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PHYSICS FOR ENGINEERSPHYSICS FOR ENGINEERS

We’ll use the book from P.A. Tipler

... and some more resources from the web

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Second lawSecond 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.

NEWTON’S LAWS

First lawFirst 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.

amF

Momentum: for a single particle of mass m with velocity v, the momentum is defined as

vmp

From Newton's second law, a force F produces a change in momentumdt

pdF

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 upon by international convention

07 - 2nd weekDynamics

stand f or simbolizar, ser las siglas de

agree upon estar de acuerdo, concertar

act on infl uir sobre

act upon infl uir sobre, af ectar

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Example. How much horizontal net force is required to accelerate a 1200 kg car at 2 m·s-2 from the rest on a horizontal plane?

07 - 2nd week

1200 kg2 m·s-2

N 2400maF

Dynamics

To do yourself: Example 4-1 (5 min)To do yourself: Example 4-1 (5 min)

strand varar, dejar desamparadoTo do yourself: Example 4-2 (5 min)To do yourself: Example 4-2 (5 min)

To do yourself: Example 4-3 (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

W0

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?

mgW 0

The box is inside a lift a

Which is the apparent weight of the box in case the lift moves up with a constant velocity?

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Also related with Newton’s second law: impulse.

An impulse is an instantaneous change in momentum vmdpd

which can be found by integrating a force F over a characteristic time t, giving

dtFpd

t

dtFp

0

07 - 2nd weekDynamics

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

Can you pose some exemples on impulse?

Third lawThird 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)

Also related with Newton’s second law: friction forces.

http://www.sc.ehu.es/sbweb/fisica/dinamica/rozamiento/general/rozamiento.htm

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07 - 2nd weekDynamics

FRICTION FORCES

Static Kinetic

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07 - 2nd weekDynamics

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: To do yourself: Example 4-6 (10 min)Example 4-6 (10 min)

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MORE PROBLEMS

To do yourself: To do yourself: Example 4-9 (10 min)Example 4-9 (10 min)

To do yourself: To do yourself: Example 4-10 (10 min)Example 4-10 (10 min)

To do yourself: To do yourself: Example 4-12 (10 min)Example 4-12 (10 min)

07 - 2nd weekDynamics

Find out the acceleration: Atwood’s Machine (consider an ideal pulley)

m2

m1

m1g

T

m2g

T

amgmTF 111

amTgmF 222 +)

ammgmm 2112

gmm

mma

21

12

gmm

mmT

21

212

What about the tension of the rope?

The block m2 has an inhabitant:it has a hollow, which is the home of a bug.

Let mb be the mass of the bug. What about its weight when it is at home?

Try it yourself!

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

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:

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.

kick dar patadas hurt hacer daño

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.

SOME MORE QUESTIONS & PROBLEMS07 - 2nd weekDynamics