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CH 3—Forces

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  1. CH 3—Forces

  2. Force, Mass, and Acceleration • For any object, the greater the force is that’s applied to it, the greater its acceleration will be. • The acceleration of an object depends on its mass, as well as the force exerted on it.

  3. Newton’s 2nd Law of Motion • The net force acting on an object causes the object to accelerate in the direction of the net force. • Force = mass x acceleration F = ma • SI unit—kg m/s2 = Newton (N) F m x a

  4. You are pushing a friend on a sled. You push with a force of 40 N. Your friend and the sled together have a mass of 80 kg. Ignoring friction, what is the acceleration of your friend the sled? • 0.5 m/s2 • A tennis ball with a mass of 0.06 kg had an acceleration of 5,500 m/s2 as it raced across the court. How much force would the tennis racket have to exert to give the ball this acceleration? • 330 N

  5. Friction • friction—force that opposes motion between two surfaces that are touching each other • The amount of friction between two surfaces depends on two factors: • kind of surfaces • force pressing the surfaces together

  6. Air Resistance • When an object falls toward Earth, it is pulled downward by the force of gravity. Another force, called air resistance, acts in the upward direction on all falling objects. • The amount of air resistance depends on the speed, size, and shape of the object.

  7. Terminal Velocity • The force of air resistance increases until it becomes large enough to cancel the force of gravity. • The forces on the falling objects are balanced, so the object no longer accelerates. • It then falls at a constant speed called the terminal velocity (highest velocity a falling object will reach).

  8. Section 2—Gravity • According to the law of gravitation, any two masses exert an attractive force on each other. • The attractive force depends on: • Mass of two objects • Distance between two objects

  9. Acceleration Due to Gravity • Near Earth’s surface, the gravitational attraction of the Earth causes all falling objects to have an acceleration of 9.8 m/s2. • acceleration due to gravity = 9.8 m/s2 • velocity of a falling object = acceleration due to gravity x time • v = a x t

  10. An acorn falls from a tree and hits the ground in 1.5 s. What is the acorn’s velocity? • v = a x t • (9.8 m/s2) (1.5 s) • 14.7 m/s • Josh is climbing on the rafters. Brittany distracts him and he falls to the ground in 0.5 s. What is Josh’s velocity when he hits the ground? • 4.9 m/s

  11. Weight • Whether you are standing, jumping, or falling, Earth exerts a gravitational force on you. • weight—gravitational force exerted on an object • You can determine the weight of anything by using Newton’s 2nd Law: • F = m x a  W = m x g

  12. How much does a person weigh on Earth if he has a mass of 70 kg? • (70 kg) (9.8 m/s2) = 686 N • Gravity on the moon is 1/6 what it is on Earth, so your weight on the moon would be 1/6 what it is on Earth. • A person weighs 480 N on Earth. What does he weigh on the moon? • w = (480 N)(1/6) = 80 N

  13. Weight and Mass • Weight and mass are not the same. Weight is a force. Mass is the amount of matter in an object. • The mass of an object remains the same on any planet and with or without gravity.

  14. Projectile Motion • projectile—anything that is thrown or shot through the air • Because of Earth’s gravitational pull and their own inertia, projectiles follow a curved path.

  15. horizontal velocity—velocity parallel to Earth’s surface • vertical velocity—velocity perpendicular to Earth’s surface • Figure 16 page 80

  16. Centripetal Force • centripetal acceleration—acceleration toward the center of a curved or circular path • centripetal force—force that causes acceleration toward the center of the circle • In the case of a car rounding the curve, the centripetal force is the friction between the tires and the road surface.

  17. Section 3—Newton’s 3rd Law of Motion • Newton’s 3rd Law of Motion—When one object exerts a force on a second object, the second object exerts a force on the first that is equal in size but opposite in direction. • “To every action, there is an equal and opposite reaction.”

  18. Describe a person walking on a sidewalk using Newton’s 3rd Law of Motion. • A person exerts a force on the sidewalk and the sidewalk pushes back on the person, moving the person forward. • Describe a fish swimming in the ocean using Newton’s 3rd Law of Motion. • A fish pushes on the water with his fins and the reaction force of the water on the fish propels the fish forward.

  19. Momentum • A moving object has a property called momentum that is related to how much force is needed to change its motion. • momentum = mass x velocity p = m x v • SI unit: kg m/s

  20. Law of Conservation of Momentum • The momentum of an object does not change unless its mass, velocity or both change. Momentum can, however, be transferred from one object to another.

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