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phy2048-ch5

# phy2048-ch5 - Chapter 5 Force and Motion I I Newtons first...

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1 I. Newton’s first law. II. Newton’s second law. III. Particular forces: -Gravitational - Weight - Normal - Friction - Tension IV. Newton’s third law. Chapter 5 – Force and Motion I I. Newton’s first law: If no net force acts on a body, then the body’s velocity cannot change; the body cannot accelerate barb2right v = constant in magnitude and direction. - Principle of superposition: when two or more forces act on a body, the net force can be obtained by adding the individual forces vectorially. Newton mechanics laws cannot be applied when: 1) The speed of the interacting bodies are a fraction of the speed of light barb2right Einstein’s special theory of relativity. 2) The interacting bodies are on the scale of the atomic structure barb2right Quantum mechanics - Inertial reference frame: where Newton’s laws hold.

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2 II. Newton’s second law: The net force on a body is equal to the product of the body’s mass and its acceleration. ) 1 . 5 ( a m F net arrowrightnosp arrowrightnosp = ) 2 . 5 ( , , , , , z z net y y net x x net ma F ma F ma F = = = - The acceleration component along a given axis is caused only by the sum of the force components along the same axis, and not by force components along any other axis. - System: collection of bodies. - External force: any force on the bodies inside the system. III. Particular forces: -Gravitational: pull directed towards a second body, normally the Earth barb2right ) 3 . 5 ( g m F g arrowrightnosp arrowrightnosp = ) 4 . 5 ( mg W = - Weight: magnitude of the upward force needed to balance the gravitational force on the body due to an astronomical body barb2right ) 5 . 5 ( mg N = - Normal force: perpendicular force on a body from a surface against which the body presses. - Frictional force: force on a body when the body attempts to slide along a surface. It is parallel to the surface and opposite to the motion. -Tension: pull on a body directed away from the body along a massless cord.
3 IV. Newton’s third law : When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. ) 6 . 5 ( CB BC F F arrowrightnosp arrowrightnosp - = QUESTIONS Q2. Two horizontal forces F 1 , F 2 pull a banana split across a frictionless counter. Without using a calculator, determine which of the vectors in the free body diagram below best represent: a) F 1 , b)F 2 . What is the net force component along (c) the x-axis, (d) the y-axis? Into which quadrant do (e) the net-force vector and (f) the split’s acceleration vector point? j N i N F j N i N F ˆ ) 2 ( ˆ ) 1 ( ˆ ) 4 ( ˆ ) 3 ( 2 1 - - = - = arrowrightnosp arrowrightnosp j N i N F F F net ˆ ) 6 ( ˆ ) 2 ( 2 1 - = + = arrowrightnosp arrowrightnosp arrowrightnosp Same quadrant, 4 F 1 F 2 I. Frictional force -Kinetic: (f k ) appears after a large enough external force is

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