Physics - Motion

Distance

How far an object moves, doesn’t involve direction. It is a scalar quantity.

Scalar quantity

Quantities with a magnitude (size) only. Not a direction. E.g. distance, speed, time.

Displacement

The distance an object moves, measured in a straight line from a start point to a finish point, and the direction of that straight line. It’s a vector quantity. Expressed in terms of magnitude and direction.

Vector quantity

Quantity with a size and direction. E.g. velocity, displacement, acceleration, force

Speed

Does not involve direction. Scalar quantity. It is a measure of the distance an object travels in a given length of time. Speed = distance divided by time.

Velocity

An objects speed in a given direction. Vector quantity.

Example of an object that has a constant speed but at the same time a changing velocity

Anything going in a circle

I.e. a car travelling on a roundabout will move at a constant speed, but with a changing velocity, as its direction is constantly changing.

planets around a solar system

What happens when an object is moving in a circle?

The direction is constantly changing. If the object is moving at a constant speed, it has a continually changing velocity.

Moving objects

The speed of a moving object is constantly changing and rarely constant. When people walk, run or travel in a car their speed is constantly changing.

Typical values of walking, running and cycling

Walking - 1.5 m/s

Running - 3m/s

Cycling - 6m/s

Typical values of train, bus, aeroplane

Train - 36m/s

Bus - 13m/s

Plane - 245m/s

Wind - 4.2 m/s

When do you use a distance - time graph?

If an object is moving along a straight line, use a distance time graph to represent how far it has travelled.

How to find the speed of an object in a distance time graph?

Determined from the gradient of the line

How to find the acceleration of an object on a distance time graph?

Drawing a tangent and measuring the gradient of the distance time graph at the particular time.

Acceleration equation

Acceleration = final velocity- initial velocity, divided by the time taken

Deceleration

An object that shows a negative acceleration is decelerating.

How to calculate the acceleration on a velocity time graph?

From the gradient of the line

How to calculate the distance travelled on a velocity time graph?

The area under the graph

Motion equation

V² - u² = 2xaxs

V= final velocity (m/s)

U = initial velocity (m/s)

A = acceleration (m/s²)

S = distance (m)

Gravity of an object near the earths surface

Near the earths surface any object falling freely under gravity has an acceleration of about 9.8 m/s²

Distance equation

Distance = speed x time