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Chapter 4
Newton's Laws
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2. Force
A force is an action exerted on an object which may change the object’s state of rest or motion.
Forces can cause accelerations.
The SI unit of force is the newton, N (kg m/s2).
Forces can act through contact or at a distance.

3. The gravitational force (Fg) exerted on an object by Earth is a vector quantity, directed toward the center of Earth.
The magnitude of this force (Fg) is a scalar quantity called weight.
Weight changes with the location of an object in the universe.

4. Calculating weight at any location:
Fg = mag
ag = free-fall acceleration at that location
Calculating weight on Earth's surface:
ag = g = 9.81 m/s2
Fg = mg = m(9.81 m/s2)

5. Comparing Mass and Weight http://www. exploratorium

6. Force Diagrams
The effect of a force depends on both magnitude and direction.Thus, force is a vector quantity.
Diagrams that show force vectors as arrows are called force diagrams.
Force diagrams that show only the forces acting on a single object are called free-body diagrams.

7. Free-Body Diagram Force Diagram
A free-body diagram shows only the forces acting on the object of interest—in this case, the car.
In a force diagram, vector arrows represent all the forces acting in a situation.

8. What happens if you don’t understand Newton’s Laws!

9. Newton’s First Law
Newton’s first law is often called the law of inertia: Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.

11. Net Force
Newton's first law refers to the net force on an object.The net force is the vector sum of all forces acting on an object.
The net force on an object can be found by using the methods for finding resultant vectors.
Although several forces are acting on this car, the vector sum of the forces is zero. Thus, the net force is zero, and the car moves at a constant velocity.

12. Inertia
Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction.
Newton’s first law is often referred to as the law of inertia because it states that in the absence of a net force, a body will preserve its state of motion.
Mass is a measure of inertia.

13. Inertia and the Operation of a Seat Belt
While inertia causes passengers in a car to continue moving forward as the car slows down, inertia also causes seat belts to lock into place.
The illustration shows how one type of shoulder harness operates.
When the car suddenly slows down, inertia causes the large mass under the seat to continue moving, which activates the lock on the safety belt.

14. Equilibrium
Equilibrium is the state in which the net force on an object is zero.
Objects that are either at rest or moving with constant velocity are said to be in equilibrium.
Newton’s first law describes objects in equilibrium.
Tip: To determine whether a body is in equilibrium, find the net force. If the net force is zero, the body is in equilibrium. If there is a net force, a second force equal and opposite to this net force will put the body in equilibrium.

15. net force = mass  acceleration
Newton’s Second Law
The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass.
SF = ma
net force = mass  acceleration
SF represents the vector of the net force.

16. Newton’s Third Law
If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.
In simpler terms:
For every action, there is an equal and opposite reaction.
Because the forces coexist, either force can be called the action or the reaction.

17. Examples: Equal & Opposite

18. Action and Reaction Forces
Action-reaction pairs do not imply that the net force on either object is zero.
The action-reaction forces are equal and opposite, but either object may still have a net force on it.
Consider driving a nail into wood with a hammer. The force that the nail exerts on the hammer is equal and opposite to the force that the hammer exerts on the nail. But there is a net force acting on the nail, which drives the nail into the wood.

19. Momentum

20. Canon ball Frank

21. momentum = mass  velocity
Linear Momentum
Momentum is defined as mass times velocity.
Momentum is represented by the symbol p, and is a vector quantity.
p = mv
momentum = mass  velocity

22. force  time interval = change in momentum
Impulse
The product of the force and the time over which the force acts on an object is called impulse.
The impulse-momentum theorem states that when a net force is applied to an object over a certain time interval, the force will cause a change in the object’s momentum.
F∆t = ∆p = mvf – mvi
force  time interval = change in momentum

23. Linear Momentum…
Stopping times and distances depend on the impulse-momentum theorem.
Force is reduced when the time interval of an impact is increased.

24. Impulse-Momentum Theorem
Section 1 Momentum and Impulse
Impulse-Momentum Theorem
Chapter 6

25. total initial momentum = total final momentum
Momentum is Conserved!!!!
The Law of Conservation of Momentum:
The total momentum of all objects interacting with one another remains constant regardless of the nature of the forces between the objects.
m1v1,i + m2v2,i = m1v1,f + m2v2,f
total initial momentum = total final momentum