Chapter 12 Section 3 Notes

Force: a push or pull on an object

Force is not something that an object possesses

An object only exerts force on another object

But when that object exerts its force, the second object always

exerts one back

Every force is part of a pair of forces

An interaction

We call this a Force Pair

One is called an Action

The other is a Reaction

Whenever one object exerts a force on a second object, the second object exerts a force that is…◦Equal in Magnitude ◦Opposite in Direction

To every action there is an equal and opposite reaction

Wall hits fist

Hey! Why don’t you try some on your own?

When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Forces are equal in size and opposite in direction◦ Hand against a wall.

Forces do not cancel if they act on different objects

Forces only cancel if they act of the same object

Momentum: Property possessed by an object.

Directly proportional to mass of the object.

Directly proportional to velocity of the object at that moment.

(instantaneous motion)

Momentum is abbreviated as p (Greek ‘rho’)

m is the symbol for mass

v is the symbol for velocity

Momentum is the product of the mass & the velocity of an object

Calculate the momentum of a 12 kg object traveling at 4 m/s.

P = ? m = 12 kg

v = 4 m s

p = mv

p = 12 kg x 4 m s

p = 48 kg x m = 48kgm s s

Hyperphysics: calculate various momenta

1.  Calculate the momentum of a 0.15 kg ball that is moving toward home plate at a velocity of 40m/s.

2.  Which has greater momentum, a 2.0kg hockey puck moving east at 2.5m/s or a 1.3kg hockey puck moving south at 3.0m/s?

3.  A track athlete throws a 2kg discus into a field with a velocity of 21m/s.  What is the momentum of the discus?

4.  Calculate the momentum of a 700g ball that is rolling down a ramp at 4.6m/s.

Hey! That looks like Newton’s 2nd Law. Force = mass * acceleration

Acceleration is the change in velocity over time

So Newton’s 2nd Law could read Force = change in momentum

Time

p = mv MomentumF = ma Newton’s Second Law

But we can use v for acceleration. tF = mv = t

OR

F =Newton’s second law as change in momentum over time.

Rearrange to a new form,Ftp Impulse Momentum-Relationship

p

t

pt

Momentum is the product of the velocity and mass of an object

Momentum = mass X velocity

So, the bigger it is and the faster it goes the greater the momentum.

In a closed system, the loss of momentum of one object equals the gain in momentum of another object

Therefore, momentum is conserved!

Bill Nye!