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Big Idea
Big Idea
WHEN AN OBJECT RESISTS A FORCE THAT IS APPLIED TO IT, IT APPLIES A FORCE BACK THAT IS EQUAL IN SIZE BUT OPPOSITE IN DIRECTION.
WHEN AN OBJECT RESISTS A FORCE THAT IS APPLIED TO IT, IT APPLIES A FORCE BACK THAT IS EQUAL IN SIZE BUT OPPOSITE IN DIRECTION.
All objects resist a change in their motion. Based on their mass, they resist when forces are applied. This inertia causes an equal magnitude and opposite direction force to be applied back on the first object. When objects do not have a large resistance to applied forces, it is difficult to apply large forces to them. For example, when you push a real car, you may be able to apply hundreds of Newtons of force to it to get it to roll, but a toy car would move quickly away from you before you applied anywhere near one hundred Newtons of force to it. When you feel an object resist the force you are applying to it, you are feeling this reaction force.
What should I know?
What should I know?
When a force is applied to an object, it resists the effects of that force due to it inertia (i.e., its mass.)
This resistance causes a reaction force, applied to the pushing object by the object being pushed. Both the action and reaction forces are the same size (magnitude) and exist simultaneously.
NEWTON'S THIRD LAW: For every force applied by object A on object B, there is an equal magnitude and opposite direction force applied back on object A by object B.
Internal action-reaction force pairs always cancel (e.g., if your car has run out of gas, you cannot push your car to the next gas station from inside the car.)
When an object causes itself to change its motion -- i.e., it causes a net force to be applied to itself -- it often does so using an action-reaction force pair (e.g., when a human jumps off the ground by pushing down on the floor, the floor applies a reaction force to the human.)
As forces are rarely constant, the average net force and the time over which it is applied can be used to find the object's change in velocity.
The product of the average net force and the time over which it is applied is called the impulse. When F = ma is rearranged, it describes the impulse mathematically: F Δt = m Δv, where the F is the average net force, Δt is the time it is applied, F Δt represents the impulse (sometimes represented with a J,) m is the mass of the object, and Δv is the change in its velocity.
The units for impulse are Newtons x seconds, or N·s.
As a consequence of Newton's third law of motion, the impulse applied by object A on object B requires that object B applies the same impulse back on object A.
When considering how much impulse it will take to cause a certain change in an object's motion, both the inertia (mass) of the object and its initial velocity are important, as seen in the impulse formula above. The product of an object's mass and its velocity is called its momentum.
Momentum is a vector quantity that encapsulates both the ideas of the current motion of the object and also how difficult it is to change that motion. The formula is p = m v.
The units for momentum are kilograms x meters/second, or kg·m/s.
When momentum does change, it is typically because the velocity of the object has changed, not its mass. The formula for the change in momentum is Δp = m Δv.
If the impulse formula is analyzed, it can be seen that the quantity on the right is actually the change in momentum. That makes the true name of that formula the impulse-momentum theorem.
In a closed system of objects -- in which no forces or energy can enter or leave -- any change in momentum for one object must correspond to a change in the momentum (or momenta) of other objects. This principle is called the LAW OF CONSERVATION OF MOMENTUM.
Typically objects in systems apply impulses on each other during collisions.
Because the same sized impulse is applied to both objects during a collision, each object experiences the same change in momentum.
If little to no motion energy is lost to heat during a collision and the objects bounce off each other, it is an elastic collision.
If some motion energy is lost to heat during a collision and there is some deformation of the objects, or if they stick together, it is an inelastic collision.
Vocabulary
Vocabulary
Interaction -- a more complete description of a push on an object. It includes the force that Object 1 applies to Object 2, and also the equally-sized force that Object 2 applies back on Object 1 in an opposite direction. It is important to remember that these two forces ALWAYS act on different objects.
Newton's Third Law -- to every force applied by one object on another object, there is an equal but opposite reaction force applied by the second back on the first.
Vector Components -- Any vector can be broken down into its purely horizontal and vertical parts to make it easier to use in vector addition.
Momentum -- inertia in motion. It is symbolized with a lowercase p and found by multiplying the mass and velocity of an object. It is measured in strange units: kg * m/s. It is a vector quantity.
Impulse -- a quantity that describes both the size of the average net force acting on an object and the time over which it acts. It is measured in N * s. It is symbolized by a capital J.
Impulse - Momentum Theorem -- states that the impulse acting on an object causes an equivalent change in momentum for that object.
System -- a collection of objects that can be analyzed as a whole.
Open System -- a collection of objects in which forces and energy are allowed both into and out of the collection, making analysis much more difficult.
Closed System -- a collection of objects in which forces and energy are (nearly completely) kept inside the system, without allowing forces or energy to come into the system. As a result, this type of system is much easier to analyze.
Law of Conservation of Momentum -- a rule of physics that states that vector sum of all the momentum vectors at one particular time in a closed system will be the same at any other particular time. It is used in the analyses of systems containing collisions and/or explosions. In all three -- elastic collisions, inelastic collisions and explosions -- the total system momentum is conserved.
Elastic Collision -- a collision in which objects rebound without any lasting deformation or the generation of heat.
Inelastic Collision -- a collision in which objects become distorted, generate heat and possible stick together.
Explosion -- an inelastic collision in reverse.
LINK to sample problems.
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