Newton’s 3 rd Law

1. Newton’s 3rd Law Physical Science Section 3.3

2. Newton’s third law of motion • when 1 object exerts a force on a 2nd object, the 2nd object exerts a force that is equal in size and opposite in direction to the force from the 1st object • For every action, there is an equal & opposite reaction • Swimming- swimmer exerts force on water; water exerts force on swimmer • Action-reaction pairs act on different object • Rocket engine- rocket engine exerts force on the hot gases produced by the fuel • gases exert a force on the rocket and push it forward

3. Newton’s 3rd Law • Orbit of earth in space influenced by: • gravitational forces between the Sun and the Earth and between other planets and the Earth • Discovery of Neptune • Orbit of Uranus could not be explained by the planets known at the time • Bicycle’s easier to stop than a car- less mass • Less mass = less inertia and less momentum

4. Object with less speed = less momentum • Less speed = easier to stop • Momentum- product of mass and velocity- influences how easily an object can be stopped • P = m x v • P : momentum Units kg*m/s2 • Has direction because velocity has direction • Indicate direction of momentum • Force can be calculated by using the final and initial momentum when an object changes its velocity. • F = (mvf – mvi)/t)

5. Law of conservation of momentum- total momentum does not change in a collision • Momentum before collision = momentum after collision • 2 objects collide when moving in the same direction • first object slows and the second speeds up in the same direction • 2 objects collide when moving in opposite directions with the same momentum • Start with 0 momentum, collide, bounce off in opposite directions, still have 0 momentum