Chapter 4 Force and Motion Force and Motion
- Slides: 21
Chapter 4 Force and Motion
Force and Motion �A force is a push or a pull exerted on an object. �Can cause objects to speed up, slow down, or change direction �Pg 89 “A force exerted on an object causes the object’s velocity to change; that is a forces causes an acceleration (or deceleration)”
Forces and Motion �Everything around the object that exerts forces on it is called the external world.
Contact and Field Forces �Field Forces are exerted without contact ◦ Gravity ◦ Magnetism �Contact forces are exerted with contact. ◦ Push, pull, drag, touch �An agent is the thing that is exerting the force.
Free Body Diagrams �Help you analyze how forces affect motion �Create a Pictorial Model ◦ Go to Page 89 �Read “ To make a physical representation…. . ” paragraph
Drawing Free-body Diagrams Represent the object with a dot 2. Represent each force with a arrow that points in the direction that the force is applied. Make the length of each arrow proportional to the size of the force. 3. Draw the force arrows pointing away from the particle. Use the symbol F with a subscript label to identify both the agent and the object on which the force is exerted 1.
Combining Forces �Net Force: the vector sum of all the forces on an object. �Pg 92 Figure 4 -5
Newton’s 2 nd Law F = ma The sum of the forces acting on the object is equal to the mass of the object times the acceleration of an object. F = force a = acceleration � Force = Newtons or (kg * (m/s 2)) � accel = m/s 2 � mass = kg m = mass
Newton’s 1 st Law �An object at rest will remain at rest, and an object in motion will continue moving in a straight line with a constant speed until the net force of the object isn’t zero. �AKA: Law of Inertia: an object’s ability to resist change �Equilibrium ◦ When net force = 0 ◦ Object is at rest or moving at a constant velocity
Different Forces we will use Pg 94 table 4 -2 Do Pg 93 7 and 8 What is the magnitude of a force on a 3 kg that is acceleration a 2 m/s 2 A force of 15 N is applied to a chair that weighs 10 kg. What is the acceleration of the chair? What is the acceleration if there is an opposing force of 10 N too?
Free Falling Object �No touching anything �Air resistance is neglected �Gravity is the only force �Ball’s acceleration is g, 9. 8 m/s 2 �F = ma �Think about how g is difference in other places.
Scales (bathroom) �Based on springs �The scale exerts an upward force on your b/c you are in contract with it. You are not accelerating so spring force must equal your weight (another type of force due to gravity) �Scales measure weight
Mass vs. weight �Weight is dependent on gravity �Mass isn’t �Example: Your mass is the same on every planet because the amount of matter inside you stays the same. �Weight, however, depends on the magnitude of the gravity � 15 -18
Apparent Weight �Ex. Standing on a scale in an elevator �How would you feel when you start moving up? Would you scale increase in force or decrease? �How would you feel when you start moving down? Or if you slow down? What would the scale say?
Apparent Weight �The force an object experiences as a result of all the forces acting on it, giving the object an acceleration �What if the cable breaks? You and the scale would accelerate with a = -g. You would be weightless �Weightlessness does not mean that an object’s weight is actually zero. But no contact forces so no apparent weight. � 19 -20
Drag Force and Terminal Velocity �Air exerts a balanced force on all sidesno net force �Drag Force is the force exerted by a fluid on the object moving through fluid. �As the speed of the object increases, so does the magnitude of the drag force. �Size and shape of the object also affects the drag force �It is also affected by the properties of the fluid (ie. Viscosity and temperature)
Terminal Velocity �Pg 101 Read first paragraph �The constant velocity that is reached when the drag force equals the force of gravity is called terminal velocity. �How can we apply this idea to life situations? �Competitive skiers, sky divers, bikers � 21 -27
Interaction Forces �Forces come in pairs �Interaction pair is two forces that are in opposite directions and have equal magnitudes. ◦ Forces exist together or not at all. ◦ Ex. Roller Bladers
Newton’s 3 rd Law �All forces come in pairs. F A on B = -F B on A The force of A on B is equal in magnitude and opposite in direction of the force on B on A �Example: Holding a book �Pg 104 28 -31
Forces of Ropes and Strings �Tension is the force exerted by a string or rope �Strings/ropes are considered massless �Ex bucket tried to string in equilibrium. �The tension in the rope is equal to the weight of the bucket (like a scale) �Example: Tug-of-War �Pg 106 32 -33
Normal Force �Any time two objects are in contact, they each exert a force on each other. �The normal force is the perpendicular contact force exerted by a surface on another object. �Always equal to weight of the object
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