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### 403-B: Newton's Model 1

• Topic Cluster: Dynamics
• Topic: Vector Force Analysis (Newton's First Law)
• Objective: Identify when net force is zero and when it is not; determine whether an abstract object accelerates, decelerates of moves at a constant speed based on its free-body diagrams
• Content: When an object is not moving or moving at a constant velocity, the net force acting on that object is zero; when an object is accelerating, the net force is not zero; accelerating in physics can mean speeding up, slowing down, or changing direction; when the net force on an object is zero we say it is 'in equilibrium'
• Level: 1

#### BACK to Ladder Vector Force Analysis (Newton's First Law)

##### Net Force $$\left( \Sigma F \right)$$

The vector sum of all forces on an object.

The symbol for net force is $$\Sigma F$$. (Pronounced "Sigma F.")

##### Newton's First Law (restated)

Any object that is not moving has a net force of zero acting on it.

Any object whose velocity is constant as a net force of zero acting on it.

Any object whose velocity is not constant has a net force greater than zero acting on it.

For the following problems, state if net force = 0 or if net force > 0.
$$\left( \Sigma F = 0 \textrm{ or } \Sigma F > 0 \right)$$

1. Anything that is not moving.
2. Anything that is moving at a constant velocity.
3. A car, when the driver is pressing the accelerator.
4. A glass or water resting on a table.
5. A ball rolling down a ramp.
6. A car driving on the highway at a constant velocity.
7. A ball that has been dropped off a building.
8. A car when the driver has pressed the breaks.

#### Interpreting Free-Body Diagrams Using Newton's Model

The free-body diagrams below illustrate the force acting on an object. For each of the following free-body diagrams, information on forces is given as well as information on the velocity of the object. Note that velocity is NOT a force, and this it is indicated separately from the free-body diagram. I wrote Newton's Model by compiling Newton's First and Second Laws with the rules of vector addition, to create a guide for qualitatively relating a free-body diagram to motion. Use Newton's Model to predict the motion for each of the free-body diagrams below.

##### Newton's Model
Net Force Motion of Object
Net Force = 0 and object is not moving Object remains still
Net Force = 0 and the object is moving Object moves with a constant velocity
Net Force is in the same direction as velocity Object increases in speed.
Net force is in the opposite direciton as velocity Object decreases in speed.
1.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.
2.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.
3.  No forces act on this object: The object moves at a constant velocity. The object increases in speed. The object decreases in speed.

If more than one force acts on an object, the forces must be combined, and the net force determines the motion of the object. In each of the following examples, first find the net force acting on the object, and then determine the motion of the object.

1.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.
2.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.
3.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.
4.  The object moves at a constant velocity. The object increases in speed. The object decreases in speed.

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