Newton’s Laws of Motion (and force!!) A force is a push or pull on an object control over movement: desired motion or preventing undesirable movement EXAMPLES of forces: Contact Forces muscles machines/mechanical ropes and wires Friction adhesion EXAMPLES of FORCES TOO: Non-contact forces: Nuclear Electrical/magnetic Gravity weight Non-contact Forces such as gravity, magnetic, and electric forces. Like Velocity and Acceleration….. FORCE has both magnitude AND Direction!!! Net FORCE (unbalanced force) results in motion (for an object at rest), or a change in velocity of the object already in motion Equilibrant Force Net Force = 0 …..means, an object at rest remains at rest OR an object moving with constant motion will continue at that constant motion. FORCE IS MEASURED IN NEWTONS One NEWTON (N) 1 kg x 1m s2 Think about it…. Force = mass X acceleration thatgives a 1 kg object, an acceleration of 1m/s2 Inertia tendency of an object to RESIST a change in motion Example: velocity of an object remains constant unless a force changes it the greater the mass, the greater the inertia Newton’s First Law of Motion An object continues in a state of rest or in a state of motion at constant speed along a straight line… unless compelled to change that state by a net force. an object moving at a constant velocity remains at that velocity unless a NET FORCE (>0) acts upon it Inertia plays a central role in one type of seat belt mechanism. Friction force of adhesion depends on the surface and force acting between surfaces EXAMPLES OF FRICTION: air, fluid, solids The Normal Force & Friction The Normal Force & Friction Newton’s Second Law of Motion When a net force F acts on an object of mass m, the acceleration a that results is directly proportional to the net force and is inversely proportional F to the mass. If: net F = m X a then…. a m direction of acceleration = direction of the net force SI Unit of Force: kg • m/s2 = newton (N) AND ONCE AGAAAAAINNN!!! A NEWTON (not the “fig” variety) ….is the amount of force required to accelerate a 1 kg object 1 m/s2 m = 1850 kg With two guys pushing……. What are the net forces??? F + 275 N + 395 N 560 N = +110 N a F 110 N 0.059 m/s 2 m 1850 kg If the airplane’s mass is 13 300 kg, what is the magnitude of the net force that the catapult and jet engine exert on the plane? F ma (13 300 kg)(31m/s ) 2 = 4.1105 N The Normal Force GRAVITY (gravitational force) the pull an object exerts on another object the amount of gravitational force is dependent on: 1) Mass of the two objects 2) distance between objects the greater the mass, the greater the gravitational pull on that object Weight measure of gravitational force varies dependent on proximity to EARTH Weight vs Mass Mass is the measure of… the matter of an object AND an object’s resistance to change in motion Newton’s Third Law of Motion Whenever a body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. Newton’s 3 rd Law…… action force = reaction force for every action, there is an equal and opposite reaction!! EXAMPLES: tires of a car push against the road, and the road in turn pushes back on the tires, OR a swimmer pushes the water backward, and the water pushes swimmer forward in both examples, the net Force is > Zero, otherwise the object wouldn’t move!!! Newton’s Third Law For every action there is an equal and opposite reaction. Free Body Diagrams A Free Body Diagram distills the problem’s complexity down to only those actions that are interacting with the object of interest. Consider the following situation: You are standing in the middle of the room. Are there any forces acting on you? Is there net force acting on you? Draw the forces acting on you. Justify your answer by referring to Newton’s Laws. Now remove the floor and draw the forces acting on you. What is your state of motion now? Characterize it by stating something that we can calculate.
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