5 Newton’s Laws of Motion5-1 Force and Mass

Question: How is acceleration induced? Δv/Δt

V=20 m/s

wall

V=5 m/s V=-5m/s

V=0

Baseball:

Force; influence that causes a body to accelerationMass; how difficult to change its velocity quantity of matter in object

5-2 Newton’s first law of Motion

An object at rest remains at rest as long as no net force acts on it.

An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it.

Law of inertia:

if an object is already moving with constant velocity, it won’t alter its speed or direction , unless a force causes the change.

5-3 Newton’s Second Law of Motion

Mass constant :)'(FF

)'(aa

Fa

Force constant :

'a

F

ma

1

sum of force vectors = FFnet

m

Fa

Vector: acceleration and force , Direction: acceleration is same as net force.Force Unit:1 N=(1kg)(1 m/s2)

Ex1.

Mass of a car 2000kg, the accelerate is 4 m/s2, find force?

Ex2.

A box from 0 to 0.7 m/s in 3s, m=20kg, find net force?

Net force

321 FFFF

Ex3. F

F

F

2F

1F

2F

Newton’s second law

m

Fa

m

Fa xx

m

Fa yy

m

Fa zz

• Isolate the object of interest (point particle)

• Sketch the forces (every external force acting on a given object)

• Choose a convenient coordinate system

• Resolve the force into components

• Apply Newton’s second law to each coordinate direction

Free-Body diagram