What is another name for Newton's First Law of Motion and what is the law?

Another name: the Law of Inertia

The Law: An object in motion (or at rest) will tend to stay in motion (or at rest) until it is acted upon by an outside force.

Remember: unless acted upon by an outside force, such as friction, an object would continue to move indefinitely.

Define:

Inertia

The tendency of an object to resist changes in its velocity.

Define:

Friction

A force that opposes motion, resulting from the contact of two surfaces.

Remember: on the atomic scale, there is no such thing as a smooth surface.

Question related to Newton's First Law:

A car is traveling down the road at 30 miles/hour.  A truck, driven by a reckless driver, comes up from behind with a speed of 50 miles/hour.  The truck slams into the back of the car.  Will the car's passengers be flung forward or backward in their seats?

Answer: The passengers will be flung backwards until the backs of their seats apply enough force to accelerate them to the same velocity as the car.

Initially, the passengers are traveling with the car at 30 miles/hour.  When the truck hits the car, the car gets pushed forward, accelerating the car faster.  However, the people will remain traveling at their initial 30 miles/hour until their backs hit the seats which are accelerating towards the bodies.  At the point that the seats hit the backs, the people will also accelerate faster with the car.

What is Newton's Second Law of Motion?

When an object is acted on by one or more outside forces, the total force is equal to the mass of the object times the resulting acceleration.

total force = (mass) * (acceleration)

OR

total force = mass * (final velocity - initial velocity)

                                               (time)

Remember: acceleration is expressed as distance

                                                             time²

Define:

Force

Force is a push or pull exerted on an object in an effort to change that object's velocity.

Remember:

Force is a vector quantity whose magnitude is usually measured in Newtons.

A Newton is the standard unit of measurement of force.

                  kg*m

A Newton = sec.²

Remember:

1. mass is usually expressed in kg in physics

2. acceleration is a distance unit over a time unit squared, thus the m/sec²

Define:

total force

Total force is equal to mass * acceleration, but

Remember:  Acceleration must take into account all the forces pusing in one direction - friction and any other force pusing in  the other direction.

E.g. if two men are pushing a stalled car,

            the total force equals F_m1 + F_m2 - F_f

where: m1 = man 1

m2 = man 2 and

f = friction

Define:

Kinetic Friction

Friction that opposes motion once the motion has already started.

Define:

Static Friction

Friction that opposes the initiation (start) of motion.

Solve the following problem:

A man needs to push a desk (mass = 100kg) across the floor.  The static frictional force can resist motion with as much as 196 Newtons of force, while the kinetic frictional force is only 110 Newtons. 

1.How much force is necessary to get the desk moving?

2. If the desk accelerates at 1.0 meters/sec², how much force did the man use?

1.  Answer: To get the desk moving, the man must apply a little more than 196 Newtons of force. In other words, he must apply more force than what static frictional force is capable of supplying.

2. Answer: 210 Newtons to the west.

How to solve:

a) total force = (mass) * (acceleration)

                        (100 kg) * (1.0   meters)

                                                second²

                     = 100 Newtons

b) Since        total force = F_m - F_f,

                   100 Newtons = F_m - F_f

                   100 Newtons = F_m - 110 Newtons

c) Solve by adding 110 Newtons to both sides. Thus,

                    100 N + 110 N = F_m

                                210 N = F_m

d) Don't forget Direction! Since the desk is moving West, the direction of the man's force must be west.

Newton's Second Law Practice Problem #2:

A box (mass = 15 kg) is given an initial shove and allowed to slide acorss the floor with no person pushing on it.  If the box slides north and experiences an acceleration of 1.1 meters/sec² to the south, what is the frictional force between the box and the floor?

Answer: 16.5 Newtons south

How to solve:

a) total force = (mass) * (acceleration)

                   = (15 kg) * (1.1   meters)

                                    (         sec.² )

                   = 16.5 Newtons

b) Remember: since there is nothing pushing the box, friction is the only force operating.  Since the box is sliding north, but friction is the only force operating, the direction of the force is south.

Define:

Newton's Third Law of Motion

For every action, there is an equal and opposite reaction.

Remember: the equal and opposite forces talked about here do not act on the same object.

Is force a scalar or vector quantity?

Force is a vector quantity; it requires a direction.