Momentum & Inertia

This unit we learned all about momentum and inertia. One of the most challenging things to remember is that the symbols for momentum and inertia are not M & I. In fact, momentum=P and impulse=J. An impulse is the change in momentum (∆P) or a force over some amount of time.

Important Formulas

F∆t=m∆v

F=m∆v/∆t

J=∆P

mava + mbvb = mava + mbvb unstuck

mava + mbvb = (ma + mb) (Vab) stuck

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.

1. While being thrown, a net force of 100 Newtons acts on a baseball with a mass of 0.5 kg for a period of 1 second.  What's the change in momentum of the ball?

F∆t = J

(100N) (1) = 100NS

J = p

p = 100kgm/s

2. Determine the impulse needed to increase a car’s speed from 30 m/s to 35 m/s.

 P = M change in V

 (1000) (35-30)

 P = 5,000 kg(m) /s

 3. A 2kg block slides at 4 m/s.

(2 kg)(4 m/s) = 8 kg(m) /s

4. How do airbags keep you safe?

Airbags try to increase the time in order to decrease the force. The ∆P is the same so the impulse ∆J is also the same.