NEWTON'S THIRD LAW OF MOTION

THIRD LAW LAW OF INTERACTION For every action, there is an equal but opposite reaction. When a force a applied to an object, the object exerts a force that has the same magnitude but opposite in direction. These are called action-reaction forces (Two forces + two objects). Box on table Table on box

THIRD LAW LAW OF INTERACTION

ACTION-REACTION FORCES

Action-reaction forces present on a moving car-Earth system Normal force of the Earth pushing up on the car The road’s friction on the car’s tires The road pushing the car forward Earth’s Gravity pulling the car down The car’s tire pushing on the road The car’s gravity pulling the Earth up Contact force of the car pushing down on the Earth The cars friction on the road

Steps to construct a free-body diagram 1. Sketch the Forces – Identify and sketch all of the external forces acting on an object. 2. Isolate the Object of Interest – Replace the object with a box. Apply each of the forces acting on the object to that point. 3. Choose a Convenient Coordinate System – If the object moves in a known direction, it is often convenient to pick that direction for one of the coordinate axes. 4. Resolve the Forces into Components – Determine the components of each force in the free-body diagram. 5. Apply Newton's Second Law to each Coordinate (up, down, left, right) Directions – Analyze motion in each coordinate direction using the component form of Newton's Second Law.

FREE BODY DIAGRAM

Foamcrete is a substance designed to stop an airplane that has run off the end of a runway, without causing injury to passengers. It is solid enough to support a car, but crumbles under the weight of a large airplane. By crumbling, it slows the plane to a safe stop. For example, suppose a 747 jetliner with a mass of 1.75 X 105 kg and an initial speed of 26.8 m/s is slowed down to a stop in 122 m. What is the magnitude of the retarding force exerted by the Foamcrete on the plane?

We need to find the acceleration of the plane first. Given:              This leads to the kinematics equation to find the acceleration. Plugging in the given values and solving for a gives:                                                                                               

The force will be negative because we are slowing the plane down. Now we need to use Newton's Second Law to find the force. The only direction we are worried about is the x direction. Therefore, all of the force in that direction needs to be added up and set equal to the mass and acceleration.                                 The force will be negative because we are slowing the plane down.                                                                                                             -

RELEASED TAKS 2004

RELEASED TAKS 2006