Forces and Newton’s Laws

Forces• Forces are ________ (magnitude and direction)• Contact forces result from ________ ________• Field forces act ___ __ __________

Fig. 4.2, p. 84

4 Fundamental Forces

• Strong nuclear force• Electromagnetic force• Weak nuclear force• Gravitational force

• All are ______ forces• Nuclear are _______

______ (femtometers)• Electromagnetic and

gravitational range is _________

Newton’s First Law

• An object moves with a constant velocity, unless acted on by a ________ net force.– Velocity is a vector: magnitude and direction

• Net force: _______ _____ of all forces exerted on object

Inertia vs. Mass

• Inertia: tendency of object to continue its _______ in the __________ of forces

• Mass: measure of object’s __________ to ________ in _______ due to force

Newton’s Second Law

• Acceleration of an object is – directly __________ to net force acting on it– _________ proportional to object’s mass

m

Fa

1.4aF

m

This is a vector equation

xx maF yy maF 2.4zz maF

Unit of Force, Newton (N)

2smkg1N1

lb225.0N1

slug1085.6kg1 2

US unit of mass

The Gravitational Force• Newton’s Universal Law of gravitation:• Every particle in the Universe attracts every other

particle with a force that is:– Directly proportional to the product of the particles’

___________– Inversely proportional to the _______ of the __________

between them

5.42

21

r

mmGFg

Fig. 4.6, p. 90

Weight

6.4mgw

• Magnitude of Fg acting on object near Earth’s

surface

• g is acceleration of gravity (of Earth)

8.42r

MGg E

• r is radius of Earth (RE) plus height (y) above

surface of Earth

yRr E

Newton’s Third Law• Forces in nature always exist ___ _______• A _______ __________ force can’t exist

• The force F12 exerted by object 1 on object 2 is

equal in _________ but opposite in _________ to the force F21 exerted by object 2 on object 1

2112 FF

Fig. 4.7, p. 92

Force Pairs & the Normal Force• TV resting on table

attracted by Earth (Fg)

• Pair: ______ attracted to ______ (Fg’)

• But a = 0, so another force is present. n is _________ force of table on TV

• Pair: normal force of _____ on _____ (n’)

Fig. 4.8, p. 93

gFn

n is ______________ to contact surface and is _____________ in origin