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FORCES

FORCES. Newton’s First Law of Motion. -An object moving at a constant velocity(constant speed and direction) keeps moving at that velocity unless an unbalanced net force acts on it. -AKA: The law of Inertia. Newton’s Second Law.

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FORCES

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  1. FORCES

  2. Newton’s First Law of Motion -An object moving at a constant velocity(constant speed and direction) keeps moving at that velocity unless an unbalanced net force acts on it. -AKA: The law of Inertia

  3. Newton’s Second Law • States: The acceleration of an object is in the same direction as the Net Force on the object and can be calculated from this equation: • acceleration = Net force (N) • Mass (kg) -It basically tells us how forces, mass, and acceleration are all related!

  4. Force and Acceleration - The greater FORCE you apply to an object = the greater ACCELERATION of the object Tapping a soccer ball with your foot produces a small acceleration but kicking it produces a large acceleration!

  5. Mass and Acceleration -The greater the mass of an object = the less acceleration of the object with the same force applied Using the same force (strength), you can hit a baseball farther than a softball

  6. You can also use Newton’s Second Law formula to calculate Net Force: just reorder it! Net Force = mass X acceleration

  7. Solve a Simple Equation You push a friend on a sled. Your friend and the sled together have a mass of 70 kg. If the net force on the sled is 35 N, what is the sled’s acceleration? Net Force = Fnet = 35 N Friend + Sled Mass = m = 70 kg Want to know Acceleration = a = m/s2 A = Fnet / m A = 35 N / 70 kg = 0.5 N/kg = 0.5 kg m X 1 = 0.5 m/s2 S2 kg

  8. FRICTION - Force that opposes the sliding motion of 2 surfaces that are touching each other Depends on Two Factors: 1. Type of Surfaces 2. Force pressing surfaces together Caused By: Microwelds (bumps on the surface of objects) which stick together

  9. Types of Friction Surfaces STICK together. Increase the force pushing objects together = Surface bumps become closer = Increase Friction. A force must be applied to break the sticking • Sticking Friction • Static Friction • Sliding Friction • Rolling Friction Frictional Force that prevents two surfaces from sliding past each other Force that opposes the sliding motion between two surfaces. Microwelds constantly break and reform as you slide. Must apply a Force to continue sliding. Friction between rolling objects and a surface. Rolling Friction causes a train to come to a stop.

  10. -Opposes the motion of objects that move through the air -Causes objects to fall at different accelerations and different speeds -This is why a feather falls at a slower speed than a bowling ball -Object = Air resistance -Depends on objects SIZE, SHAPE, and SPEED Air Resistance

  11. Terminal Velocity Term. Vel = 53m/s for humans, slows to 5-10 m/s with open chute -The highest speed a falling object will reach -It is a constant velocity (speed) thus the net force = 0 b/c air resistance balances force of gravity, and acceleration = 0 -Depends on size, shape, and mass -Ex: Open parachute v/s Closed parachute (air resistance is greater for the opened one)

  12. GRAVITY • Attractive force between any 2 objects • Depends on Masses of the objects and the distance between the objects • Force of gravity between objects increases as the objects’ mass increases or as they get closer together. Decreases as distance increases • Everything with mass exerts the force of gravity! Why don’t you feel the force of gravity from your book? Why don’t you feel the force of gravity from the sun? Why do you feel the force of gravity from the Earth?

  13. Law of Universal Gravitation • Developed by Newton (1687) • Long-range force (never reaches zero) • G = universal gravitational constant • = 6.67 X 10^-11 m3/kg s2 • F = Force of gravity between any 2 objects F = G (m1m2) d2

  14. Gravitational Acceleration • Acceleration due to gravity (w/out air resistance) = • 9.8 m/s2 F of gravity (N) = mass (kg) X acceleration of gravity (m/s2) F = mg

  15. Weight • Review: Mass = amount of matter in an object • You find your mass by standing on a scale • Gravitational force exerted on an object = object’s weight • Weight (N) = mass (kg) X acceleration of gravity (m/s2) • W = mg • Weight changes as gravitational force on objects change

  16. Weightlessness and Freefall Freefall = force of gravity stills acts on you Weightlessness = You feel weightless because you no long exert a force down. Everything in freefall falls with the same acceleration so you seem to be floating.

  17. Projectile Motion • Projectile = anything thrown/shot through the air • Gravity causes projectiles to follow a curved path • Horizontal and Vertical Motions • Independent of each other • Horizontal motion can be constant while vertical motion is changing due to gravity • Horizontal and Vertical Distance • Objects travel same vertical distance in same time even though one travels more horizontal distance

  18. Newton's 3rd Law Describes Action and Reaction When one object exerts a force on another object, the second object exerts a force on the first that is equal in strength and opposite in direction “to every action there is an equal and opposite reaction”

  19. Action and Reaction Forces Action and Reaction Forces are EQUAL So how does anything ever happen?? For instance swimming, if you push down on the water and the water pushes back, why are you propelled through the water?? Action and Reaction Forces act on different objects. Even though they are equal they are NOT Balanced.

  20. Rocket Propulsion As a rocket burns fuel it produces hot gases. The rocket exerts a force on the gasses and causes them to escape out the back. The gases in turn exert a force on the rocket and cause it to…..??? Liftoff!!

  21. Momentum • The product of an object’s mass and velocity • Related to how much force is needed to change an object’s motion • Symbol: p • Equation: • momentum (p) = mass (kg) X Velocity (m/s) • p = mv

  22. So how can a football player increase his/her momentum?? Run Faster or Increase his/her Mass p = m X v

  23. Force and Changin Momentum Review: Acceleration Newton’s Second Law a = Vf – Vi F = m X a t Put these two together and you write the 2nd law this way: F = (mvf – mvi) t mvf and mvi are both momentum (p) So: You can calculate Net Force if you have momentum

  24. Conservation of Momentum Momentum can be transferred from one object to another: it isn’t lost or created Ex: Pool You hit the cue ball into the group of balls and they all begin to move. The momentum the cue ball had is transferred to the other balls.

  25. Collisions If a hockey puck is moving to the right and gets hit from behind by a second hockey puck what happens? The hockey puck that got hit increases its speed. It gains momentum from the second puck. If two hockey pucks collide going opposite directions what happens? They reverse their direction and move with the same speed. Total momentum is Zero

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