- The speed of a body is the distance that it has travelled in unit time.
-> v = s/t
-> Average speed = total distance travelled/ time taken
- Velocity is the distance travelled in unit time in a given direction
- The velocity of a body is uniform or constant if it moves with a steady speed in a straight line. It is not uniform if it moves in a curved path. -The units of speed and velocity are the same, km/h, m/s.
-Distance moved in a stated direction is called the displacement
-Velocity may also be defined as :-
Velocity = change in displacement/ time taken
-Speed is a scalar quantity and velocity a vector.
- Displacement is a vector, unlike distance which is a scalar.
-When the velocity of an object changes, we say the object accelerates
-Acceleration is defined as the change of velocity in unit time or acceleration = change of velocity/ time taken for change
-Acceleration is also a vector and both its magnitude and direction should be stated
-An acceleration is positive if the velocity increases and negative if it decreases. A negative acceleration is also called a deceleration or retardation.
Speed-time graphs
- If the speed of an object is plotted against the time, the graph obtained is a speed-time graph
Using the gradient of a speed-time graph to calculate acceleration
-The gradient of a speed-time graph represents the acceleration of the object
-An object is accelerating if the speed increases with time and decelerating if the speed decreases with time
Distance-time graphs
-An object travelling with constant speed covers equal distances in equal times
-Its distance-time graph is a straight line
- The speed of the object is higher when the gradient of the graph is steeper
- Speed at any point equal the gradient of the tangent
- Distance = area of rectangle (" ") + area of triangle (" ")
Equations for constant acceleration
-> A = change of speed / time taken
at = v-u or v = u+ at
- Average speed = u + v/2, s/t = u + v /2 or s =(u+v)t/2
Acceleration of free fall
-All bodies falling freely under the force of gravity do so with uniform acceleration if air resistance is negligible
- This acceleration, called the acceleration of free fall, is denoted by the italic letter g
-Acceleration of free fall g for an object near to the surface of the Earth, this is approximately constant and is approximately 9.8m/s^2
Air resistance : terminal velocity
-When an object falls in a uniform gravitational field, the air resistance (fluid friction) opposing its motion increases as its speed rises, so reducing its acceleration
-Eventually, air resistance acting upwards equals the weight of the object acting downwards
- The object falls at a constant velocity, called its terminal velocity, whose value depends on the size, shape and weight of the object
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