Momentum Conservation of Force. Impulse and Momentum Impulse and Momentum.

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Transcript of Momentum Conservation of Force. Impulse and Momentum Impulse and Momentum.
Momentum
Conservation of Force
Impulse and Momentum
Impulse and Momentum
Impulse and Momentum
The right side of the equation FΔt = mΔv, mΔv involves the change in velocity: Δv = vf − vi
Therefore, mΔv = mvf − mvi
The product of the object’s mass, m, and the object’s velocity, v, is defined as the momentum of the object. Momentum is measured in kg·m/s. An object’s momentum, also known as linear momentum, is represented by the following equation:
Impulse and Momentum
p = mv
Impulse and Momentum
Because mvf = pf and mvi = pi:
pf − pi, describes change in momentum of an object
The right side of this equation, pf − pi, describes the change in momentum of an object. Thus, the impulse on an object is equal to the change in its momentum, which is called the impulsemomentum theorem
The impulsemomentum theorem is represented by the following equation:
Impulse and Momentum
FΔt = mΔv = pf − pi
FΔt = pf − pi
Impulse and Momentum Impulse and Momentum
If the force on an object is constant, the impulse is the product of the force multiplied by the time interval over which it acts
Because velocity is a vector, momentum also is a vector
Similarly, impulse is a vector because force is a vector
This means that signs will be important for motion in one dimension
Impulse and Momentum ImpulseMomentum Theorem
Look at the change in momentum of a baseball. The impulse, that is the area under the curve, is approximately 13.1 N·s. The direction of the impulse is in the direction of the force. Therefore, the change in momentum of the ball is also 13.1 N·s
Impulse and Momentum ImpulseMomentum Theorem
What is the final momentum of ball after collision if initial momentum is 5.5 kg m/s?
What is the baseball’s final velocity?
Impulse and Momentum Angular Momentum
The angular velocity of a rotating object changes only if torque is applied to it
This is a statement of Newton’s law for rotating motion, τ = IΔω/Δt
This equation can be rearranged in the same way as Newton’s second law of motion was, to produce τΔt = IΔω
The left side of this equation is the angular impulse of the rotating object and the right side can be rewritten as Δω = ωf− ωi
Impulse and Momentum Angular Momentum
The angular momentum of an object is equal to the product of a rotating object’s moment of inertia and angular velocity. Angular momentum is measured in kg·m2/s
L = Iω
Impulse and Momentum Angular Momentum
Just as the linear momentum of an object changes when an impulse acts on it, the angular momentum of an object changes when an angular impulse acts on it
Thus, the angular impulse on the object is equal to the change in the object’s angular momentum, which is called the angular impulseangular momentum theorem
The angular impulseangular momentum theorem is represented by the following equation:
τΔt = Lf − Li
Impulse and Momentum Angular Momentum
If there are no forces acting on an object, its linear momentum is constant
If there are no torques acting on an object, its angular momentum is also constant
Because an object’s mass cannot be changed, if its momentum is constant, then its velocity is also constant
Impulse and Momentum Angular Momentum
In the case of angular momentum, however, the object’s angular velocity does not remain constant.
This is because the moment of inertia
depends on the object’s mass and the
way it is distributed about the axis of
rotation or revolution
Thus, the angular velocity of an object
can change even if no torques are acting
on it
Impulse and Momentum Angular Momentum
The diver uses the diving board to apply an external torque to her body
Then, the diver moves her center of mass in front of her feet and uses the board to give a final upward push to her feet
This torque acts over time, Δt, and thus increases the angular momentum of the diver
Before the diver reaches the water,
she can change her angular velocity
by changing her moment of inertia.
She may go into a tuck position,
grabbing her knees with her hands
Impulse and Momentum Angular Momentum
By moving her mass closer to the axis of rotation, the diver decreases her moment of inertia and increases her angular velocity
When she nears the water, she stretches her
body straight, thereby increasing the moment
of inertia and reducing the angular velocity
As a result, she goes straight into the water
Impulse and Momentum Angular Momentum
Impulse and Momentum Impulse and Momentum
A golfer uses a club to hit a 45 g golf ball resting on an elevated tee, so that the golf ball leaves the tee at a horizontal speed of 38 m/s.
What is the impulse on the golf ball?
What is the average force that the club exerts on the golf ball if they are in contact for 2.0 x 103 s?
What average force does the golf ball exert on the club during this time interval?
Impulse and Momentum Impulse and Momentum
A single uranium atom has a mass of 3.97 x 1025 kg. It decays into the nucleus of a thorium atom by emitting an alpha particle at a speed of 2.10 x 107 m/s. The mass of an alpha particle is 6.68 x 1027 kg. What is the recoil speed of the thorium nucleus?
Two cars enter an icy intersection. Car 1, with a mass of 2.50 x 103 kg, is heading east at 20.0 m/s, and car 2, with a mass of 1.45 x 103 kg is going north at 30.0 m/s. The two vehicles collide and stick together. What is the speed and direction of the cars as they skid away together just after colliding?
Conservation of Momentum TwoParticle Collisions
Conservation of Momentum Momentum in a Closed, Isolated System
Under what conditions is momentum of system of two balls conserved?
First condition: no balls lost and no balls gained. Closed system: one which does not gain or lose mass
Second condition: forces internal (no forces acting on system by objects outside it)
When net external force on closed system zero, system is an isolated system
Impulse and Momentum Momentum in a Closed, Isolated System
No system on Earth absolutely isolated, because always some interactions between system and its surroundings
Often interactions small enough to be ignored
Systems contain any number of objects, and objects stick together or come apart in collision
Law of conservation of momentum: momentum of any closed, isolated system does not change
Conservation of Momentum Momentum in a Closed, Isolated System
A 1875kg car going 23 m/s rearends a 1025kg compact car going 17 m/s on ice in the same direction. The two cars stick together. How fast do the two cars move together immediately after the collision?
Conservation of Momentum Recoil
The momentum of a baseball changes when the external force of a bat is exerted on it. The baseball, therefore, is not an isolated system
On the other hand, the total momentum of two colliding balls within an isolated system does not change because all forces are between the objects within the system
Conservation of Momentum Recoil
Assume that a girl and a boy are skating on a smooth surface with no external forces. They both start at rest, one behind the other. Skater C, the boy, gives skater D, the girl, a push. Find the final velocities of the two inline skaters
Conservation of Momentum Recoil
After clashing with each other, both skaters are moving, making this situation similar to that of an explosion. Because the push was an internal force, you can use the law of conservation of momentum to find the skaters’ relative velocities
The total momentum of the system was zero before the push. Therefore, it must be zero after the push
Conservation of Momentum Recoil
Before
pCi + pDi = pCf + pDf
0 = pCf + pDf
pCf = −pDf
mCvCf = −mDvDf
After
Conservation of Momentum Recoil
Are the skaters’ velocities equal and opposite?
The last equation, for the velocity of skater C, can be rewritten as follows:
Velocities depend on skaters’ relative masses. Less massive skater moves at greater velocity
Conservation of Momentum Propulsion in Space
How does a rocket in space change its velocity?
Rocket carries both fuel and oxidizer. When the fuel and oxidizer combine in rocket motor, resulting hot gases leave exhaust nozzle at high speed
Rocket and chemicals are closed system
Forces that expel gases internal forces, so system also isolated
Thus, objects in space accelerate using law of conservation of momentum and Newton’s third law of motion
Conservation of Momentum Propulsion in Space
Deep Space 1 performed flyby of asteroid Braille in 1999
Had ion engine that exerted as much force as sheet of paper resting on person’s hand
In ion engine, xenon atoms expelled at speed of 30 km/s, produced force of only 0.092 N. Runs continuously for days, weeks, or months
Impulse delivered large enough to increase momentum
Conservation of Momentum TwoDimensional Collisions
Two billiard balls system
Original momentum of moving ball pCi and momentum of the stationary ball zero
Momentum of system before collision
equal to pCi
After collision, both billiard balls
moving and have momenta
If friction ignored, system closed and
isolated
Conservation of Momentum TwoDimensional Collisions
Law of conservation of momentum used
Initial momentum equals vector sum of final momenta. So:
Components of vectors before and after collision equal. Xaxis in direction of initial momentum. Ycomponent of initial momentum zero
Sum of final ycomponents also zero
Sum of horizontal components equal to initial momentum
pCi = pCf + pDf
pCf, y + pDf, y = 0
pCi = pCf, x + pDf, x
Conservation of Momentum Conservation of Angular Momentum
Like linear momentum, angular momentum can be conserved
The law of conservation of angular momentum states that if no net external torque acts on an object, then its angular momentum does not change
Initial angular momentum equal final angular momentum
Earth’s angular momentum constant and conserved. So, length of a day does not change
Lf = Li
Conservation of Momentum Conservation of Angular Momentum
When ice skater pulls in arms, he begins spinning faster
Without external torque, angular momentum does not change; L = Iω constant
Increased angular velocity makes decreased moment of inertia
By pulling arms close to body, iceskater brings more mass closer to axis of rotation, decreasing radius of rotation and decreasing his moment of inertia
Li = Lf
so, Iiωi = Ifωf
Conservation of Momentum Conservation of Angular Momentum
Frequency is f = ω/2π, so:
Conservation of Momentum Conservation of Angular Momentum
If torquefree object starts with no angular momentum, must continue with no angular momentum
Thus, if part of an object rotates in one direction, another part must rotate in the opposite direction
For example, if you switch on a loosely held electric drill, the drill body will rotate in the direction opposite to the rotation of the motor and bit
Conservation of Momentum Tops and Gyroscopes
Because of conservation of angular momentum, direction of rotation of a spinning object can be changed only by applying a torque
When a top is vertical, there is no
torque on it, and the direction of
its rotation does not change
If the top is tipped a torque tries
to rotate it downward. Rather than
tipping over, however, the upper end
of the top revolves, or precesses
slowly about the vertical axis
Conservation of Momentum Tops and Gyroscopes
Gyroscope – wheel or disk that spins rapidly around one axis while being free to rotate around one or two other axes
Direction of large angular momentum changed only by applying appropriate torque
Without torque, direction of axis
of rotation does not change
Conservation of Momentum Tops and Gyroscopes
Gyroscopes are used in airplanes, submarines, and spacecraft to keep an unchanging reference direction
Giant gyroscopes are used in cruise ships to reduce their motion in rough water. Gyroscopic compasses, unlike magnetic compasses, maintain direction even when they are not on a level surface
Conservation of Momentum Momentum
At 9.0 s after takeoff, a 250 kg rocket attains a vertical velocity of 120 m/s.
What is the impulse on the rocket?
What is the average force on the rocket?
What is its altitude?
Conservation of Momentum Momentum
In a circus act, a 18 kg dog is trained to jump onto a 3.0 kg skateboard moving with a velocity of 0.14 m/s. At what velocity does the dog jump onto the skateboard if afterward the velocity of the dog and skateboard is 10.0 m/s?