Transcript Chapter 3
Chapter 3 - Forces Section 1 – Newton’s 2nd Law • Objects accelerate in the direction of the net force. Section 1 – Newton’s 2nd Law • Force, mass, and acceleration are related ▫ The harder you kick it, the faster it moves ▫ The more massive it is, the slower it moves • F=ma ▫ F is Force ▫ m is mass ▫ a is acceleration Calculate • You are pushing a friend on a sled. You push with 40N. Your friend and the sled together have a mass of 80kg. Ignoring friction, what is the acceleration of your friend on the sled? ▫ ▫ ▫ ▫ F=ma 40N = (80kg)a Divide both sides by 80kg 0.5 m/s2 = acceleration Calculate • A student pedaling a bike applies a net force of 200N. The mass of the rider and bike is 50kg. What is the acceleration of the bike and rider? ▫ F=ma ▫ 200N = (50kg)a ▫ 4m/s2 = acceleration Friction • Reduces motion • Rough surfaces have more friction • Three types: ▫ Static—Keeps it from moving ▫ Sliding—Must apply constant force to keep it moving ▫ Rolling—Curved objects roll Air Resistance • Friction in air • More surface area = more air resistance • Without air resistance, everything falls at same rate. • Terminal Velocity – stops accelerating; falls at constant speed Prove it! • Friction Mini-lab • Air Resistance Mini-lab Section 2 - Gravity • EVERYTHING with mass has gravity • Determined by mass and distance ▫ Small = less gravity ▫ Far away = less gravity • A constant 9.8m/s2 on Earth Weight • Gravity’s pull on you is your weight • F=ma ▫ Force can be gravitational force too! ▫ a can be gravitational acceleration too! • On Earth, an object with 100kg of mass weighs 980Newtons ▫ F=(100kg)(9.8m/s2) • Can loose weight by loosing mass or lowering gravity’s pull. Mass Weight • Amount of matter • Gravitational Force • SI is kilograms (kg) • SI is Newton (N) • Measured on balance • Measured on scale • DOESN’T change with gravity • CHANGES with gravity • All matter has mass • Can feel weightless Prove It! Projectile Motion • Thrown objects curve downwards ▫ Have forward and downward force • Without gravity, objects would go straight ▫ Have only forward force • BOTH fall at the same rate Centripetal Force • Accelerate towards center of a spin • A car making donuts—hold on to accelerate toward center—let go to move away from center • If you let go, you travel in a STRAIGHT LINE Prove It • Horizontal and Vertical Motion Mini-Lab ▫ Watch demonstration and perform lab ▫ Draw the lab to explain • Centripetal Mini-Lab ▫ Watch demonstration ▫ Draw the lab to explain Centripetal Examples • • • • • Water through curled hose Amusement Park Rides Moon around Earth Satellite around Earth Earth around the sun Section 3 – Newton’s 3rd Law of Motion • Every action has an equal but opposite reaction ▫ You push down on Earth; Earth pushes up on you ▫ You push ground backward; it pushes you forward ▫ Bird pushes air down and back; air pushes bird up and forward Momentum • Force needed to change motion • Is conserved (Not gained or lost just changed) • p = mv ▫ p is momentum ▫ m is mass ▫ v is velocity • Faster = more momentum • More mass = more momentum p = mv • Compare the momentum of a 50kg dolphin swimming 10.4m/s and a 6,300kg elephant walking 0.11m/s. ▫ Dolphin’s momentum is 520kg m/s ▫ Elephant’s momentum is 693kg m/s • Which is harder to change their motion? Conservation of Momentum • Momentum is transferred • Body transfers momentum to a bowling ball. The ball transfers momentum to the pins. Pins transfer momentum to the ground.