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Motion (Velocity and acceleration (Acceleration (The acceleration of an…
Motion
Velocity and acceleration
Velocity= Speed in a given direction E.g two moving objects can have the same speed but different velocities as they are moving in opposite directions
An object that travels at a constant velocity travels at a constant speed without changing its direction.So it travels in a straight line in a given direction.
Displacement is the word used for the distance travelled in a given direction
Acceleration
The acceleration of an object is its change of velocity per second
The unit of acceleration is the metre per second squared
Any object with a changing velocity is accelerating. You can work out its average by using this equation;
acceleration= change in velocity÷ time taken for change
m/s²= m/s ÷ s
For an object that accelerates (a) steadily from an initial velocity to a final velocity in time(t);
its change of velocity= final velocity (v)- initial velocity (u)
Equation= a= v-u ÷ t
Deceleration
A car decelerates when the driver brakes. The term is used for any suituation where an object slows down
EXAMPLE QUESTION
initial velocity u= 28m/s, final velocity v= 0, time taken t= 8.0s
acceleration a= 0m/s-28m/s ÷ 8.0s= 3.5m/s²
More about velocity time graphs
The gradient of the line on a velocity time graph represents acceleration
Braking
Braking reduces the velocity of a vehicle.
-A horizontal line shows constant velocity. If the gradient of a line is 0 than the acceleration in this section is 0.
When the bakes are applied the vehicle will decelerate and its velocity decreases to 0. The gradient of a line will be negative in this section So the acceleration is negative.
Speed and velocity can also be described in km/h (kilometers per hour)
It's important to remember that if speed and velocity are given in km/h their values must be converted to metres/s before plotting them on a graph or using them in an equation
Positive gradient= positive acceleration
Negative gradient= represents deceleration
Speed and distance-time graphs
Equation for constant speed
Speed= distance travelled ÷time taken
(metres per second m/s) = metres(m)÷seconds (s)
This equation can be used to calculate the average speed of an object whose speed varies.
E.g if a motorist in a traffic queue took 50s to travel a distance of 300m, the cars average speed was 6.0 m/s (=300m÷50s)
The formula can be written in a triangle;
Speed in action
The distance- time graph for any object that is:
Stationary; is a horizontal line
moving at constant speed, is a straight line that sloped upwards
The gradient of a distance- time graph for an object represents the object's speed
Analysing motion graphs
Using distance-time graphs
For an object moving at a constant speed the distance- time graph is a straight line sloping upwards.
The speed of the object is represented by the gradient of the line .To the find the gradient you draw a triangle under the line and divide the height by the base of the triangle.
Using velocity time graphs
Velocity time graph of an object moving with a constant acceleration is a straight line with a constant gradient.
Acceleration= Gradient of the line
The height underneath the gradient triangle represents the change of velocity and the base represents time taken so gradient represents acceleration because;
acceleration= change of velocity ÷ time taken
