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Physics Chapter 4: Forces and the Laws of Motion

Physics Chapter 4: Forces and the Laws of Motion. Section 4.2 Newton ’ s First Law. Inertia. --Objects at the Earth ’ s surface come to rest if no external force is applied to them because of friction. --The natural tendency of all matter to remain

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Physics Chapter 4: Forces and the Laws of Motion

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  1. PhysicsChapter 4: Forces and the Laws of Motion Section 4.2 Newton’s First Law

  2. Inertia • --Objects at the Earth’s surface come to rest • if no external force is applied to them because • of friction. • --The natural tendency of all matter to remain • moving at a constant velocity, or to remain at • rest (acceleration = 0) is called inertia. • --Inertia is the tendency of objects to maintain • their state of motion.

  3. Galileo • --Galileo first proposed the concept of inertia • in the 1630’s when he noticed that a ball rolled • down an incline tended to rise to the same • height on another incline. • --If the second incline had a lesser slope the • ball had to roll farther to reach the same • height. • --He reasoned that if the ball did not come to a • 2nd incline, it would roll forever (to infinity), • if there were no friction.

  4. Isaac Newton • --Isaac Newton (1687) stated the concept of • inertia in his First Law of Motion: An object at rest remains at rest, and an object in motion continues in motion with a constant velocity (that is, a constant speed in a straight line) unless the object experiences a net external force.

  5. --When the net external force acting on an • object is zero, its acceleration is zero. • --An object remains at rest, or in motion with • a constant velocity, unless an outside force • acts on the object.

  6. Net Force and Inertia • --What is the net force acting on each object • in the following cases: (Fnet = 0) 1. A car that is parked? 2. A car that is moving at a constant velocity? (Fnet = 0) 3. A car whose speed is increasing at a constant rate? (Fnet is constant)

  7. 4. A car whose speed is decreasing at a constant rate? (Fnet is constant) 5. A car whose speed is increasing at changing rate? (Fnet is increasing)

  8. Inertia and Mass • --Objects that have a greater mass have a greater • inertia; the velocity is more difficult to change if the • mass of the object is greater. • --The inertia of an object is therefore directly related • to its mass, and mass can be defined as a measure of • the amount of inertia of an object.

  9. --Which has a greater inertia, a car moving at 50 km/h or a train • that is at rest? --The train, because it has a greater mass.

  10. Equilibrium • --An object which is at rest or moving with • a constant velocity is said to be in a state • of equilibrium. • --The net external force acting on an object • in a state of equilibrium is zero.

  11. --A force that causes an accelerating object • to reach a state of equilibrium must be equal • and opposite to the net external force that • was causing the acceleration.

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