Circular Motion

Circular motion requires an inward force, centripetal force, to create a change in direction resulting in a circular pattern.

Two Types of Circular Motion

A rotation is when an object spins around an internal axis.  A revolution is when an object turns around an external point beyond the axis.  See how the stickman is revolving around the central axis.  The axis is a central point around which rotation takes place.

rotation revolution

Tangential Velocity

The tangent is a straight line that would just touch the curve of the circle as seen in the animation.

Because velocity is a vector, it includes magnitude and direction.  Since direction changes, circular motion is considered accelerated.  So will use the term tangential velocity to represent what the velocity would be if released.  If you lost the centripetal force the object would continue in a straight line tangent to the circle as seen.

Tangent to Circle

Circular Motion is the Result of Centripetal Force

Newton's First Law of Motion, inertia, states that an object moving would continue in a straight line path.  To have circular motion an inward force is necessary to create the continuous turning.  Centripetal Force is a force towards the center of a circle keeping it in a circular path.

Centripetal Force Equation Variables

Variable: Name (Unit)

  • FC: Centripetal Force (N)
  • m: Mass (kg)
  • v: Tangential Velocity (m/s)
  • r: Radius (m)
types of centripetal force

What could provide the centripetal force causing circular motion?

  • A rope: Fc = FT tension in the rope
  • Gravity: Fc = Fg gravity
  • Friction: Fc = Ff friction

Important Note: The problems in this unit may not state centripetal force directly.  They may ask what is the tension, gravity, or friction keeping an object in circular motion.  If stated this way, solve for centripetal force (Fc).

Centripetal Acceleration (ac)

Acceleration is a vector which includes magnitude and direction.  Changing either direction, speed, or both is considered acceleration.  So going from 10 m/s north to 10 m/s east would require an acceleration.

In circular motion it’s centripetal force (Fc) that causes a centripetal acceleration (ac) seen in the equation (FC = maC).

The variable for centripetal acceleration is ac and its standard unit like other accelerations is m/s2.

When you plug in the centripetal acceleration equation aC = (v2/r) into (FC = maC) it becomes our standard centripetal force equation (FC = mv2/r) as seen on most equation sheets.

centripetal acceleration

Tangential Velocity Equation

Tangential velocity is calculated by taking the circular distance divided by the time it takes to go around the circle or period (T).  The distance around the circle is found by calculating the circumference of a circle.

2πr = circle circumference (unit: meters (m))

T = Period: Time it takes for one rotation (unit: seconds (s))

 

tangental velocity

What is a Period?

Period (T) is the time it takes for a specific motion to occur.  See in the animation how this could be one swing, rotation, wave, segment, or osculation.  In this unit it will be a rotation.

period

Period and Frequency

A period (T) is the amount of time per cycle.

A frequency (f) is the number of cycles per time.  The unit is the hertz (Hz) which means cycles per second.

You can find either with cycles and time or the other.  If you have frequency just take the inverse of it.  If you have period you can take the inverse of it to find frequency as seen in the animation.

period and frequency

A greater radius gives an object a greater velocity when having the same period.  You can analyze the equations in this unit along with any other using the rule of onesClick here if you want a reminder on how to use this.  Notice in the image that the fastest circle is green which is the furthest out and the slowest is red.

greater radius greater tangental velocity

Centripetal Force Example Problems

1. What would happen to the centripetal force required to keep an object going in a circle if the radius of a circle was doubled?

circular motion problem 1
(Click on the picture to enlarge it)

2. How many times the centripetal force would you have if a car slowed from 60 mph to 20 mph going around a curve?

60 -> 20 mph = 20/60 = 1/3 the speed

circular motion problem 2

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3. Kent spins in his chair with a frequency of 0.5 Hz. What is the period of his spin?

F = 0.5 Hz

T = 1/f =1/0.5 = 2 s

(Click on the picture to enlarge it)

4. A record takes 1.3 s to make one complete rotation. An object on this record is 0.12 m from the center.  What is its velocity?

T = 1.3 s

r = 0.12 m

V = ?

circular motion problem 3

(Click on the picture to enlarge it)

5a. The pilot of a 60,500 kg jet plane is flying in circles whose radius is 5.00 x 104 m.  It takes 1.8 x 103 s to make one rotation.  What is the velocity of the plane?

m = 60,500 kg

r =  5.00 x 10m

T = 1.8 x 103 s

circular motion problem 5a

(Click on the picture to enlarge it)

5b. How much centripetal force would there be?

m = 60,500 kg

r =  5.00 x 10m

T = 1.8 x 103 s

v =175 m/s

circular motion problem 5b

(Click on the picture to enlarge it)

6. What is the centripetal acceleration of a bike traveling a tangential speed of 8 meters per second in a circle that has a radius of 5 meters?

aC = ?

v = 8 m/s

r = 5 m

circular motion problem 6

(Click on the picture to enlarge it)

(Click Here for All Circular Motion Problems and Solutions)

Circular Motion Practice Quiz

240

Circular Motion Quiz

1 / 10

What happens to tangential velocity if you increase the radius?
It _________

2 / 10

The time it takes for one rotation is the ___________.

3 / 10

The number of rotations per time is the ___________.

4 / 10

The inward force that causes circular motion is _________________.

5 / 10

ball on string

How would a ball travel if you were spinning a ball on a rope above your head and the rope broke?

6 / 10

Why would a coffee cup sitting on your car fall off when taking a quick a turn?

7 / 10

What will happen to the centripetal force required to keep an object going in a circle if the mass of the object is doubled and the radius of the circle is cut in half?

It would become ___________ times the original.

8 / 10

What will happen to the centripetal force required to keep an object going in a circle if mass, radius, and velocity are all doubled?

It would become ___________ times the original.

9 / 10

What is the tangential velocity of a 50 kg girl is bicycling in circles with a radius of 5.00 m when it takes 12 s to make one rotation?

10 / 10

What is the centripetal force of a 50 kg girl is bicycling in circles with a radius of 5.00 m when it takes 12 s to make one rotation?

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