Newton 3

Newton's Third Law of Motion

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Are Force Pairs the same as Balanced Forces?

NO! Force pairs are 2 forces acting to two DIFFERENT objects. When you high-five someone in space there is the force you put on your partner and the force they put onto you. The forces do not balance, which is why you both accelerate in opposite directions.

Balanced forces are 2 forces acting on the SAME object. Because it's no the same object they cancel out so there is no acceleration. For example, if your applied force is the same as the force of friction the object won't move. If your pull on a rope is equal to someone else pulling on the same rope in the opposite direction, there is no acceleration because they balance!

Identifying Force Pairs:

• When you push on a wall, eventually your feet slide backwards because the wall is pushing back against you!
• Balloons: Letting go of a balloon, as the air rushes out with a force it causes the balloon to move forward with a force.
• Rockets: Exhaust flow pushes one way, the rocket is pushed the opposite way!
• Pushing someone on ice skates/ roller skates/ skate boards

Momentum: The quantity of motion of a moving body, measured as a product of its mass and velocity. (Vector Quantity - has direction!) This can also be defined as how hard it would be to stop a moving object. More momentum means it's harder to stop.

• p = mv
• momentum = mass * velocity (speed)
• Units: kg * m/s
• Momentum is conserved - your starting momentum equals the momentum at the end.

Examples of Momentum:

• Momentum:
  • Bullets - Small mass but move at such high speeds that they have high momentum (they are VERY hard to stop!)
  • Red Rover - When playing Red Rover, the faster you are going the harder you are to stop! Also, the bigger the person, the harder it is to stop them.