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Chapter 6: Momentum
Transcript of Chapter 6: Momentum
topic to YOU! "One way to keep momentum going is to have constantly greater goals."
Concepts we will
Cover in this Chapter Introduction Momentum Questions to ask ourselves: Got images? 1. What exactly is Momentum?
2. What is Impulse?
3. How does any of this relate to me? Before we continue further here are some words we'll need to remember! 1. Mass 2. Velocity 3. speed 4. time 5. Acceleration 6. Force 7. Inertia 1. Mass: The quantity of matter in an object. 2. Velocity: Speed of an object and its direction
of motion. 3. Speed: How fast something moves.
Speed= Distance/Time 4. Time: The indefinite continued progress of existence and events in the past, present, and future as a whole. 5. Acceleration: (a) rate at which an objects velocity changes with time. 6. Force: Any influence that tends to accelerate an object. 7. Inertia: Sluggishness or apparent resistance of an object to change its state of motion. 1. Momentum 2. Impulse 3. Impulse Changes Momentum (All Cases) 4. Bouncing 5. Conservation of Momentum 6. Collisions 7. More Complicated Collisions Let's
Begin... What is momentum? Momentum is the product of an object and its velocity. Momentum= mass x velocity or mv In short... When direction is not an important factor... Momentum= mass x speed An object can have large momentum if either its mass or velocity is large or if both its mass and velocity are large. Impulse What is Impulse, you ask... -Impulse is the product of the force acting on an object and the time during which it acts. Formula: The quantity force x time interval= Impulse If the momentum of an object changes then so could the mass or velocity.
The greater the force the greater the momentum
Time is also a main factor in momentum change You can push with the same force but when you force for twice the time, the amount of impulse and momentum is doubled. If a flowerpot falls from a shelf onto your head, you may be in trouble. If it bounces from your head, you may be in more serious trouble. Why? Bouncing If the boxer moves away from the force then time extends, which then diminishes the force. If he moves towards the force he then faces a greater impact. Figure 6.8 It takes the same impulse to decrease your momentum to zero.
The same impulse does not mean the same amount of force or the same amount of time; rather it means the same product of force and time. If you were in a car that was out of control and you had to choose between hitting a concrete wall or a haystack, you wouldn’t have to call on your knowledge of physics to make up your mind. Case 2 : Decreasing Momentum A golfer teeing off and a baseball player trying for a home run do both of these things when they swing as hard as possible and follow through with their swings. Following through extends the time of contact. To increase the momentum of an object, it makes sense to apply the greatest force possible for as long as possible. Case 1: Increasing Momentum In short impact times, the impact forces are large.
An object brought to rest, the impulse is the same, no matter how it is stopped. But, if the time is short, the force will be large. When boxing, if you move into a punch instead of away you’re in trouble.
Or catch a high speed baseball while your hand moves toward the ball instead of away upon contact. Case 3: Decreasing Momentum Over a Short Time Figure 6.6: If the change in momentum occurs in a small amount of time the hitting force then is big. Figure 6.5: If the change in momentum occurs over a large amount of time then the hitting force will be small. The impulse momentum relationship helps us to analyze many examples in which forces act and motion changes.
Impulse and change of momentum are always linked. The greater the impulse exerted on something, the greater will be the change in momentum. The exact relationship is:
Impulse = change in momentum
Ft = (mv) Impulse Changes Momentum - Because impulses are greater when an object bounces. Collisions
Momentum is conserved in collisions,
meaning that the net momentum of a system of colliding objects is unchanged
before, during, and after the collision.
The forces that act during the collision
are internal forces.
In any collision we can say:
Net momentum before collision = net momentum after collision
This is true no matter how the objects
may be moving before they collide. When the momentum, or any quantity in physics, doesn’t change,
we say it is conserved.
This idea that momentum is conserved when no external force acts are elevated to a central law of mechanics
called the law of conservation of momentum.
This basically states,
that in the absence of an external force,
the momentum of a system remains unchanged More complicated collisions
The net momentum remains unchanged in any collisions,
regardless of the angle between the paths of the colliding objects.
The parallelogram rule of vector addition
is used to express the net momentum when different directions are involved.
Example figure 6.18(picture)
Whatever the nature of the collision
or however complicated it is,
the total momentum
before, during, and after the collision
Elastic collision is when a collision in which colliding objects rebound without lasting deformation or the generation of heat.
An example of this is figure 6.13 (picture)
But momentum is conserved
even when the colliding objects become entangled during the collision.
This is an inelastic collision.
Collisions in which the colliding objects become distorted, generate heat, and possibly stick together.
In a perfectly inelastic collision, both objects stick together.
Example 6.14(picture) Momentum, like the quantities velocity and force, has both direction and magnitude.
It is a vector quantity. Just like velocity and force, it can be canceled. If no net force or net impulse acts on a system, the momentum of the system cannot change. Example figure 6.11 (picture)
The momentum before firing is zero. After firing, the net momentum is still zero,
because the momentum of the cannon is equal and opposite to the momentum of the cannonball.
The net momentum is neither gained nor lost. Momentum is conserved for all collisions, elastic, and inelastic (whenever external forces don’t interfere). Conservation of Momentum
From Newton’s Second law, we know that in order to accelerate an object there must be a net force applied to it.
If we wish to change the momentum of an object, we must exert an impulse on it.
Only an impulse external to a system can change the momentum of the system.
Internal forces and impulses won’t work. We hope you enjoyed this presentation!