Newton’s 2nd & 3rd Laws

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…

(Good formula to remember = F(grav) = m x -9.8) 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 -9.8)

Standard Units of Force Force measured in Newtons (N) 1.0 N = 1.0 kg x m/s2 1.0 Kg weighs approximately 2.2 lbs 1.0 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) / Applied Force (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 = m x -9.8

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