Chapter 8: Motion Revision Notes
When an object moves over time, it is said to be in motion.
When a body moves uniformly, it covers the same distance in the same amount of time.
When a body travels an unequal distance in the same amount of time. It moves in a non-uniform manner.
In explaining motion, both distance and time are relevant.
Even if you don’t see anything, you can tell when a motion has occurred.
When two objects move in a plane (either in the same direction or in the opposite direction), each has a relative motion to the other.
For example, if a person is sitting on a train and watching a tree, the tree is steady but is assumed to be moving in relation to the train.
Displacement vs. Distance
- The distance travelled by an object.
- It only has magnitude and no direction.
- Displacement refers to how far and how far an object has moved from its initial position. i.e. the straight distance between the starting and ending places.
- The distance travelled by an object in a certain amount of time is called speed.
- Distance divided by time equals speed.
- The metric unit of speed is the metre per second (m/s).
- Constant speed means that the speed does not alter.
- Changing speeds: 5 kilometres on a bicycle. Take off and pick up speed, slow down up hill, speed up down hill, and come to a complete stop at the stop sign. It took you 15 minutes to complete the journey (.25 h)
- Total speed divided by total time equals average speed.
- Speed at any given time is referred to as instantaneous speed.
- Velocity is a term that encompasses both speed and direction.
- The storm is moving at a speed of 20 kilometers per hour.
- Assume that two trains are travelling at the same speed but in opposite directions, resulting in different velocities.
- A race car travelling around an oval track may maintain a constant speed yet have varying velocities at different points.
- Acceleration is defined as a change in velocity over a period of time.
- The direction is indicated by the (+ or -) sign. The + symbol indicates acceleration, while the – sign indicates deceleration.
Equation of uniform (constant) acceleration
a = v/t a = v/t a = v/t
- Galileo Galilei was an Italian physicist and astronomer who lived from 1564 to 1642. Formulated laws of motion for free-falling objects.
- Any object traveling freely under the effect of gravity alone is referred to as a freely falling object.
- It is independent of the object’s initial motion.
- Regardless of the beginning velocity, the acceleration of an item in free fall is downward.
- The acceleration of free fall (gravitational acceleration) is g = 9.80 m/s
- With rising altitude, g lowers.
- g is affected by latitude, height, and depth below the earth’s surface.
- 9.80 metres per second square is the average on the surface of the Earth
- The acceleration owing to gravity will be represented by the italics g.
- Not to be confused with the letter g, which stands for grams.
- Falling items can be regarded freely falling if there is no air resistance. Objects of various forms accelerate at varying rates (stone vs feather)
- Both upward and downward acceleration is possible.
- The journey follows a symmetrical pattern.
- The rate of acceleration is constant.
- At identical heights, the magnitude of the velocities is the same.
- As time passes, the images get closer together.
- When the ball moves upward, the acceleration and velocity are in opposite directions.
- The rate of acceleration is consistent (violet arrows maintain the same length)
- In an upward motion, the velocity is decreasing (red arrows are getting shorter)
- Negative acceleration and positive velocity
- At maximum height, velocity is zero.
- The time it takes to get upward and back is always the same.
GRAPHICAL REPRESENTATION OF MOTION
Graph of Distance and Time
- Distance-Time graphs depict how an object’s position changes over time.
- Non-linear fluctuations imply non-uniform motion, while linear variations imply uniform motion.
- We gain speed because of the slope.
Graph of Distance vs. Time
- Because the slope is constant, OA implies uniform motion with constant speed.
- Because the slope is zero, AB denotes that the body is at rest.
- The motion from B to C is non-uniform.
Graph of Velocity and Time
- The change in velocity with respect to time is represented by a velocity-time graph.
- Acceleration is provided by the slope.
- The displacement is determined by the area under the curve.
- Constant velocity is implied by a line parallel to the x-axis.
EQUATIONS OF MOTION
Three equations can be used to describe the motion of an item travelling with uniform acceleration, namely
v= u + at
v2 – u2 = 2as
s = (1/2)at2 + ut
UNIFORM CIRCULAR MOTION
- Uniform circular motion occurs when an object moves in a circular route at a constant speed.
- As the direction shifts, so does the velocity.
- The rate of acceleration is constant.