1N the force that a 100g bar of chocolate exerts on your hand.

Transcription

1 Forces: - - > cause change in motions Newton's first law = law of inertia In absence of a net external force acting upon it, a body will either remain at rest or continue in its rectilinear uniform motion. unit of force: 1N the force that a 100g bar of chocolate exerts on your hand.

2 1. Gravitational force F g G = 6.67 x N x m2 kg 2 Example: Two people have masses of 60 and 70 kg and are standing 40 cm apart from each other. G =

3 gravitational force, weight and mass: what's my weight? weight: = gravitational force an object experiences in proximity to a celestrial body measured in mass: measure for quantity of matter measured in measured with a scale measured with a dynamometer

4 Normal force why is this book not accelerating downwards? normal force: - the surface of a body exerts a counter force when a force is applied to the surface (= normal force = resistance to deformation) - normal force always orientated perpendicular to surface: F push F normal on a curved surface, normal force is perpendicular to the plain that touches the surface in one point only

5 Normal force on inclined plane May 11, 2015

6 Force of friction - particles on two different surfaces are attracted to one another and oppose sliding motion - contact force - to... F N F f = µ F N µ = friction coefficient - - > depends on nature of two surfaces It takes more force to get an object moving than to keep it moving. (Imagine yourself pushing a heavy object - once you get going it is a little easier) µ s : friction coefficient for static friction - - > object not moving or starting to move µ k : friction coefficient for kinetic friction - - > object already moving always: µ k < µ s

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9 example: - 24 kg chest to be pulled across floor - how much force needs to be applied to start moving if µ s = 0.5 practice P.277 # 1,2,4,5,6

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11 Tension force = force that is transmitted through a string, rope, or cable, when it is pulled tight by forces pulling in opposite direction on either end. the force is directed along the length of the wire and pulls equally on the objects on both ends of the wire. F T F T tension is the ropes resistance to stretching; it is usually equal to the force that is pulling on the rope when the rope changes direction, only the direction of the tension force changes, not its magnitude example: what is the tension force in a rope that holds up a mass of 10 kg?

12 Centripetal force All moving bodies want to continue moving at constant velocity in the same direction. In order to force them onto a curved path a force has to be applied continuously = centripetal force ; points towards centre of rotation Centripetal force can take on different forms depending on the situation: = tension force in rope when swinging a sling shot = gravitational force for moon circulating earth = frictional force between tire and road when a car drives into a curve Equation: Fc = mv2 r

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16 If an object is not moving => net force =0 => "all forces have to cancel each other out" tension force on each of the cables? tension force on each of the cables?

17 worksheet May 11, 2015

18 Analyzing multiple forces acting on a body free body diagrams: diagram in which all forces exerted on an object are represented by arrows how to draw a free body diagram: represent object under study and draw a dot as the centre of the object every force acting on the object is represented by an arrow anchored at the objects centre. draw the length of the arrow proportional to the magnitude of the force place the origin of the coordinate system into the objects centre point (if on an incline tilt coordinate system so that one axis of the coordinate system parallel to movement axis.) example: A book is pushed across a desk with constant velocity. Consider frictional forces but neglect air resistance. practice: P.289 # 1, 2

19 Analyzing multiple forces acting on a body free body diagrams: diagram in which all forces exerted on an object are represented by arrows how to draw a free body diagram: represent object under study and draw a dot as the centre of the object every force acting on the object is represented by an arrow anchored at the objects centre. draw the length of the arrow proportional to the magnitude of the force place the origin of the coordinate system into the objects centre point (if on an incline tilt coordinate system so that one axis of the coordinate system parallel to movement axis.) example: A book is pushed across a desk with constant velocity. Consider frictional forces but neglect air resistance. practice: P.289 # 1, 2

20 Resultant force / net force net force / resultant force = vector sum of all forces exerted on a body: F R = F 1 + F F n decompose all force vectors into component vectors - > then add the components: example: A boat's engine exerts a driving force of 1000 N at 30º, the wind is pushing the boat with a force of 150 N at 60º and the water current is exerting a force of 250 N at 210º. What is the net force?

21 Newton's second law: F net = m a May 11, 2015

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25 7 HW: P.293 # 1 and 2c)

26 Equilibrium A body is in a state of equilibrium if the resultant / net force is 0. a) static equilibrium b) dynamic equilibrium equilibrant force: the force that will put the system to equilibrium cancels out the net / resultant force = opposite vector of resultant force: F eq = - F r

27 inclined plane weight of the object will do two things: press object onto the surface pull object down the incline need to figure out how much of each - - > depends on angle of incline procedure: 1.) draw a coordinate system with the x- axis parallel to and pointing in direction of movement 2.) decompose weight force (F g ) vector into components on x and y axis y F gx x F gy F gy presses object onto surface F gx pulls object down the incline F g Normal force counteracts F gy : Frictional force counteracts F gx : y F N x y F gx F f x F gy F g F g F gx = sin F g F gy = cos F g

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29 example: A 50 kg object slides down an incline with angle 30º. What must the force of friction be for it to slide down at constant velocity?

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31 Newton's second law: F R = m a if we decompose the force vector into x- and y- component: F Rx = m a x example: F Ry = m a y Using a rope with an incline of 30 º a 4 kg object is pulled over a horizontal surface. The friction is equivalent to 12 N. What is the objects acceleration? 30º

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34 Newtons third law: actio = reactio If body A exerts a force on body B, B will exert a force on A that is equal in magnitude but opposite in direction. example 1: example 2: A girl (on skateboard) is pushing a boy (on skateboard) - > she is exerting a force on him. His body exerts a counter force of equal magnitude to her. both of them start moving backwards example 3: foot kicks ball - - > ball kicks foot why then does the ball start to move?

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