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Introduction toUniform Circular Motion
Uniform Circular Motion
An object moves at uniform speed in a circle of constant radius.Uniform circular motion is accelerated motion. Why?
When a ball on the end of a string is swung in a vertical circle, the ball is accelerating because
A. the speed is changing.B. the direction is changing.C. the speed and the direction are changing.D. the ball is not accelerating.
Checking Understanding
Slide 6-13
AnswerWhen a ball on the end of a string is swung in a vertical circle, the ball is accelerating because
A. the speed is changing.B. the direction is changing.C. the speed and the direction are changing.D. the ball is not accelerating.
Slide 6-14
Centrifugal ForceIt’s a myth!We need to go back to Newton’s Laws to properly explain the feeling you get on a merry-go-round or in a turning car.
When a car accelerates
You, as a passenger, feel as if you are flung backward.Your inertia (mass) resists acceleration.You are NOT flung backward. Your body wants to remain at rest as the car accelerates forward.
When a car decelerates
You, as a passenger, feel as if you are flung forward.Your inertia (mass) resists the negative acceleration.You are NOT flung forward. Your body wants to remain in motion at constant velocity as the car accelerates backwards.
When a car turns
You feel as if you are flung to the outside. Your inertia resists acceleration.You are not flung out, your body simply wants to keep moving in straight line motion!
As a general rule
Whenever you feel you are flung in a certain direction, you can bet the acceleration is pointing in the opposite direction.Remember the elevator problems? When you feel you are flying up, acceleration of the elevator is down. When you feel you are sinking down, acceleration is up.
Acceleration in Uniform Circular Motion
The velocity vector at any given point is subjected to an acceleration that turns it, but does not speed it up or slow it down.The acceleration vector is always at right angles to the velocity.The acceleration points toward the center of the circle.
Acceleration in Uniform Circular Motion
The acceleration responsible for uniform circular motion is referred to as centripetal acceleration.
When a ball on the end of a string is swung in a vertical circle:
What is the direction of the acceleration of the ball?
A. Tangent to the circle, in the direction of the ball’s motion
B. Toward the center of the circle
Checking Understanding
Slide 6-15
AnswerWhen a ball on the end of a string is swung in a vertical
circle:What is the direction of the acceleration of the ball?
A. Tangent to the circle, in the direction of the ball’s motion
B. Toward the center of the circle
Slide 6-16
Centripetal Acceleration
• ac = v2/rac: centripetal
acceleration in m/s2
v: tangential speed in m/sr: radius in metersv
V = 2πr / T T=period
v
ac
v acac
v
Centripetal acceleration always points toward center of circle!
Force in Uniform Circular Motion
A force responsible for uniform circular motion is referred to as a centripetal force.Centripetal force is simply mass times centripetal acceleration.
Fc = mac
Centripetal Force•Fc = m ac
•Fc = m v2 / rFc: centripetal force in N
v: tangential speed in m/s
r: radius in meters
vFc
v Fc
Fc
vAlways toward center of circle!
More on Centripetal Force
Centripetal force is not a unique type of force.Centripetal forces always arise from other forces.You can always identify the real force which is causing the centripetal acceleration.Nearly any kind of force can act as a centripetal force.
Friction as centripetal force
As a car makes a turn, the force of friction acting upon the turned wheels of the car provide the centripetal force required for circular motion.
Example Problem A level curve on a country road has a radius of 150 m. What is the maximum speed at which this curve can be safely negotiated on a rainy day when the coefficient of friction between the tires on a car and the road is 0.40?
Slide 6-28
Tension as centripetal forceAs a bucket of water is tied to a string and spun in a circle, the force of tension acting upon the bucket provides the centripetal force required for circular motion.
Gravity as centripetal force
As the moon orbits the Earth, the force of gravity acting upon the moon provides the centripetal force required for circular motion.
Normal force as centripetal force
An automobile turning on a banked curve uses the normal force to provide the necessary centripetal force.
Example Problem
A curve on a racetrack of radius 70 m is banked at a 15 degree angle. At what speed can a car take this curve without assistance from friction?
Weight on a string moving in vertical
circleCentripetal force arises
from combination of tension and gravity.
Tennessee Tornado at Dollywood
Centripetal force when you are
upside down arises from a combination of normal force and
gravity.
Centripetal Force can do no work
A centripetal force alters the direction of the object without altering its speed. Since speed remains constant, kinetic energy remains constant, and work is zero.
The Universal Law of Gravity
Fg = Gm1m2/r2
Fg: Force due to gravity (N)G: Universal gravitational constant
6.67 x 10-11N m2/kg2
m1 and m2: the two masses (kg)r: the distance between the centers of the masses (m)
The Force of Gravity
Slide 6-35
Acceleration due to gravity
Fg = mg = GmME/r2
What is g equivalent to?
g = GME/r2
1) A typical bowling ball is spherical, weighs 16 kgs, and has a diameter of 8.5 m. Suppose two bowling balls are right next to each other in the rack. What is the gravitational force between the two—magnitude and direction?
2) What is the magnitude and direction of the force of gravity on a 60 kg person? (Mearth = 5.98x1024 kg, Rearth = 6.37 x 106 m)
Example Problems
Slide 6-36
Acceleration and distance
Planet Radius(m Mass (kg) g (m/s2)
Mercury 2.43 x 106 3.2 x 1023 3.61
Venus 6.073 x 106
4.88 x1024 8.83
Mars 3.38 x 106 6.42 x 1023 3.75
Jupiter 6.98 x 106 1.901 x 1027
26.0
Saturn 5.82 x 107 5.68 x 1026 11.2
Uranus 2.35 x 107 8.68 x 1025 10.5
Neptune 2.27 x 107 1.03 x 1026 13.3
Pluto 1.15 x 106 1.2 x 1022 0.61
Kepler’s Laws
1. Planets orbit the sun in elliptical orbits.
2. Planets orbiting the sun carve out equal area triangles in equal times.
3. The planet’s year is related to its distance from the sun in a predictable way.
Kepler’s Laws
Satellites
Orbital Motion• Gmems / r2 = mev2 / r =
• The mass of the orbiting body does not affect the orbital motion!
Consider the see saw
Consider the see saw
Consider the see saw
Consider the see sawThe weight of each child is a downward force that causes the see saw to twist.The force is more effective at causing the twist if it is greater OR if it is further from the point of rotation.
Consider the see saw
The twisting force, coupled with the distance from the point of rotation is called a torque.
What is Torque?
Torque is a “twist” (whereas force is a push or pull).Torque is called “moment” by engineers.The larger the torque, the more easily it causes a system to twist.
Torque
r
F
Hinge (rotates)
Direction of rotation
Consider a beam connected to a wall by a hinge.
Now consider a force F on the beam that is applied a distance r from the hinge. What happens? A rotation occurs due to
the combination of r and F. In this case, the direction is clockwise.
Torque
= F r sin is torqueis forcer is “moment arm” is angle between F and r
r
F
Hinge (rotates)
Direction of rotation
If we know the angle the force acts at, we can calculate torque!
Torque equation: simplified
If is 90o… = F r is torqueF is forcer is “moment arm
r
F
Hinge: rotates
Direction of rotation
We use torque every day
Consider the door to the classroom. We use torque to open it.Identify the following:The point of rotation.The moment arm.The point of application of force.
Question
Why is the doorknob far from the hinges of the door? Why is it not in the middle of the door? Or near the hinges?
Torque Units
What are the SI units for torque?mN or Nm.
Can you substitute Joule for Nm?No. Even though a Joule is a Nm, it is
a scalar. Torque is a vector and cannot be ascribed energy units.
Now consider a balanced situation
If the weights are equal, and the moment arms are equal, then the clockwise and counterclockwise torques are equal and no net rotation will occur. The kids can balance!
40 kg 40 kg
Now consider a balanced situation
ccw = cw
This is called rotational equilibrium!
40 kg 40 kg
Periodic Motion
• Repeats itself over a fixed and reproducible period of time.
• Mechanical devices that do this are known as oscillators.
An example of periodic motion…
• A weight attached to a spring which has been stretched and released.
• If you were to plot distance the vs time you would get a graph that resembled a sine or cosine function.
3
-3
2 4 6 t(s)
x(m)
An example of periodic motion…
Simple Harmonic Motion (SHM)
• Periodic motion which can be described by a sine or cosine function.
• Springs and pendulums are common examples of Simple Harmonic Oscillators (SHOs).
Equilibrium
• The midpoint of the oscillation of a simple harmonic oscillator.
• Position of minimum potential energy and maximum kinetic energy.
All oscillators obey…
Law of Conservation
of Energy
Amplitude (A)
• How far the wave is from equilibrium at its maximum displacement.
• Waves with high amplitude have more energy than waves with low amplitude.
Period (T)
• The length of time it takes for one cycle of periodic motion to complete itself.
Frequency (f):
• How fast the oscillation is occurring.
• Frequency is inversely related to period.
• f = 1/T• The units of frequency is the
Herz (Hz) where 1 Hz = 1 s-1.
3
-3
2 4 6 t(s)
x(m)
A
T
Parts of a Wave
Equilibrium point
Springs
A very common type of Simple Harmonic Oscillator.Our springs will be ideal springs.They are massless.They are compressible and
extensible.
They will follow a law known as Hooke’s Law.
Restoring force
The restoring force is the secret behind simple harmonic motion.
The force is always directed so as to push or pull the system back to its equilibrium (normal rest) position.
Hooke’s LawA restoring force directly
proportional to displacement is responsible for the motion of a
spring.
F = -kxwhere
F: restoring forcek: force constant
x: displacement from equilibrium
Hooke’s Law
The force constant of a spring can be determined by attaching a weight and seeing how far it stretches.mg
Fsm
Fs = -kx
Hooke’s Law
m
m
m xF
x
F
F = -kxSpring compressed, restoring force out
Spring at equilibrium, restoring force zero
Spring stretched, restoring force in
Equilibrium position
Period of a spring
T = 2m/kT: period (s)m: mass (kg)k: force constant (N/m)
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Pendulums
The pendulum can be thought of as an oscillator.The displacement needs to be small for it to work properly.
Pendulum Forces
T
mg
mg sin
Period of a pendulum
T = 2l/gT: period (s)l: length of string (m)g: gravitational acceleration (m/s2)
