Unit Summary Blog

Unit Meta-cognition: 

•  impulses of two things colliding are always equal, the momentum is always equal to the impulse, but the acceleration changes
•  In order to follow Newton's 3rd law, things must move equally fast
•  when two objects end together, use the equation: MaVa+MbVb = (Ma+Mb) Vab
• When two objects end apart, use the formula p total Vi= MaVb +MbVb
•  When 2 things collide, the momentum is equal and opposite, therefore have equal impulses.
•  P represents momentum
•  J represents impulse
•  the change in J= change in P
• increasing the time will only affect the force if the J's are constant
• the impulse (J) is the same when the change in momentum is constant (smaller force= safer)
• the units of measurement for the law of conservation of matter is kgm/s
• Law of Conservation of Matter says that Cart A+ Cart B =0 (ptotal before= ptotal after)
• Why do airbags always keep you safe?
• the J's are always the same
• change in P= mass times change in velocity
• increases time because the J's are the same, by increasing the time it decreases the velocity
• the velocity goes from change in V to 0 

 What is impulse?
The simplest answer is that impulse is the change in momentum and the force upon something and how long the force is applied.
You can solve for momentum (P) using this formula: (Keep in mind if there is a negative sign in this equation, then it means the impulse was going in the negative direction.)

Δp = mΔv

Newton's Third Law:
For every action, there is an equal and opposite reaction. 
Fa= -Fb

Newton's Second Law: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Conservation of Momentum Equation:

What is momentum?
Momentum is the motion of a moving body, measured as a product of its mass and velocity.

Impulse = Change in momentum
 

Example Problems:
1. Why do Otterbox cell phone cases prevent cell phones from breaking when they are dropped?

•  When a collision occurs, an objects experiences a force for an amount of time that results in mass undergoes a change in velocity, that therefore results in a momentum change. The time increases because the J's are the same. The J's are the same, because the change in velocity is the same. ( change in P= mass times change in V)

Why do Climbers use stretchy ropes?

Stretchy, nylon ropes are used by climbers because the momentum is slowed by the ropes, therefore preventing the climber from falling. The ropes are made to be stretchy so if the climber falls, it will apply a force upon the climber for a longer period of time. By increasing the time that the climber would be falling, the force exerted on the climber is reduced. Therefore the ropes increase the stopping time and decrease the stopping force.


How do air bags keep you from getting hurt?

•  Change in momentum is the same regardless of whether you stop quickly or slowly
• impulse = force times time
• The car will go from moving to not moving no matter how the car is stopped. Therefore the change in momentum is the same. The impulse will also be the same regardless of how the car stops. ( change in P= impulse ) 
• The airbag increases the time of the impulse, therefore the force on you is less. (Small force = less injury)
• If the change in P=J, the change in momentum is the same, therefore the impulse is the same too. Since the J is the same, the airbag increases the time of the impulse. This cause the force to decrease. The smaller force keeps you safe.

Impulse Momentum Theorem:
A 2kg block slides at 4 m/s. What is the momentum?

p=mv
p= 2 (4)
p= 8 m/s


A gun fires a bullet. On which object is the force greater?
 The force on both is the same because of Newton's 3rd law.

On which object is the impulse greater?
 The change in P equals the J, so the impulses are equal because the force and time is equal.

On which object is the change in momentum greater?
 The change in momentum is the same. No matter how fast the bullet moves, the change in momentum would always be the same because impulses=force times time of force applied.

Which object undergoes a greater acceleration?
 Because of Newton's 2nd law, the bullet undergoes a greater acceleration because it has a smaller mass.