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**Chapter 2: Force and Newton’s Laws**

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**Section 1 Newton’s First Law**

Force – push or pull on an object The combination of all the forces acting on an object is the net force

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When forces are balanced forces, they cancel each other out and do not change an object’s motion; when forces are unbalanced forces, the motion of an object changes.

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Newton’s first law of motion – an object will remain at rest or move with constant speed unless a force is applied

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**Friction is a force that resists sliding between two touching surfaces or through air or water**

Friction slows down an object’s motion Static friction – the type of friction that prevents an object from moving when a force is applied Sliding friction is due to the microscopic roughness of two surfaces; it slows down a sliding object Rolling friction between the ground and a wheel allows the wheel to roll

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**Write down where is friction is at work**

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**Section 2 Newton’s second Law**

Newton’s second law of motion connects force, acceleration, and mass; it explains that an object acted upon by a force will accelerate in the direction of the force; acceleration equals net force divided by mass. Acceleration = net force / mass

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**Units of Force Measured in newtons (N)**

Equal to mass x acceleration (kg x m/s2)

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Gravity – attractive force between two objects; depends on the mass of the objects and distance between them; gravitational force is also called weight. Weight: F= ma = m x (9.8 m/s2) *your weight is the gravitational force between you and the Earth Mass and weight are different!

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The second law explains how to calculate the acceleration of an object if its mass and the forces acting on it are both known Acceleration = net force / mass = N/kg = (kg x m/s2 )/kg = m/s2

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**In circular motion, the centripetal force is always perpendicular to the motion**

What does perpendicular mean?

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Terminal velocity is reached when the force of gravity is balanced by air resistance; the size of the air resistance force depends on the shape of an object and its speed Air resistance = air pushing against an object’s motion A form of friction The force of air resistance pushing up on the object is increasing as the object moves faster

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An object can speed up, slow down, or turn in the direction of the net force when unbalanced forces act on it Speeding Up – force in same direction as motion Slowing Down – force in opposite direction as motion Turning- force at an angle to direction of motion

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**Section 3 Newton’s Third Law**

Newton’s third law of motion states that forces always act in equal but opposite pairs; for every action there is an equal and opposite reaction Action-reaction forces are always the same size but are in opposite directions and act on different objects.

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When the mass of one object is considerably larger than the mass of another object, the action-reaction force is not noticeable Air and water exert action-reaction forces with objects such as hands or canoe paddles A rocket launches due to the equal but opposite forces of the burning fuel.

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Weightlessness The condition that occurs in free fall when the weight of an object seems to be zero Free fall = when the only force acting on a falling object is gravity With no force pushing up on you, you have no sensation of weight

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