Ch. 3 Forces
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Ch. 3 Forces. I. Newton’s 2 nd Law:. A. Newton’s 2 nd Law:. Different forces and masses affect the acceleration of objects. 1. Greater Forces cause greater accelerations!. (Directly related). 2. Greater masses cause smaller accelerations!. (Inversely related).

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Ch. 3 Forces

I. Newton’s 2nd Law:

A. Newton’s 2nd Law:

Different forces and masses affect the acceleration of objects

1. Greater Forces cause greater accelerations!

(Directly related)

2. Greater masses cause smaller accelerations!

(Inversely related)

3. Acceleration equals net force divided by mass

( Equation: a = F/m)

4. Example: A Force of 25 N on a mass of 10 kg. a = ?

a. Equation: a = F/m

b. F = 25 N, m = 10 kg

c. a = 25/10

d. a = 2.5 m/s2

B. Forces opposing Motion

1. Friction: the force between two touching objects opposing motion

a. Static friction: friction between two non-moving objects

b. Sliding friction: friction between objects sliding past each other

c. Rolling friction: friction with a rolling object and a surface

2. Air Resistance: the force opposing motion of objects moving through the air

a. The amount depends on the object’s size, shape and speed

  • Terminal Velocity: When the weight of a falling object and the upward

  • force of air resistance are equal. Net force = 0, acceleration is 0, and

  • speed is now constant!!

II. Gravity

A. Law of gravity: There is a force of attraction between any two objects

[9.8 m/s2 ]

B. Due to inertia, all objects fall with the same acceleration (w/out air resistance)

C. Weight: Measurement of gravity’s pull on an object

1. The greater the gravity, the greater the weight of the object.

2. Weight is the force pulling on the object, mass is the stuff in the object.

D. Orbiting

1. Objects in orbit float because all forces are cancelled

2. They have horizontal speeds so fast that they fall around the Earth

  • Centripetal force holds them in the circular motion with centripetal acceleration,

  • but their speed keeps them from being pulled in!

Orbiting Diagram

Diagram – Mars Orbit

III. Newton’s 3rd Law of Motion

A. Newton’s 3rd Law:

For every action force exerted on an object, that object will

exert a reaction force back which is equal and opposite!

  • Action / Reaction forces are always on two separate objects, that is why they

  • don’t cancel out.

  • Rockets use this law – the rocket pushes off of the fuel…and the fuel pushes

  • off of the rocket

B. Momentum:

Inertia of an object (mass) in motion (velocity)

1. Momentum = mass x velocity (Equation: p = mv) [ SI units = kgm/s]

  • Example: An object with a mass of 25 kg and a velocity of 5 m/s. p = ?

a. p = mv

b. m = 25 kg , v = 5 m/s

c. p = 25 x 5

d. p = 125 kgm/s

3. Law of Conservation of Momentum:

The momentum of an object cannot be lost

or gained, but can be transferred to a different

object… (pool balls)