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**Forces in One Dimension: Force and Motion 4.1**

Pg. 86

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**Force and Motion Force is a push or pull exerted on an object.**

Can cause objects to speed up, show down, or change directions as they move The object of interest is called the system . The object exerting the force is called the agent. Everything else is called the external world. “F” is a vector and represents the size and direction of a force “F” represents only the magnitude

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**Contact Forces and Field Forces**

Contact forces exists when an object from the external world touches a system and exerts a force on it. Field forces are exerted without contact (gravitational force).

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**Free-Body Diagrams Read pg. 89**

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**Free-Body Diagrams Things to Remember…**

Create a pictorial model. Circle the system and identify every place where the system touches the external world (this is where contact forces are exerted). Identify contact forces. Identify any field forces on the system. Apply the particle model and represent the object with a dot. Represent each force with a blue arrow that points in the direction that the force is applied. (make arrow the length proportional to the size of the force). Always draw the force arrows pointing away from the particle. Label each force. Use the “F” symbol with a subscript label to identify agent and object on which force is exerted. Choose a direction to be positive.

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**Force and Acceleration**

They have a relationship! It is linear! Greater force means greater acceleration! You can apply the straight line equation… y = mx + b

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**Force and Acceleration**

See graphs pg.91 The graph shows that if the same force is applied in each situation… The acceleration of two carts is ½ acceleration of one cart. The acceleration of three carts is 1/3 acceleration of one cart.

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Therefore… As the number of carts increases a greater force is needed to produce the same acceleration. The slope depends on the total mass of the carts. If the slope (k), in this case, is defined as the reciprocal of the mass (k=1/m), then a = F/m or F=ma (Newton’s Second Law)

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**a = F/m This equation tells you that…**

a force applied to an object causes that object to experience a change in motion – the force causes the object to accelerate. If you double the force, you will double the object’s acceleration. If you apply the same force to several different objects, the one with the most mass will have the smallest acceleration and the one with the least mass will have the greatest acceleration.

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**Units for Force F = ma Mass is measured in kg**

Acceleration is measured in m/s² That makes Force be measured in kg•m/s² The unit 1 kg•m/s² is called a Newton (N)

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Combining Forces

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**Combining Forces Explained…**

In the free-body diagram a vector represents the total result of the two forces. When force vectors are in the same direction they can be replaced with one vector with an equal length of their combined lengths. When the forces are in opposite directions, the resulting vector is the length of the difference between the two vectors. Net Force – the sum of all the vectors on an object.

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Newton’s Second Law The acceleration of an object is proportional to the net force and inversely proportional to the mass of the object being accelerated. A net force is the cause of a change in velocity (an acceleration).

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Newton’s First Law Galileo was the first to recognize that the general principles of motion could be found by extrapolating experimental results to the ideal case in which there is no resistance to slow down an object’s motion.

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Newton’s First Law Newton generalized Galileo’s results in the following statement: “an object that is at rest will remain at rest, and an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero”.

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**Ff FN Fsp FT Fthrust Fg Force Symbol Definition Direction Friction**

The contact force that acts to oppose sliding motion between surfaces Parallel to the surface and opposite the direction of sliding Normal FN The contact force exerted by a surface on an object Perpendicular to and away from the surface Spring Fsp A restoring force; that is, the push or pull a spring exerts on an object Opposite the displacement of the object at the end of the spring Tension FT The pull exerted by a string, rope, or cable when attached to a body and pulled taut Away from the object and parallel to the string, rope, or cable at the point of attachment Thrust Fthrust A general term for the forces that move objects such as rockets, planes, cars, and people In the same direction as the acceleration of the object, barring any resistive forces Weight Fg A field force due to gravitational attraction between two objects, generally Earth and an object Straight down toward the center of the Earth

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**Newton’s First Law Inertia Tendency of an object to resist change**

If an object is at rest, it tends to stay at rest If an object is moving at a constant velocity, it tends to continue moving at that velocity.

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**Equilibrium A net force is something that causes velocity to change.**

If net force is zero, the object is in equilibrium. The object is at rest (0 velocity) or constant velocity. A net force is something that disturbs a state of equilibrium If there is no net force acting on an object, the object does not experience a change in speed or direction (velocity)

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