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Projectile Motion A projectile is an object on which the only force acting is ______ and the direction of this force is always _____. (In real life, a projectile will also have ______ or ___________ acting on it and the direction of this force is always _____________________.)

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Projectile Motion A projectile is an object on which the only force acting is gravity and the direction of this force is always down. (In real life, a projectile will also have friction or air resistance acting on it and the direction of this force is always opposite the direction of motion.)

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An Introduction to Forces: Student Learning Goal The student will analyse, in quantitative terms, the forces acting on an object, and use free-body diagrams to determine net force on the object in one dimension. (B2.9) The student will analyse, in quantitative terms, the forces acting on an object, and use free-body diagrams to determine net force on the object in one dimension. (B2.9)

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An Introduction to Forces SPH4C

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A Definition A force is a push or a pull.

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A Definition A force is a push or a pull. It is a vector quantity and is symbolized by:

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A Definition A force is a push or a pull. It is a vector quantity and is symbolized by:

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A Definition A force is a push or a pull. It is a vector quantity and is symbolized by: In the SI system, force is measured in Newtons (N). 1 N = 1 kg m/s 2

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Applied Force Applied Force is a general term for any contact force, e.g.

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Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension

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Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension Friction Friction

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Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension Friction Friction Normal Force Normal Force

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Tension Tension is the force exerted by strings, ropes, cables, etc. attached to an object.

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Tension The tension along the string is constant.

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Friction Friction acts to oppose any (attempted) motion.

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Friction Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move

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Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion

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Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion Air resistance (drag): friction on an object moving through air Air resistance (drag): friction on an object moving through air

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Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion Air resistance (drag): friction on an object moving through air (many physics problems with neglect this) Air resistance (drag): friction on an object moving through air (many physics problems with neglect this)

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Normal Force The normal force acts to keep objects apart.

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Normal Force The normal force acts to keep objects apart. i.e. if you push on a wall, the wall will exert a normal force on your hand that prevents your hand from pushing through the wall.

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Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces.

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Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces.

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Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces. One example is gravitational force, the force of attraction between all objects with mass.

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Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces. One example is gravitational force, the force of attraction between all objects with mass. (The gravitational force the Earth exerts on an object is called its weight.)

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Free-Body Diagrams Usually an object will have more than one force acting upon it.

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Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object

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Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object.

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Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram

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Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram and the forces acting on it are drawn as arrows pointing outwards.

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Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram and the forces acting on it are drawn as arrows pointing outwards. The arrows must be labelled!

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FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball.

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FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball.

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FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. FgFg

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FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. FgFg F air

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book.

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book.

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FAFA

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FAFA FfFf

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FgFg FAFA FfFf

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FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FgFg FNFN FAFA FfFf

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Net Force FBDs are drawn to help determine the net force (the sum of all forces) acting on an object. Resolve all vertical and horizontal vectors and determine the resultant.

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FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book.

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FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book. F g = 10 N F A = 5 NF f = 2 N F N = 10 N

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FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book. Horizontal forces: F A + F f = 5 N + (- 2 N) = 3 N Vertical forces: F g + F N = (- 10 N) + (10 N) = 0 The net force F net = 3 N [right]

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More Practice Estimating and Measuring Forces

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