Introduction: “Everything moves. The Earth revolves on its axis as it travels around the Sun. The Sun orbits within the Milky Way. Galaxies apart. Motion and its causes are important to a study of physics. We begin by defining the meaning of everyday terms such as distance, velocity, acceleration and go on to develop models for motion that will allow us to predict the future motion of an object.”

2.1 Motion

UNDERSTANDIING

• Distance and displacement

• Speed and velocity

• Acceleration

• Graphs describing motion

• Equations of motion for uniform acceleration

• Projectile Motion

• Fluid resistance and terminal speed

Distance and displacement

Distance is the total length. It is a scalar quantity meaning it has no direction, just magnitude and unit.

With distance, the journey from a to b and b to a is the same.

Displacement is the length from point a to point b. start to finish. It is a vector quantity meaning it has magnitude + unit + direction.

With displacement, the journey from a to b is different compared with b to a; they have the same vector length but direction is opposite.

Speed and velocity

scalar speed (ms^-1 or kmh^-1) = distance travelled on the journey / time taken for the journey

vector velocity = distance travelled on the journey / time taken for the journey + [direction]

ex. 4.2 ms^-1 due north or -55 kmh^-1 at N22.5°E

Worked Q. A cyclist travels 16 km in 70 minutes. Calculate, in m s–1 , the speed of the

cyclist.

Solution: 70 minutes is 60 × 70 = 4200 s; 16 km is 16 000 m.

The speed of the cyclist is 16 000 / 4200 = 3.8 m s^–1 .

Worked Q. The speed of light in a vacuum is 3.0 × 108 m s–1 . A star is 22 light years from Earth (1 light year is the distance travelled by light in one year). Calculate the distance of the star from Earth in kilometers.

Solution: Light travels 3.0 × 10^8 m in 1 s. So in a year it travels 3.0 × 10^8 × 365 × 24 × 60 × 60 = 9.5 × 10^15 m. The distance of the star from the Earth is 22 × 9.5 × 10^15 = 2.1 × 10^17 m. The answer in kilometers, so the distance is 2.1 × 10^14 km.

Describing motion with a graph--I

The distance is plotted on the y-axis while the time is plotted on the x-axis. Use the data in the distance-time graph to calculate speed and/or velocity.

Speed would be the gradient or slope of the graph. Add the overall direction to this speed to get velocity.

Instantaneous and average values

Instantaneous speed: the value of the speed at the moment in time at which speed is determined

Acceleration

Describing motion with a graph--II

The d-t plots lead to a convenient display of speed and velocity changes. The v-t plots help display and visualize acceleration.

2.2 Forces

We depend on forces and their efforts for all aspects of our life. Forces are more than the simple description from before; they can change the motion of a body and deform the shapes of bodies.

UNDERSTANDING

• Objects as point particles

• Free-body diagrams

• Translational equilibrium

• Newton’s laws of motion

• Solid friction

Newton’s laws of motion

Newton’s first law (the law of inertia)

objects have inertia--a resistance to stopping and that, once in motion, objects continue to move.

“An object continues to remain stationary or to move at a constant velocity unless an external force acts on it.” Or most known as “An object at rest stays at rest, and an object in motion stays in motion at constant speed in a straight line unless acted on by an unbalanced force.”

Newton’s law > Galileo’s idea > Aristotle’s view

Newton’s second law

If a force does act on an object, in what way does the velocity change?

simple terms: **Force (N) = mass (kg) x acceleration (ms^-2) (**F=ma)

Two things arise:

Mass is scalar; no change in direction of acceleration if multiplied with mass. Direction of force and direction of acceleration must be the same. --So applying a force to a mass will change the velocity in the same direction as that of the force.

Mass = ratio of force required per unit of acceleration for given object → standardize our units of force

Newton’s third law

“Every action has an equal and opposite reaction.”