Force and Motion - General Physcis - Lecture Slides, Slides of Physics

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4. Newton's Laws

The Wrong Question

Newton’s 1

st

nd

laws

Forces

The Force of Gravity

Using Newton’s 2

nd

Law

Newton’s 3

rd

Law

What forces govern the motion of the sailboard?

Ans:• Human body: contact force• Wind & water: fluid pressure• Gravity: action-at-a-distance

4.2. Newton’s 1

st

nd

laws

Net force determines motion.Newton’s 1

st

law of motion:

A body at rest or in uniform motion remains sounless acted on by a nonzero net force.

F

net

^0

v

F

net

^0

v

const

GOT IT? 4.1.

On a horizontal tabletop is a curved barrier that exerts a force on a ball,guiding its motion in a circular path.After the ball leaves the barrier, which of the dashed paths shown does it follow?

Mass, Inertia, & Force

Loaded truck hasgreater mass,more inertia,less acceleration.

Inertia: resistance to changes in motion.^1

st

law = law of inertia

known

known

f^

m

a

unknown

unknown

m

a

unknown

known

known

unknown

m

a

m

a 

Operationaldefinition of mass

1 N (Newton)



force required to give a mass of 1 Kg anacceleration of 1 m / s

1 N = 1 Kg m / s

1 Dyne = 1 g cm / s

Example 4.1. Accelerating Car

A 1200 kg car accelerates constantly in a straight line from rest to 20 m/s in 7.8 s.a)What is the net force on it?b)What is the net force on it if it rounds a bend 85 m in radius at v = 20 m/s?

f^

m a 

v

m

t 

^

^

m

s

kg

s

k N

f^

m a 

2 v m

r

^

^

2

m

s

kg

m

k N

4.3. Forces

Examples of forces:• Pushes & pulls.• Car collided with truck & stopped.• Moon circles Earth.• Person sitting on chair.• Climber on rope.

(action-at-a distance)

(contact force)

(tension force)

F

c^

=



F

g

T

=



F

g

The Fundamental Forces

The fundamental forces:

• Gravity:

large scale phenomena

• Electroweak force
  • Electromagnetic force:

everyday phenomena

• Weak (nuclear) force
  • Strong (nuclear) force

1

10

25

10

36

10

38

Weightlessness

These astonauts feel “weightless” becausethey are in freefall.Their weight is about 93% of that on surface.

same

a

4.5. Using Newton’s 2

nd

Law

Tactics 4.1. Free-Body Diagram1.Identify object of interest & forces on it.2.Object



dot.

3.Draw forces on object as vectors at dot.

GOT IT? 4.4.

How’s

T

compared to

w = m g

if the elevator

a)moves upward starting at rest.b)decelerates to stop while moving upward.c)starts moving downward, accelerating from rest.d)slows to stop while moving downward.e)moves upward with constant speed.

greater greaterequal

lessless

Conceptual Example 4.1.

At the Equator

When you stand on a scale, the scale reading shows the force it pushes up to support you.If you stand on a scale at Earth’s equator, is the reading greater or less than your weight?

Ans:You’re in uniform circular motion so that the net force on you is centripetal.

2 v

m a

m

R

scale F

W

m a

W

scale

W

F

F

scale

W =

m

g

m

a

4.6. Newton’s 3

rd

Law

Newton’s 3

rd

law: Action = Reaction

Forces of 3

rd

law pair act on different objects.

Hence, they don’t cancel each other out.

Horse-Cart dilemma

Example 4.4. Pushing Books

2 books lie on frictionless horizontal surface.You push with force

F

on books of mass m

. 1

which in turn on book of mass m

. 2

What force does the 2

nd

book exert on the 1

st

?

1

2

m

m

F 

a

Acceleration of books:

12

2 m 

F

a

Net force on 2

nd

book: 21

12

F

F

2 1

2

m m

m

F

Force exerted by 2

nd

book on the 1

st

2 1

2

m m

m

F