 A force is defined simply as a push or a pull on an object  A force is a VECTOR quantity  Units: lbs or Newtons (N)  1 lb = 4.45 Newtons  What is your weight in Newtons? _________  Net Force – the sum of all the forces acting on an object.

ForceSymbol for Force Direction of Force FrictionFfFf Parallel to surface and opposing motion NormalFNFN Perpendicular and away from surface TensionFTFT Along the rope away from object WeightFWFW Straight down towards the center of Earth AppliedFWhat ever direction you push or pull the object.

 Internal Forces – we wont do math with these  Electromagnetic (electroweak) Interactions – holds atoms & molecules together, responsible for all contact forces  Strong Nuclear Interactions – hold nucleus together  Weak Nuclear Interactions – radioactive decay

 A drawing that shows all of the forces on an object  EXAMPLES:  A flowerpot falls freely from a windowsill. (Ignore air resistance.)  A cable pulls a crate at a constant speed across a horizontal surface. The surface DOES have friction  A rope lifts a bucket at a constant speed. (Ignore air resistance.)

 An object that is at rest will remain at rest or an object that is in moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero.  Law of inertia – inertia = resistance to change in motion or direction  Equilibrium – net force = 0, moving at a constant velocity

 Restate Newton’s 1 st law IN YOUR OWN WORDS.

 The force on an object is equal to the object’s mass times its acceleration  F NET = m*a  Units: ▪ F = N ▪ m = kg ▪ a = m/s 2

 What is the net force acting on a 1.0 kg ball in free-fall?  9.8 N

 Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other applies a force of 395 N. Both forces act in the same direction. A third force of 560 N also acts on the car, but in a direction opposite to that in which the people are pushing. This force is caused by friction between the tires and the pavement. Find the acceleration of the car.  m/s 2

 All forces come in pairs. The two forces in the pair act on different objects and are equal in magnitude and opposite in direction: F A on B = - F B on A  Example:  Accelerating a Car – the wheels exert a backwards force on the road. (friction) The road responds with a forward force on the car.

 When a ball has been thrown, the force of the hand that threw it remains on it. No, the force of the hand is a CONTACT force.  A force is needed to keep an object moving. No net force = constant velocity. Coasting car.  Inertia is a force. No it’s a property of matter!  Air does not exert a force. Air pressure is VERY high. But exerted on all sides.  The quantity ma is a force. No, its just been proven experimentally that the two quantities in F = ma are equal. Directly proportional.

 What is mass?  The mass of an object is a quantitative measure of inertia  How much matter an object contains  What is weight?  The downward force on an object’s mass caused by gravity.  Points directly towards the center of the earth.  Weight still exists when you are not on Earth, but decreases the further away you get

 Mass is measured with a balance. Mass does not change no matter where you measure it.  Weight is measured with a scale. Weight changes as gravity changes.  Scales – contain a spring which exert an upward force to create equilibrium. The force of the spring = the force due to gravity.

 Mass –  Calculated with density, D = m/V  Weight –  Calculated with Newton’s 2 nd Law, F = m*a ▪ Where a = g = 9.8 m/s 2

 (p 97 #16) Kamaria is learning how to ice- skate. She wants her mother to pull her along so that she has an acceleration of 0.80 m/s 2. If Kamaria’s mass is 27.2 kg, with what force does her mother need to pull her? (Neglect friction)  22 N

 Changes in weight due to other forces acting on a body  When the net force is downwards, your apparent weight DECREASES  When the net force is upwards, your apparent weight INCREASES  Would you ‘appear’ lighter or heavier?  Riding a roller coaster downwards at a high speed  When an elevator first starts to move upwards

 APPARENT weightlessness  Because you do still have a weight!  When the acceleration downwards equals the acceleration due to gravity (9.8)

 The component of force that a surface exerts on an object with which it is in contact  The normal force is ALWAYS perpendicular to the surfaces in contact.  This is the force that keeps you from falling through your chair.  Normal Force = weight

 Static Friction – no relative motion  Pushing a heavy box and it doesn’t move  Static Friction Force  The F f(static) is more than zero, but less than μ s F N  F fs = μ s F N ▪ μ s = coefficient of static friction. This is a constant that depends on the surfaces in contact.

 (p 128 #18) You need to move a 105 kg sofa to a different location in the room. It takes a force of 102 N to START it moving. What is the coefficient of static friction between the sofa and the carpet?  102N = μ s * (105kg *9.8m/s 2 ) 

 Kinetic Friction – between two surfaces in motion  Our Model for Friction – We assume (pretend) that friction depends only on the material of the 2 surfaces in contact.  F f(kinetic) = μ k F N

 (p 128 #17) A girl exerts a 36 N horizontal force as she pulls a 52 N sled across a cement sidewalk at constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners?  36N=μ k * 52N  0.69

 A 1.4 kg block is pushed with a force of 3.5 N across a rough surface such that it slows down with an acceleration of m/s 2. What is the coefficient of kinetic friction between the block and the surface?  0.13

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 Tension force is the force applied through a rope  We assume that the force applied at one end of the “massless” rope is transmitted completely to the other end of the rope

 Air resistance/drag is a type of friction force  This force DOES depend on the speed of an object and the size/shape of the object  The drag force increases as an object’s speed increases  When the drag force = g, then there is no net force and no more acceleration. This is called TERMINAL VELOCITY.  This is the highest velocity a falling object reaches!

 Exists between ALL objects  Depends on the objects masses and the distance between them  Law of Universal Gravitation