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## Newton’s 2 nd & 3 rd Laws

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**A couple things to keep straight**• Mass = the amount of “stuff” something is made of. • Weight = the measure of the force of gravity on an object.**A couple of things to remember**• You can use Newton’s 2nd Law to solve for weight. • Weight = force of gravity acting on a specific object. (acceleration due to gravity is -9.8m/s2 ) therefore…**How to solve for weight**• Fnet = m x a (2nd Law) • Weight = mass of object x acc. due to gravity • F(grav) = m x acc (due to “g”) • Weight = m x acc (due to “g”) so: F(grav) = weight (Good formula to remember = F(grav) = m x g)**Standard Units of Force**• Force measured in Newtons (N) • 1 N = 1 kg x m/s2 • 1 Kg weighs approximately 2.2 lbs • 1 kg weighs approximately 10 N • So…10 N = 2.2 lbs**Solving a Force Problem with Newton’s 2nd Law**• Fnet= sum of all forces • If there is no motion in one direction (“X” or “Y”), forces are balanced…nothing to solve. • Must determine direction of motion and solve for those net forces • Fnet= Fapp - Ff OR Fnet = Fn – Fg**A question to make you think???**• Is the force of gravity the same for all objects on Earth? Answer… NO!!! Due to mass, however acceleration is the same!**A couple things about Newton’s 3rd Law**• Forces always occur in pairs • “For every action there is an equal & opposite reaction” Fa on b = -Fb on a Ex. Car driving down the road. What are the forces?**A couple things about Newton’s 3rd Law (cont.)**What forces are acting on the car? Force of grav. (down) / Normal Force (up) Force of friction (road on car) / Force of friction (car on road)**A couple things about Newton’s 3rd Law (cont.)**• Drop a softball (mass 0.18Kg) ? What is the force on Earth due to the ball ? What is the Earth’s acceleration (mass of Earth = 6.0 x 1024 Kg) hint: use Fg = mg**Softball Answer**• F(grav) = (0.18 kg) x (-9.8 m/s2) F = -1.8 N (therefore, force on ball from Earth = 1.8 • Acceleration = 1.8 N = (6.0E24)a 3.1E-25 m/s2