Motion

Physical Science, Physical Science

Jesse Dent, Arleen Henderson-Instructors

Part 1: Part 1: Measuring MotionMeasuring Motion

Kinematics-What is it?Kinematics is the study of motion.

Motion is always defined in terms of change in location.

Locations or Positions are defined in terms of some FRAME of REFERENCE .

Our background or our surrounding environment is often our frame of reference

The Earth is our most commonly used frame of reference

Distance and DisplacementDistance and Displacement seem very similar,

but they actually have distinct meanings.Distance is a quantity which refers to how far a body moves in going from one point to another.

Displacement is a quantity which refers to the distance between two points in a particular direction. It is how far you are from your original start point!

What is distance?What is displacement?

Speed, Velocity, and Acceleration

Since forces cause an object’s motion to change, we need to be able to measure those changes.

There are three important measurements you must be able to make to understand how forces affect motion. These measurements are speed, velocity, and acceleration.

SpeedSpeed measures how fast something moves. Speed is measured in units like meters/sec or

miles/hour (always a unit for distance over a unit for time).

Speed = distance / time

D

ts

If you know any two of these values, you can solve for the third one:

D = s ts = D / tt = D / s

Velocity Velocity measures both speed and direction. Velocity is also equal to distance / time, but you must ALSO tell

the direction of the movement too. These are correct ways to describe velocity: “The car traveled at

70 miles/hour east.” “The rocket’s velocity was 34,000 m/s straight up.”

Velocity has the same units as speed. meters/second or miles/hour

D

tv

If you know any two of these values, you can solve for the third one:

D = v t

v = D / tt = D / v

Compare these formulas to the ones on the previous slide! What do you see? _____________________________

AccelerationAcceleration measures how fast you change

velocity.Acceleration happens whenever you speed up, slow

down, or change direction. When you speed up, acceleration is a positive number.When you slow down, acceleration is a negative

number, and is sometimes called “deceleration.”Acceleration is measured by change in velocity /

time. The unit for acceleration is meters/sec2.

Δ v

ta

If you know any two of these values, you can solve for the third one:

Δ v = a t

a = Δv / tt = Δv / a

Δ = “change in”

Falling Objects

If no other force acts on an object, the force of gravity will make the object fall towards the center of the earth.

As the object falls, it will accelerate (speed up) by 9.8 m/sec2 every second it falls until it hits something.

If the object falls from a very high place, like a tall building or an airplane, it may reach terminal velocity before it hits the ground.

At terminal velocity, the upward force of friction and the downward force of gravity are equal and opposite (balanced), so the object’s speed stays the same until it hits.

Projectile MotionA projectile is anything that is thrown or shot in a roughly

horizontal direction, like a baseball or a bullet.Projectile motion is always a curved path followed by

the moving object that was thrown or shot. This curved path of projectile motion is caused by the

combination of two forces: the horizontal push from the gun or the hand and the downward force of gravity.

Horizontal Force of cannon firing

Downward Pull of gravity