1. Uniform circular motion and Universal Gravitation
Unit 4 Notes
Uniform circular motion and Universal Gravitation

2. Centripetal Acceleration & Force

3. Centripetal Acceleration & Force
This unit we will investigate the special case of kinematics and dynamics of objects in uniform circular motion.
First let’s consider a mass on a string being twirled in a horizontal circle at a constant speed.

4. Centripetal Acceleration & Force
Let’s determine the speed of the object.
Remember that speed is defined as: _________________________
We define the period of motion (T) as the time it takes to complete one rotation. How far does it travel in one rotation?
We can find the circumference of the circular path (distance traveled) by: ________
Therefore the speed of an object in uniform circular motion is:
𝑣= ∆𝑥 𝑡
Circumference
𝐶=2𝜋𝑟
𝑣= 2𝜋𝑟 𝑇

5. Example
A plane makes a complete circle with a radius of 3622 m in 2.10 min. What is the speed of the plane?
𝑟=3622 𝑚
𝑣= 2𝜋𝑟 𝑇
𝑇= 2.10 𝑚𝑖𝑛 60 𝑠 1 𝑚𝑖𝑛
=126 𝑠
𝑣= (2)(𝜋)(3622 𝑚) 126 𝑠
𝑣=180.6 𝑚/𝑠

6. Centripetal Acceleration & Force
Ok so we’ve figured out its speed, but is the mass accelerating?
Remember that the mass is traveling at a constant speed. However, acceleration is defined as:
So how does the velocity of the mass change with respect to time? v
𝑎= ∆𝑣 𝑡 ac

7. Centripetal Acceleration & Force
Notice that the direction of the velocity at any time is…
So even though it may be traveling at a anything traveling in a circular path is
because the of its velocity is always changing.
The acceleration of an object in uniform circular motion is:
Tangent to the circle
Constant Speed
accelerating
direction
𝑎 𝑐 = 𝑣 2 𝑟 = 4 𝜋 2 𝑟 𝑇 2

8. Centripetal Acceleration & Force
Whenever an object is accelerated, there must be a…
This force is known as centripetal force, Fc. This is not a new force, it is simply the net force that accelerates an object toward the center of its circular path.
Net Force

9. Examples
A mass is twirled in a circle at the end of a string, the centripetal force is provided by…
When a car rounds a corner on a highway, the centripetal force is provided by…
When the Moon orbits the Earth, the centripetal force is provided by…
Tension
Friction
Gravity

10. Centripetal Acceleration & Force
Newton’s 2nd Law can help us determine a formula for centripetal force:
𝐹 𝑐 =𝑚 𝑎 𝑐 = 𝑚 𝑣 2 𝑟 = 4 𝜋 2 𝑟 𝑇 2

11. Example
A skater travels at 2.0 m/s in a circle of radius 4.0 m. What is her centripetal acceleration?
𝑟=4.0 𝑚
𝑎 𝑐 = 𝑣 2 𝑟
𝑣=2.0 𝑚 𝑠
𝑎 𝑐 =
𝑎 𝑐 =?
𝑎 𝑐 =1.0 𝑚 𝑠 2

12. Example
What is the centripetal acceleration of the Moon towards Earth?
𝑟=3.84× 10 8 𝑚
𝑎 𝑐 = 4 𝜋 2 𝑟 𝑇 2
𝑇=2.36× 10 6 𝑠
𝑎 𝑐 = (4)( 𝜋 2 )(3.84× 10 8 ) (2.36× ) 2
𝑎 𝑐 =?
𝑎 𝑐 =2.72× 10 −3 𝑚/ 𝑠 2

13. Example
A car traveling at 14 m/s goes around an unbanked curve in the road that has a radius of 96 m. What is the car’s centripetal acceleration?
𝑟=96𝑚
𝑎 𝑐 = 𝑣 2 𝑟
𝑣=14𝑚/𝑠
𝑎 𝑐 =
𝑎 𝑐 =?
𝑎 𝑐 =2.04 𝑚/ 𝑠 2

14. Example
What is the minimum coefficient of friction between the road and the car’s tires?
Combine
𝐹 𝑐 = 𝐹 𝑓 =𝑚 𝑎 𝑐
𝐹 𝑓 =𝜇𝑚𝑔
𝑚 𝑎 𝑐 =𝜇𝑚𝑔
𝑎 𝑐 =𝜇𝑔
𝜇= 𝑎 𝑐 𝑔 =
=0.21

15. Centripetal Acceleration & Force
seeking
One last note on a little thing called centrifugal force. While centripetal means center-___________________ centrifugal means center- _______________________.
An inertial frame of reference is a one where Newton’s Law’s ________ ____________.
In an inertial frame of reference, centrifugal force is actually an apparent force - it does not exist. It is simply the apparent force that causes a revolving or rotating object to move in a straight line.
However, Newton’s First Law tells us that we do not need a force to keep an object moving in a straight line, you only need a force to deflect an object from moving in a straight line.
fleeing
Are true

16. Example
When riding in the backseat of a car that is turning a corner, you slide across the seat, seeming to accelerate outwards, away from the center of the turning circle.
Explain why the force in this case is actually working towards the center of the turn and not outwards.
1st Law – Inertia makes it feel like a force is pushing outward
- The car is actually pulling you inward