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PHYSICS FOR ENGINEERS

Well use the book from P.A. Tipler

... and some more resources from the web

07 - 2nd week

Dynamics

NEWTONS LAWS

First law. An object at rest stays at rest unless

acted on by an external force.

An object in motion continues to travel with

constant velocity unless acted on by an external

force.

Remark Also called the "law of inertia,"

Newton's first law states that a body at rest

remains at rest and a body in motion continues to

move at a constant velocity unless acted upon by

an external force.

Second law. A net force F acting on a body gives

it an acceleration a which is in the direction of

the force and has magnitude inversely

proportional to the mass m of the body.

Momentum for a single particle of mass m with

velocity v, the momentum is defined as

From Newton's second law, a force F produces a

change in momentum

Which are the SI units of force and momentum?

"SI" stands for "System International" and is the

set of physical units agreed upon by

international convention

07 - 2nd week

Dynamics

Example. How much horizontal net force is

required to accelerate a 1200 kg car at 2 ms-2

from the rest on a horizontal plane?

To do yourself Example 4-1 (5 min)

To do yourself Example 4-2 (5 min)

To do yourself Example 4-3 (5 min)

For you to read The Force Due to Gravity Weight

(15 min)

Exemple. A box on a lift

The box is inside a lift

Let W0 be the weight of the box

The lift accelerates upwards, let a be its

acceleration

Which is then the apparent weight of the box?

Which is the apparent weight of the box in case

the lift moves up with a constant velocity?

07 - 2nd week

Dynamics

Also related with Newtons second law impulse.

An impulse is an instantaneous change in momentum

Which S.I. units do we use for impulse?

Can you pose some exemples on impulse?

Also related with Newtons second law friction

forces.

http//www.sc.ehu.es/sbweb/fisica/dinamica/rozamie

nto/general/rozamiento.htm

Third law. Whenever a body exerts a force on

another body, the latter exerts a force of equal

magnitude and opposite direction on the former.

This is known as the law of action and reaction.

A well knowing horse

To do yourself Exemple 4-11 (5 min)

07 - 2nd week

Dynamics

FRICTION FORCES

07 - 2nd week

Dynamics

FREE-BODY DIAGRAMS

A diagram that shows schematically all forces

acting on a system, is called free-body diagram.

It is called free-body diagram because the body

is drawn free from (without) its surroundings.

To do yourself Example 4-6 (10 min)

07 - 2nd week

Dynamics

Find out the acceleration Atwoods Machine

(consider an ideal pulley)

MORE PROBLEMS

What about the tension of the rope?

To do yourself Example 4-9 (10 min)

The block m2 has an inhabitantit has a hollow,

which is the home of a bug.

To do yourself Example 4-10 (10 min)

To do yourself Example 4-12 (10 min)

Let mb be the mass of the bug. What about its

weight when it is at home?

Try it yourself!

07 - 2nd week

Dynamics

SOME MORE QUESTIONS PROBLEMS

An astronaut on Earth kicks a bowling ball and

hurts his foot. A year later, the same astronaut

kicks the same bowling ball on the moon with the

same force.

Question

(a) more (b) less (c) the same

His foot hurts...

The masses of both the bowling ball and the

astronaut remain the same, so his foot will feel

the same resistance and hurt the same as before.

Question Where (the Earth or the Moon) is easier

for the astronaut to pick up the bowling ball

from the floor?

The weights of the bowling ball and the astronaut

are less on the Moon...

Thus it would be easier for the astronaut to pick

up the bowling ball on the Moon than on the Earth.