Forces and Newtons Laws of Motion Tension Tension
- Slides: 41
Forces and Newton’s Laws of Motion
Tension • Tension (T)- force that acts through a solid object such as a rope or chain; directed along the rope and pulls equally on the objects on either end. .
Normal Force • Normal Force (Fn)- contact force that always acts perpendicular to the surfaces that are touching
Free-Body Diagram (FBD) • Free-body diagrams are drawings used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
Basic Force Diagrams A Car on a Level Surface All forces on the car are vertical, so no horizontal force can be generated. A Car on a Banked Turn The normal force on the car due to the road is no longer vertical, so a component of the normal force acts in the horizontal direction. Normal force (N) - contact pressure that pushes surfaces together. It is easier to drag a light chair across a room than a heavy table. This is because the weight of the table exerts a greater normal force.
FBD: A book is at rest on a table
A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.
An egg is free-falling towards the ground. Neglect air resistance.
A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance.
A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.
Real Life Application
More real life… http: //www. ux 1. eiu. edu/~cfadd/1150/04 Nwtn/appl. html
Another Force Diagram… The chandelier is separated from the chain for interpretation purposes only! http: //www. ux 1. eiu. edu/~cfadd/1150/04 Nwtn/appl. html
Bring in the anglesa bit more complex http: //www. ux 1. eiu. edu/~cfadd/1150/04 Nwtn/appl. html
Box on an Incline http: //img. sparknotes. com/figures/1/174 f 9 bfb 5307940 e 1 bb 1 c 2 ddac 88 fd 8 d/problem 1_5. gif
Net Force Balanced = No Net Force = Fnet = 0 N Unbalanced = Net Force = Fnet ≠ 0 N • Determined by combining ALL forces acting on an object. – Zero net force = zero acceleration – If there is a net force, there will be an acceleration. http: //4. bp. blogspot. com/_f. DHme. Cj. Bbk/SS 4 h. RNb. Lwa. I/AAAAAHw/gx 0 p 9 HIDS 1 o/s 1600 -h/12. gif
Balanced Forces (zero net) www. physicsclassroom. com
Unbalanced Forces (non-zero net) Imagine pushing your text book so that it would slide across the desk… www. physicsclassroom. com
Equilibrium • Balance – In regards to motion an object has constant velocity if it is in equilibrium. – NO acceleration. • Types: – Static Equilibrium – Dynamic Equilibrium
Static Equilibrium Velocity is zero Scales pushing up Examples: Weighing yourself on a set of scales Car parked on an incline Friction Normal Weight down
Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocity Normal up Air resistance Force from road Terminal velocity when parachuting Weight down Friction Weight down
Friction • The force that opposes the motion between two surfaces that are in contact. • Friction is the "evil monster" of all motion. Regardless of which direction something moves in, friction pulls it the other way. – Move something left, friction pulls right. Move something up, friction pulls down. • It appears as if nature has given us friction to stop us from moving anything. • Friction is a force that “appears” when there is relative motion between two objects. – Although two objects might look smooth, microscopically, they are very rough and jagged.
Friction • A force that opposes motion. Friction acts in a direction opposite to the object's direction in motion. Without friction, the object would continue to move at a constant speed forever • Static Friction: when object is at rest • Kinetic Friction: when object is moving – Sliding Friction: – Rolling Friction:
Friction is not always a bad thing! • Walking! Your foot pushes backwards on the ground and friction opposes the push of your foot by pushing your foot forward
Coefficient of Friction • A constant that depends on the two surfaces in contact • Ff = FN • (mu)…. coefficient of friction Coefficient of Friction - Friction on an Incline
Newton’s First Law (1642 -1727) • “The Law of Inertia” • A body remains at rest or moves in a straight line at a constant speed unless acted upon by a net force. – Objects do not accelerate unless a net force is applied.
Newton’s 1 st Law of Motion Objects at rest remain at rest and objects in motion remain in motion, unless acted upon by an outside force. http: //talesfromechocanyon. blogspot. com/2007_03_02_archive. html
Newton’s 2 nd law of Motion …mathematically Net Force = (mass)(accel) Fnet = ma
NEWTON'S 2 nd LAW OF MOTION F a m F M a m m F m a F M a
When the acceleration is g we have Free Fall m 2 m F 2 F
Terminal Velocity Net Force Acceleration = g but motion is about to begin mg F Velocity = 0 Acceleration < g v increasing downward Acceleration << g v still increasing downward mg F just not as rapidly as before mg F Acceleration = 0 mg Terminal velocity
Falling with Air Resistance & Terminal Velocity • When falling the force of air resistance becomes large enough to balance the force of gravity. • At this instant in time, there is no net force — the object stops accelerating (see D below); terminal velocity has been reached.
Newton’s Third Law • Action-Reaction • When one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object. Example of Newton’s 3 rd Law: http: //bp 3. blogger. com/_v. QA 3 QRUnk 3 M/Rrgyfr. Rni. PI/AAAAAMs/DYl h. M 7 p. De. I 8/s 1600 -h/newton
Newton’s Third Law of Motion For every action, there is always a reaction of equal (magnitude) and opposite (direction) reaction. “action” or “reaction” refers to force. Action/Reaction forces do NOT act on the SAME object!
Reaction: road pushes on tire Action: tire pushes on road
Reaction: gases push on rocket Action: rocket pushes on gases
Action- Reaction Forces • Do Action-Reaction forces cancel each other? • No, they are acting on different objects. Forces can only be added together when they are acting on the same object. http: //quest. nasa. gov/space/teachers/rockets/principles. html
rd Newton’s 3 Law of Motion Reaction Force: the ball is pushing the guy rightwards Action Force: A guy is pushing a ball leftwards
Tug-a-war • If Fido and Rover play tug-a war, how do the “pulls” of the dogs compare? • If each dog pulls with 50 N of force, what is the tension force in the middle of the rope (between the dogs)?
• While driving, Mrs. Ingle observed a bug striking the windshield of her car. The bug hits the windshield and the windshield hits the bug. Which of the two forces is greater: the force on the bug or the force on the windshield? EXPLAIN!
Putting Newton's Laws of Motion Together http: //www. sp acetoday. org/i mages/Rocket s/Ariane. Rock ets/Ariane 5 La unch. Arianesp ace. jpg • An unbalanced force must be exerted for a rocket to lift off from a launch pad or for a craft in space to change speed or direction (First Law). • The amount of thrust (force) produced by a rocket engine will be determined by the rate at which the mass of the rocket fuel burns and the speed of the gas escaping the rocket (Second Law). • The reaction, or motion, of the rocket is equal to and in the opposite direction of the action, or thrust, from the engine (Third Law). http: //quest. nasa. gov/space/teachers/rockets/principles. html
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