 # FO S E C R FORCE A push

• Slides: 23 FO S E C R FORCE A push or pull acting on an object typically measured in Newtons (kg • m/s 2) is a vector (Has a magnitude And direction) can be combined to predict motion net force Forces can be balanced or Unbalanced Balanced Forces cause no change in the motion of an object. In order to make an object move it needs to be acted upon by an unbalanced force. TYPES OF FORCES Contact Forces §Friction §Tension Force §Normal Force §Compression §Buoyant Force §Air Resistance Non-Contact Forces §Gravity §Electromagnetic §Centripetal Force FRICTION Friction force is the result of two surfaces bein pressed together closely. This causes intermolecular attractive forces between molecules of different surfaces resulting in friction. The frictional forces depend upon the nature of the surfaces. The more rough the surface the more the frictional force. Types Of Friction : In which direction § Kinetic: Sliding Or Rolling is the force (friction) vector pointing? § Static § Fluid Force Motion Friction TENSION Force that is transmitted through a string, rope, cable or wire when it is PULLED tight by forces acting from opposite ends. It is directed along the length of the wire and PULLS equally on the objects on the opposite ends of the wire NORMAL FORCE This is also called the support force. If an object is resting upon a table then the table is exerting an upward force upon the object in order to support the weight of the object. If a person leans against a wall, the wall pushes horizontally on the person. the support force exerted on an object directly related to weight (gravity) Gravity is always perpendicular to the surfaces in contact 0 x 90 o B Box 900 Normal Force F ion t c ri l ma r No rce Fo COMPRESSION The force exerted by a compressed or stretched spring upon any object that is attached to it For most springs, the magnitude of the force is directly proportional to the amount of stretch or compression of the spring If both springs are the same size when not compressed, which spring will apply more force to the ball when released? Explain your reasoning. Images taken from: http: //www. lesjoforsab. com/standard-springs/compression-springs. asp AIR RESISTANCE It is friction due to air molecules It acts upon objects as they travel through the air opposes the motion of an object It ismost noticeable for objects traveling at fast Speeds and is Greater as the surface area increases Example § Sky Diver diving in the sky BUOYANT FORCE It is an upward force in a fluid(gas or liquid) GRAVITY natural force of attraction between any two objects factors: § distance – increased distance less gravitational pull or vice versa § mass – increased mass more gravitational pull or vice versa Why does the force of gravity have more of an impact on holding our solar system together compared to holding the parts of an atom together? ELECTROMAGNETIC FORCE force that moving charges exert on one another results from the repulsion of like charges and the attraction of opposites + + + - - Compare and contrast gravitational force and electromagnetic force. Notice how the particles with the same charge move apart and the particles with different charges move together. FREE BODY DIAGRAMS visuals that help show net force § use a square and draw all forces acting on the object. § remember size and direction of vector arrows are important! Practice Fnorm = 5 N What do you think the What is the net symbols w/subscripts force on this object? represent? Ffric = 3 N Fapp = 3 N Fgrav = 5 N WHAT’S THE NET FORCE Fnorm = 10 N Fapp = 20 N Ffric = 5 N Fgrav = 10 N You throw a baseball to your friend who is to your left. Ffric = 5 N Fapp = 15 N Fgrav = 10 N Your dog pulls you down the street on a skateboard in an eastward direction. WHAT’S THE NET FORCE (AN INTERESTING CASE) A skydiver is descending with a constant velocity. Consider air resistance. Ffric The same skydiver is descending after 30 seconds. Consider air resistance. Ffric Fgrav What has the skydiver reached in this scenario? CENTRIPETAL FORCE any force that keeps an object moving in a circle directed toward the center of the circle. In this case, the force of the ball as it accelerates around the circle is pointing inward, toward the center. LAWS OF MOTION Newton’s 1 st Law Newton’s 2 nd Law Newton’s 3 rd Law NEWTON’S 1 ST LAW objects at rest remain at rest, and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force also considered the Law of Inertia INERTIA the resistance of an object to a change in the speed or the direction of its motion directly related to mass NEWTON’S 2 ND LAW the acceleration of an object increases with increased force and decreases with increased massthe direction in which an object accelerates is the same as the direction of the force Formula: F = ma (or a = F/m) PRACTICE PROBLEMS - FORCE 1. What net force is needed to accelerate a 24 kg dogsled to a rate of 3 m/s 2? F = ma 2. F = (24 kg) (3 m/s 2) 72 kg·m/s 2 = or 72 N A 1. 5 kg object accelerates across a smooth table at a rate of 0. 5 m/s 2? What is the unbalanced force applied to it? F = ma F = (1. 5 kg)(0. 5 m/s 2) = 0. 75 kg·m/s 2 or 0. 75 N NEWTON’S 3 RD LAW states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object. VECTOR a quantity that has both direction and magnitude (size) drawn as an arrow which shows direction and magnitude (length of arrow) § consists of two parts: tail and head Head Tail Consider the vectors above. Describe the direction and relative magnitude (force) of each ball based on the vector. COMBINING VECTORS can be combined/added to help determine net force § the overall force acting on an object when all of the forces acting on it are combined What is the hockey puck’s net force? Gravity = 14 N 23 N Applied Force = 25 N Friction = 2 N Normal Force = 14 N