Two Chapter 4

• Newton's 2nd law tells us that an object accelerates in the direction of the force on it.
• There are two cases
  • The force is parallel to the motion.
    • The object speeds up or slows down.
    • Acceleration is in the direction of the force.
  • The force is not parallel to the motion.
    • The direction of motion changes (sometimes).
    • The magnitude of the velocity changes (sometimes).
    • Both the magnitude and the velocity change (sometimes).
    • Acceleration is in the direction of the force.
• If while helping your brother/sister to learn how to ride a bicycle, you give a strong push on the seat in the direction of motion, the bicycle speeds up.
• If you turn the steering wheel on your car so that you will go around a corner, the car will change directions, but not speed.
• If you push at an angle of 30^o to horizontal on a crate in a warehouse, the crate will accelerate parallel to the floor (change speed), but will not change direction.
• A cart of mass 24 kg has a force applied to it at 40^o to horizontal. The frictional force on the cart is 6.0 N. If the acceleration of the cart is 1.5 m/s² to the right, what is the applied force?

• There are three things to keep in mind.
  • The direction of the acceleration is the same as the direction of the force.
  • The force that causes the acceleration is the Net Force.
  • When forces prevent motions in certain directions, then net force in the allowed direction causes the acceleration in that direction

• Newton's Third Law - For every action force there exists a reaction force that is equal in magnitude but opposite in direction.

• When you push on a crate in order to move it across the floor, you apply a force. The crate applies an equal and opposite force back on you which you experience as resistance to your efforts.
• When you fly through the air in a car accident because you didn't wear your seat belts, your head will likely come in contact with something. Your head applies a force on the object (maybe the door post), and the object applies an equal but opposite force on your head.
• As you can see from these examples, all forces occur in pairs of an action and reaction force.
• Action force - the force that initiates the process
• Reaction force - the force that is present because of the action force.
• Action reaction pair - The action and reaction force.
• In order to have an action reaction pair, the action force must act on a different body than the reaction force.
  • A book applies a force on the table that it sits on. As a result, the table applies a force equal in magnitude but opposite in direction on the book.
    • Action force - The force of the book on the table.
    • Reaction force - The force of the table on the book.
    • Notice that these two forces are on different objects.
  • Gravity applies a force on a book on a table. As a result, the table applies a force on the book. These two forces are not an action reaction pair because they are both on the book.
• When more than one object is being accelerated, Newton's third law must be used to determine how the applied force affects each of the accelerating objects.
• For the diagram below, what is the tension in each string?

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October 30. 2013