lecture outline chapter 5 -...
TRANSCRIPT
LECTURE OUTLINE
CHAPTER 5
Newton’s Laws
of Motionof Motion
5-1 Force and Mass
� Force: push or pull
� Force is a vector – it has magnitude and direction
� Mass is the measure of how hard it is to change an
object’s velocity.object’s velocity.
5-2 Newton’s First Law of Motion.
� Newton's first law:
� An object at rest will remain at rest and an object in
motion will remain in motion with a constant velocity
unless acted on by a net external force.unless acted on by a net external force.
� This is also known as the law of inertia.
� In order to change the velocity of an object –
magnitude or direction – a net force is required.
5-3 Newton’s Second Law of Motion.
� Newton's second law:
� The acceleration of an object is directly
proportional to the net force acting on it and
inversely proportional to its mass.inversely proportional to its mass.
� Acceleration is inversely proportional to mass:
5-3 Newton’s Second Law of Motion.
� 2- Acceleration is proportional to force:
� Combining these two observations gives
Or
5-3 Newton’s Second Law of Motion.
� An object may have several forces acting on it; the
acceleration is due to the net force (sum of force
vectors):
5-3 Newton’s Second Law of Motion.
� A free-body diagram shows every force acting on
an object.
� Example of a free-body diagram:
x xF ma=∑
y yF ma=∑
5-3 Newton’s Second Law of Motion.
5-4 Newton’s Third Law of Motion
� Newton’s Third Law (the law of action-reaction):
� when two bodies interact, the force which body "A"
exerts on body "B" (the action force ) is equal in
magnitude and opposite in direction to the force magnitude and opposite in direction to the force
which body "B" exerts on body "A" (the reaction
force).
5-4 Newton’s Third Law of Motion
� the action force and the reaction force act on
different objects.
� If object 1 exerts a force on object 2, then object
2 exerts a force – on object 1.
F�
F�
2 exerts a force – on object 1.F
�Weight & Normal force
� The weight of an object on the Earth’s surface is the
gravitational force exerted on it by the Earth.
� The normal force
force exerted by a surface on an object.
The normal force is always perpendicular
to the surface.
Weight & Normal force
�EXAMPLES:
� Example 5-1: three forces
� The boat’s Mass= 752kg
�
� Fined in the two cases
80.5M L CF F F N= = =
a�
�EXAMPLES:
2.0x m∆ =
�EXAMPLES:
�Contact force
� The action reaction force when objects are touching
one another.
� For object 1
21F ←21 1F F m a− =� For object 2
� is the contact force
12F→21F ←21 1F F m a− =
12 2F m a=
21 12F F−=� �
12F
1 2a a a= =
� Solution
� Part a21F ←
12F→21 1
20 20 ( )
F F m a
m a m a m m a
− =− = ⇒ = +
� Part b:
2 1 1 2
2
20 20 ( )
201.33 /
15
m a m a m m a
a m s
− = ⇒ = +
= =
21 12 2 5 1.33 6.67F F m a N= = = × =