A 300 Car Travels at 11m/s What Is Its Momentum
9 Linear Momentum and Collisions
9.5 Collisions in Multiple Dimensions
Learning Objectives
By the finish of this section, you lot will be able to:
- Express momentum as a two-dimensional vector
- Write equations for momentum conservation in component form
- Calculate momentum in two dimensions, as a vector quantity
Information technology is far more than common for collisions to occur in two dimensions; that is, the bending between the initial velocity vectors is neither zero nor
. Let's encounter what complications arise from this.
The first idea we need is that momentum is a vector; like all vectors, it can be expressed as a sum of perpendicular components (usually, though not e'er, an x-component and a y-component, and a z-component if necessary). Thus, when nosotros write downward the statement of conservation of momentum for a trouble, our momentum vectors can be, and usually will be, expressed in component form.
The 2d idea we demand comes from the fact that momentum is related to forcefulness:
Expressing both the force and the momentum in component class,
Call up, these equations are merely Newton'south 2d law, in vector form and in component class. We know that Newton'south second law is truthful in each direction, independently of the others. Information technology follows therefore (via Newton's third law) that conservation of momentum is also true in each direction independently.
These two ideas motivate the solution to two-dimensional problems: We write down the expression for conservation of momentum twice: once in the ten-direction and once in the y-direction.
This process is shown graphically in (Figure).
We solve each of these 2 component equations independently to obtain the 10– and y-components of the desired velocity vector:
(Hither, m represents the total mass of the system.) Finally, combine these components using the Pythagorean theorem,
Problem-Solving Strategy: Conservation of Momentum in Two Dimensions
The method for solving a two-dimensional (or even iii-dimensional) conservation of momentum problem is by and large the same as the method for solving a i-dimensional trouble, except that yous have to conserve momentum in both (or all three) dimensions simultaneously:
- Identify a closed system.
- Write downwards the equation that represents conservation of momentum in the x-direction, and solve it for the desired quantity. If you are calculating a vector quantity (velocity, ordinarily), this will give y'all the x-component of the vector.
- Write downward the equation that represents conservation of momentum in the y-direction, and solve. This will give you the y-component of your vector quantity.
- Assuming you are calculating a vector quantity, apply the Pythagorean theorem to calculate its magnitude, using the results of steps 3 and 4.
Instance
Traffic Standoff
A small car of mass 1200 kg traveling east at 60 km/60 minutes collides at an intersection with a truck of mass 3000 kg that is traveling due due north at xl km/hr ((Figure)). The two vehicles are locked together. What is the velocity of the combined wreckage?
Strategy
Offset off, we need a closed system. The natural system to choose is the (machine + truck), just this organisation is not closed; friction from the road acts on both vehicles. We avoid this problem by restricting the question to finding the velocity at the instant just after the collision, so that friction has non yet had any effect on the arrangement. With that restriction, momentum is conserved for this system.
Since there are two directions involved, we exercise conservation of momentum twice: once in the x-direction and once in the y-direction.
SolutionBefore the collision the total momentum is
Later the collision, the wreckage has momentum
Since the system is closed, momentum must be conserved, so nosotros have
Nosotros have to be conscientious; the two initial momenta are not parallel. We must add together vectorially ((Figure)).
If we define the +ten-direction to point eastward and the +y-management to signal due north, as in the figure, and so (conveniently),
Therefore, in the x-direction:
and in the y-direction:
Applying the Pythagorean theorem gives
As for its direction, using the angle shown in the figure,
This bending is east of north, or
counterclockwise from the +ten-direction.
Significance
Every bit a practical matter, blow investigators usually piece of work in the "opposite direction"; they measure the distance of skid marks on the road (which gives the stopping distance) and apply the work-free energy theorem along with conservation of momentum to make up one's mind the speeds and directions of the cars prior to the standoff. We saw that analysis in an earlier section.
Check Your Understanding
Suppose the initial velocities were not at correct angles to each other. How would this modify both the physical result and the mathematical analysis of the collision?
[reveal-answer q="fs-id1167133455456″]Show Solution[/reveal-answer]
[hidden-respond a="fs-id1167133455456″]
Were the initial velocities not at correct angles, then one or both of the velocities would take to be expressed in component form. The mathematical analysis of the problem would be slightly more involved, but the physical issue would not change.
[/hidden-answer]
Example
Exploding Scuba Tank
A mutual scuba tank is an aluminum cylinder that weighs 31.7 pounds empty ((Figure)). When full of compressed air, the internal pressure is between 2500 and 3000 psi (pounds per square inch). Suppose such a tank, which had been sitting motionless, suddenly explodes into three pieces. The kickoff piece, weighing 10 pounds, shoots off horizontally at 235 miles per 60 minutes; the second piece (7 pounds) shoots off at 172 miles per 60 minutes, also in the horizontal plane, but at a
angle to the first piece. What is the mass and initial velocity of the tertiary piece? (Do all piece of work, and limited your concluding respond, in SI units.)
Strategy
To use conservation of momentum, we need a airtight organization. If we define the arrangement to be the scuba tank, this is not a closed system, since gravity is an external forcefulness. However, the problem asks for the just the initial velocity of the third slice, and then nosotros can fail the issue of gravity and consider the tank past itself as a closed arrangement. Detect that, for this system, the initial momentum vector is zero.
Nosotros choose a coordinate organisation where all the move happens in the xy-plane. Nosotros and then write down the equations for conservation of momentum in each direction, thus obtaining the x– and y-components of the momentum of the third piece, from which we obtain its magnitude (via the Pythagorean theorem) and its management. Finally, dividing this momentum by the mass of the third slice gives us the velocity.
Solution
First, let'south go all the conversions to SI units out of the way:
Now apply conservation of momentum in each direction.
x-management:
y-direction:
From our chosen coordinate system, we write the 10-components as
For the y-direction, we have
This gives the magnitude of
:
The velocity of the 3rd piece is therefore
The management of its velocity vector is the same as the management of its momentum vector:
Because
is beneath the
-axis, the actual angle is
from the +x-direction.
Significance
The enormous velocities here are typical; an exploding tank of any compressed gas can easily punch through the wall of a house and crusade pregnant injury, or death. Fortunately, such explosions are extremely rare, on a percentage ground.
Check Your Understanding
Discover that the mass of the air in the tank was neglected in the analysis and solution. How would the solution method inverse if the air was included? How large a departure do you think it would make in the final respond?
[reveal-answer q="fs-id1167133523735″]Show Solution[/reveal-answer]
[hidden-reply a="fs-id1167133523735″]
The volume of a scuba tank is virtually 11 L. Assuming air is an ideal gas, the number of gas molecules in the tank is
The average molecular mass of air is 29 grand/mol, so the mass of air independent in the tank is most 2.2 kg. This is about x times less than the mass of the tank, so it is condom to neglect it. Also, the initial force of the air pressure level is roughly proportional to the surface area of each piece, which is in turn proportional to the mass of each slice (assuming uniform thickness). Thus, the initial dispatch of each slice would change very little if we explicitly consider the air.
[/hidden-answer]
Summary
- The approach to ii-dimensional collisions is to choose a user-friendly coordinate system and interruption the motion into components along perpendicular axes.
- Momentum is conserved in both directions simultaneously and independently.
- The Pythagorean theorem gives the magnitude of the momentum vector using the x– and y-components, calculated using conservation of momentum in each direction.
Conceptual Questions
Momentum for a system tin be conserved in one direction while non existence conserved in some other. What is the angle between the directions? Give an example.
[reveal-answer q="fs-id1167132326318″]Prove Solution[/reveal-reply]
[hidden-reply a="fs-id1167132326318″]
The angle betwixt the directions must be 90°. Whatever arrangement that has nix internet external force in one management and nonzero net external strength in a perpendicular direction volition satisfy these weather condition.
[/hidden-answer]
Problems
A ane.lxxx-kg falcon is diving at 28.0 m/s at a down angle of
. Information technology catches a 0.650-kg dove from behind in midair. What is their combined velocity later bear on if the pigeon's initial velocity was 7.00 1000/s directed horizontally? Note that
is a unit vector pointing in the direction in which the hawk is initially flying.
[reveal-reply q="501216″]Prove Answer[/reveal-answer]
[hidden-reply a="501216″]22.i k/s at
below the horizontal[/hidden-answer]
A billiard ball, labeled 1, moving horizontally strikes another billiard ball, labeled 2, at rest. Before impact, brawl 1 was moving at a speed of 3.00 m/s, and afterwards impact it is moving at 0.50 thou/s at l° from the original direction. If the ii balls take equal masses of 300 g, what is the velocity of the ball two after the touch on?
A projectile of mass 2.0 kg is fired in the air at an angle of 40.0
to the horizon at a speed of 50.0 m/s. At the highest point in its flight, the projectile breaks into three parts of mass 1.0 kg, 0.7 kg, and 0.three kg. The one.0-kg part falls directly down after breakup with an initial speed of 10.0 g/due south, the 0.vii-kg role moves in the original forward direction, and the 0.3-kg function goes direct up.
- Find the speeds of the 0.3-kg and 0.7-kg pieces immediately after the break-up.
- How high from the break-upward betoken does the 0.iii-kg piece go before coming to remainder?
- Where does the 0.7-kg piece state relative to where it was fired from?
[reveal-answer q="446915″]Show Answer[/reveal-reply]
[subconscious-answer a="446915″]a. 33 thousand/south and 110 g/s; b. 57 m; c. 480 chiliad[/hidden-answer]
Two asteroids collide and stick together. The first asteroid has mass of
and is initially moving at 770 m/due south. The second asteroid has mass of
and is moving at 1020 m/s. Their initial velocities made an angle of 20° with respect to each other. What is the final speed and direction with respect to the velocity of the first asteroid?
A 200-kg rocket in deep space moves with a velocity of
. Suddenly, it explodes into three pieces, with the first (78 kg) moving at
and the second (56 kg) moving at
. Find the velocity of the 3rd slice.
[reveal-reply q="fs-id1167132464579″]Show Solution[/reveal-reply]
[hidden-answer a="fs-id1167132464579″]
[/hidden-answer]
A proton traveling at
scatters elastically from an initially stationary blastoff particle and is deflected at an angle of 85° with respect to its initial velocity. Given that the alpha particle has four times the mass of the proton, what pct of its initial kinetic energy does the proton retain after the collision?
Three 70-kg deer are continuing on a flat 200-kg rock that is on an ice-covered pond. A gunshot goes off and the love scatter, with deer A running at
, deer B running at
, and deer C running at
. What is the velocity of the rock on which they were standing?
[reveal-answer q="fs-id1167132215564″]Testify Solution[/reveal-answer]
[hidden-answer a="fs-id1167132215564″]
[/hidden-respond]
A family is skating. The father (75 kg) skates at eight.2 1000/southward and collides and sticks to the female parent (50 kg), who was initially moving at 3.3 m/s and at 45° with respect to the father's velocity. The pair then collides with their daughter (30 kg), who was stationary, and the 3 slide off together. What is their final velocity?
An oxygen cantlet (mass 16 u) moving at 733 m/southward at 15.0° with respect to the
management collides and sticks to an oxygen molecule (mass 32 u) moving at 528 m/s at 128° with respect to the
management. The ii stick together to class ozone. What is the final velocity of the ozone molecule?
[reveal-reply q="fs-id1167132276387″]Show Solution[/reveal-answer]
[hidden-answer a="fs-id1167132276387″]
341 thou/s at 86.viii° with respect to the
axis.
[/subconscious-answer]
2 cars arroyo an extremely icy four-way perpendicular intersection. Motorcar A travels due north at xxx k/due south and car B is travelling e. They collide and stick together, traveling at 28° northward of east. What was the initial velocity of car B?
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