Notice that Newton’s first law and second law talk about the forces on one body (net force =0
on that body, it will rest or move with constant speed 1
st
law; net force is not zero, then that
body will have acceleration2
nd
law).

Notice that Newton’s third law (an action force and its reaction force) talk about the forces
on different objects
!
2
Chapter 2 Practice
If mass and net force is tripled, how is the new acceleration compared to the original
one?
The same because acceleration is net force divided by the mass so if they are both
tripled, it is the same ratio, just scaled (cancel each other out)
Why doesn’t a heavy object accelerate more than a lighter object when free falling?
Free
falling is the same acceleration, it does not matter the weight (Newton’s 2
nd
law)
Apply force to wall, fingers bend, what is the force?
Reaction force, the wall pushing the
fingers back (Newton’s 3
rd
law)
Allison exerts steady net force of 50 on 20 shopping cart initially at rest for 2 seconds.
Find the acceleration of the cart and shows that it moves a distance of 5m.
Newton’s 2
nd
law
Lenny has mass of 100, on skateboarding going 9, comes to dead stop in .2 seconds.
Initial speed is 9, final speed is 0.
(09)
45= n/100
.2
Chapter 3 Momentum and Energy
Know the definition of momentum (=mv)
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