Newtons Laws of Motion

Forces

Types Range

Gravitational Unlimited

Electromagnetic Unlimited

Weak Nuclear ? 10-12 m

Strong Nuclear ? 10-15 m

Newtons First Law (Law of Inertia)

If no external force acts, an object maintains

a constant velocity.

Mass Gravitational mass mg

Inertial mass mi

mg mi

Newtons Second Law

Units of Force

System Mass Acceleration Force

SI kg m/s2

N kg m/s2 British slug

ft/s2 lb slug ft/s2

Force of Gravity and Weight

Weight mg

Newtons Third Law

Action/Reaction Forces

Normal Force

Normal Force

N

mg

Normal Force

N

mg

A man stands on a scale inside a stationary

elevator.

Forces acting on the man

Reading on scale

When Moving Upward With Constant Velocity

Forces acting on the man

Reading on scale

When Moving Upward With Constant Acceleration

Forces acting on the man

Reading on scale

When Moving Downward With Constant Acceleration

Forces acting on the man

a

Reading on scale

Applications of Newtons Laws

Tension in strings

q

m

q 60o

m 20 kg

mg 196 N

Applications of Newtons Laws

Another method

q

T1

T2

mg

m

Applications of Newtons Laws

F 20 N

vo 0

m 5 kg

Dt 5 s

v ?

F

m

Find velocity of block after Dt

Applications of Newtons Laws

F -10 N

vo 20 m/s

m 5 kg

v 0

F

m

x

Find distance block moves

Find the acceleration of the two block system

Forces on m1

m1

m2

Forces on m2

Find the acceleration of the two block system

Mass 1

Mass 2

Acceleration down a frictionless incline

q

q

Three mass system - find acceleration

T1

T2

F

m

3m

2m

Three mass system - find T2

T2

T1

F

m

3m

2m

Three mass system - find T1

T1

T2

F

m

3m

2m

Three mass system - find T1

Or

T1

T2

F

m

3m

2m

Friction

Force of Static Friction

m

Friction

Force of Kinetic Friction

v

m

Table 5.2

Coefficients Of Friction

Steel on steel 0.74 0.57 Aluminum on steel

0.61 0.47 Copper on steel 0.53

0.36 Rubber on concrete 1.0 0.8 Wood on

wood 0.25-0.5 0.2 Glass on glass 0.94

0.4 Waxed wood on wet snow 0.14 0.1 Waxed

wood on dry snow ------ 0.04 Metal on metal

(lubricated) 0.15 0.06 Ice on ice 0.1

0.03 Teflon on Teflon 0.04 0.04

q

q

Acceleration on a rough incline

Pulling a block

Pulling a block with constant speed

m

The normal force

Pulling a block with constant speed

y

N

F

N

mk

F

q

f

q

m

f

x

mg

mg

The frictional force

Pulling a block with constant speed

y

N

F

N

mk

F

q

f

q

m

f

x

mg

mg

The coefficient of friction

Frictionless

Find the final speed and the time to side to the

bottom.

mg sinq

mg cosq

q

With friction

f mkN

N mg cosq

Find the final speed and the time to side to the

bottom.

f mkmg cosq

mg sinq

mg cosq

q

END