of weight W attached to a string under tension T is ‘whirling’ in a vertical circle as shown below.

Determine a relationship for the centripetal force acting on the ball:

1. at the bottom.
2. at the sides.
3. at the top.
A
1. Centripetal force = Tension - Weight
2. Centripetal force = Tension
3. Centripetal force = Tension + Weight
35
Q

An expression for the displacement of an object with respect to time is given as

s = 3.1t2 + 4.1t + 6

Determine expressions for the object’s velocity and acceleration.

A

v = 6.2t + 4.1

a = 6.2 (ms-2)

36
Q

Describe how to convert an angular velocity in rpm (revolutions per minute) into rad s-1 (radians per second).

A

revolutions per minute → radians per second:

× 2π then ÷ 60

37
Q

State the law of conservation of angular momentum.

A

In the absence of external torques, the total angular momentum of object(s) before a collision is equal to the total angular momentum of the object(s) after the collision.

38
Q

What does the prefix n stand for?

A

n = nano = x 10-9

39
Q