Chapter 3 - Forces

Section 1 – Newton’s 2 nd Law Objects accelerate in the direction of the net force.

Section 1 – Newton’s 2 nd 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/s 2 = 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/s 2 = 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/s 2 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/s 2 ) Can loose weight by loosing mass or lowering gravity’s pull.

MassWeight Amount of matter SI is kilograms (kg) Measured on balance DOESN’T change with gravity All matter has mass Gravitational Force SI is Newton (N) Measured on scale CHANGES with gravity 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 3 rd 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.