Chapter 3 Newton’s Laws. Every object possesses inertia. Inertia is the tendency of a body at rest to remain at rest, and of a body in motion to continue moving with unchanged velocity. or Inertia is the sluggishness of an object to changes in its state of motion.
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Demo – Hoop and chalk
(Only the net force is shown in the figures on this slide.)
This symbol means
The acceleration of an object is directly proportional to the net force.
Consider the same net force applied to different mass objects.
The acceleration is inversely proportional to the mass of the object.
A less rigorous form of the second law will be used here.
If the net external force acting on an object is not zero, then the acceleration of the object is directly proportional to the net external force and inversely proportional to the mass of the object.NEWTON’S SECOND LAW
The force in each of these equations represents the vector sum of all of the forces acting on the object of mass m.
This vector equation can be written out as three separate scalar equations.
Therefore, forces come in pairs.
For each force exerted on one body, there is an equal, but oppositely directed, force on some other body interacting with it.
“You cannot touch without being touched.” Paul Hewitt
This is often called the Law of Action and Reaction.NEWTON’S THIRD LAW
Yet it is the weakest.
Let’s look at the inverse-square nature for the force of gravity.
Because the Earth is not a perfect uniform sphere, and because it is spinning, the weight measured by a scale (often called the effective weight) will be very slightly different from that defined here.THE WEIGHT
We call that the weight of the object.
A 1 kg object would weigh 9.81 N
or 2.20 lb.RELATIONSHIP BETWEEN MASS AND WEIGHT
It’s magnitude is called the tension.THE TENSILE FORCE
The friction force is parallel to the surface and opposite to the direction of motion or of impending motion.
Only when the applied force exceeds the maximum static friction force will an object begin to slide.THE FRICTION FORCE
The normal force is perpendicular to the surfaces in contact.
The frictional force is parallel to the surfaces in contact.THE NORMAL FORCE
Friction opposes the motion between surfaces in contact with one another.
When there is a tendency for movement between two surfaces and yet there is no motion, the friction is static friction.
Static friction has an upper limit.
When there is motion between the two surfaces, the friction is kinetic (sliding) friction.
On the verge of slipping
Maximum Static Friction
Kinetic (sliding) Friction
Applied Force, FA
Length - L
Mass - m
Time - t
Derived dimensions are combinations of the fundamental dimensions.
For example: length/time2
Remember that you can only add or subtract things that are alike.
For example velocity plus velocity
When numbers are placed into equations, their units must appear with them.
Units undergo the same mathematical operation as the numbers do.
Conversion from one form of the unit to another may be necessary.
For example meters to centimeters.
See examples in text, page 30.
This is a diagram showing some object and the forces applied to it.
It contains only forces and coordinate information, nothing else.
There are only two kinds of forces to be considered in mechanics:
Force of gravity