ACCELERATION

What is Acceleration

The increase in the rate or speed of something

More Scientifically

We can define acceleration as the change in velocity per unit in time

If an object is speeding up, we can say that it is accelerating, or going faster per unit in time

If an object is slowing down, we can say that it is decelerating, or going slower per unit in time

In the real world

Acceleration is very common. It is very rare for an object to start off at a certain velocity

In real life, to reach our desired velocity, we must accelerate

Examples:

Vehicles

Humans

Animals

Weather

Formula for Acceleration

Because we describe it as the change in velocity per unit in time, the formula for acceleration is:

a = Δv / Δ t

a = Δv / Δ t Recall: Velocity is measured in m/s and time is measured in s

So, our unit of measurement for acceleration is m/s/s or m/s²

Example A constant force changes the speed of a car from 8 m/s to 20 m/s in 4s. What

is average acceleration?

+

v1 = +8 m/s

t = 4 s

v2 = +20 m/s

Force

Example (Continued): What is the average acceleration of the car?

Recall definition of average acceleration.

2 1

2 1avg

v v va

t t t

20 m/s - 8 m/s3 m/s

4 sa

3 m/s, rightwarda

+

v1 = +8 m/s

t = 4 s

v2 = +20 m/s

Force

• A wagon moving east at 20 m/s encounters a very strong head-wind, causing it to change directions. After 5 s, it is traveling west at 5 m/s. What is the average acceleration? (Be careful of signs.)

+ Force

vo = +20 m/s

vf = -5 m/s

E

Example

• A wagon moving east at 20 m/s encounters a head-wind, causing it to change directions. Five seconds later, it is traveling west at 5 m/s. What is the average acceleration?

Choose the eastward direction as positive.

Initial velocity, vo = +20 m/s, east (+)

Final velocity, vf = -5 m/s, west (-)

The change in velocity, Dv = vf - v0

Dv = (-5 m/s) - (+20 m/s) = -25 m/s

Choose the eastward direction as positive.

Initial velocity, vo = +20 m/s, east (+)

Final velocity, vf = -5 m/s, west (-)

The change in velocity, Dv = vf - v0

Dv = (-5 m/s) - (+20 m/s) = -25 m/s

Example Cont

Example Cont

aavg = =Dv

Dt

vf - vo

tf - to

a =-25 m/s

5 s

a = - 5 m/s2

a = - 5 m/s2

Acceleration is directed to left, west (same as F).

+ Force

vo = +20 m/s vf = -5 m/s

E

Dv = (-5 m/s) - (+20 m/s) = -25 m/s

GRAVITY

What is Gravity?

The force by which objects tend to fall towards the centre of the earth

What is Gravity?

Gravity is what holds the planets in orbit around the Sun and what keeps the Moon in orbit around Earth.

The closer you are to an object, the stronger its gravitational pull is.

What is Gravity?

How much gravity an object has depends on how big it is.

To be specific, how much mass it has.

Who discovered gravity?

The first person who dropped something heavy on their toe knew something was going on, but gravity was first mathematically described by the scientist Isaac Newton.

His theory is called Newton's law of universal gravitation.

Later, Albert Einstein would make some improvements on this theory in his theory of relativity.

What is the rate of gravity?

For the purposes of calculations on Earth, objects fall at a rate of 9.80m/s²

80

Acceleration Due to Gravity

Every object on the earth experiences a common force: the force due to gravity.

This force is always directed toward the center of the earth (downward).

The acceleration due to gravity is relatively constant near the Earth’s surface.

Every object on the earth experiences a common force: the force due to gravity.

This force is always directed toward the center of the earth (downward).

The acceleration due to gravity is relatively constant near the Earth’s surface.

Earth

Wg

Gravitational Acceleration

In a vacuum, all objects fall with same acceleration.

Equations for constant acceleration apply as usual.

Near the Earth’s surface:

In a vacuum, all objects fall with same acceleration.

Equations for constant acceleration apply as usual.

Near the Earth’s surface:

a = g = 9.80 m/s2 or 32 ft/s2

Directed downward (usually negative).