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= = = × =