Unit 4 - Introduction to Dynamics

dynamics

The study of why objects move

force

sometimes described as a push or a pull.

Gravity

an attractive force which governs the shape of the universe. It is exerted by all bodies on all bodies in the universe. Gravity depends on the mass and separation between bodies, and can act over very long distances. It acts through fields. Different planets and moons have different acceleration due to gravity.

Newton’s First Law

An object continues in a state of rest or in a state of motion at a constant speed along a straight path unless acted on by a net force.

Net force

the overall force. It is the sum of all forces acting on a body from all directions.

Inertia

a scientific idea, a property of matter, the natural tendency of an object at rest to stay at rest and an object moving to stay moving. Inertia cannot be measured, but can be thought of in qualitative ways. resistance to changes in velocity.

mass

More massive objects have more inertia. Mass (symbol: $m$ ,

unit: $\operatorname{kg}$) is difficult to define at this stage, but it can be roughly described as the amount of space an object occupies. Mass is an intrinsic property of matter, it remains the same everywhere in the universe. Quantity of matter containted within an object, a measure of a body’s inertia (resistance to changes in velocity), it is a scalar

weight

Weight (symbol: $F_{g}$ , unit: kg m/s² or $N$) is the amount of force due to gravity acting on an object, or the amount of pull. Weight is a force! It can very from planet to planet. weight is the magnitude of the force due to gravity, it is scalar

Newton’s Second Law

Any net force produces an acceleration in the direction of the force. The magnitude of the acceleration is directly proportional to the force, and inversely proportional to the mass of the object.

$\overrightarrow{F}=m\overrightarrow{a}$ is directly proportional, because:

as the force increases, the acceleration increases

$\overrightarrow{F}=m\overrightarrow{a}$ is also inversely proportional, because:

as the mass increases, the acceleration decreases

Vector Addition Diagram

• Add vectors tip to tail

• Net (resultant) force is a single vector going from start to finish.

• Advantage:

  • Can graphically determine magnitude of $F_{net}$

• Disadvantages:

  • Forces do not act sequentially

  • Not a good representation of what is happening

  • Can look confusing if many forces are involved

Free Body Diagram

• show all forces acting on an object

• All force vectors originate at the object’s centre of mass.

• Advantage:

  • Better representation of what is happening (All forces act on the object at the same time)

• Disadvantage:

  • Difficult to determine Fnet graphically from free body diagram

Balanced and Unbalanced Forces

Unit Analysis

Using Newton’s 2nd Law to write the Newton (unit of force) in terms of fundamental (base) SI units (m, kg, s)

What is the difference between mass and weight?

mass can be roughly described as the amount of space an object occupies. Mass is an intrinsic property of matter, it remains the same everywhere in the universe. Quantity of matter containted within an object, a measure of a body’s inertia (resistance to changes in velocity). it is scalar.

Weight (symbol: $F_{g}$ , unit: kg m/s² or $N$) is the amount of force due to gravity acting on an object, or the amount of pull. Weight is a force! It can very from planet to planet. weight is the magnitude of the force due to gravity, it is scalar

What is the difference between newton’s first and second law?

Newtons first law: An object continues in a state of rest or in a state of motion at a constant speed along a straight path unless acted on by a net force.

Newtons second law: Any net force produces an acceleration in the direction of the force. The magnitude of the acceleration is directly proportional to the force, and inversely proportional to the mass of the object.

normal force

The normal (perpendicular) force is the force exerted by a surface on an object. There is no universal formula to calculate the normal force (it depends on what other forces are acting).