A distance-time graph is a graph which shows how the distance of an object moving in a straight line varies over time, telling us if it’s moving at a constant speed and what that speed is. The straight line means that there is a constant speed. The slope of the line represents the magnitude of the speed (ie steep = fast).
If the object is moving at a changing speed, then a curve is used. An increasing slope is acceleration (positive acceleration) and a decreasing slope is deceleration (negative acceleration). The gradient of a distance-time graph represents the speed of the object.
The equation for speed is speed = distance / time (v=s/t).
Acceleration is the rate of change of velocity. It describes how much an object’s velocity changes each second.
Velocity-time graphs show how the velocity of a movie object will vary over time, either increasing or decreasing. It indicates whether there is acceleration, deceleration, and the magnitude of that acceleration/deceleration. Straight lines are constant (aka uniform) acceleration (steep slope is large acceleration or deceleration). The actual value for the acceleration can be found by using the gradient. The area underneath a velocity-time graph represents the displacement of the object. Displacement is the distance travelled / the distance moved.
When there is uniform acceleration, you can use the formula v^2 - u^2 = 2as to calculate velocity, distance and acceleration.
All moving objects have something called momentum. We use the equation p = m * v to calculate momentum (p). The units for momentum are kilogram metre per second (kg m/s). Momentum is what keeps objects moving in the same direction, which makes it difficult to change direction without large changes in momentum. When the object is at rest, it has no momentum. Momentum does depend on the direction of the object’s travel (it can be positive or negative). There are three factors which can affect change in momentum:
Acceleration/Deceleration
Change in direction
Change in mass
The Principle of Conservation of Momentum states that in a closed system, the total momentum before an event is equal to the total momentum after the event. There is a simple equation for this - total momentum before the collision = total momentum after the collision. Momentum is always conserved over time.
Force is the rate of change in momentum. This means that when force acts on moving objects, the object will either accelerate or decelerate. The formula for this is F = (change in momentum) / (change in time). The same change in momentum over a longer period of time will produce less force.
Newton’s first law is that objects remain at rest, or move with a constant velocity unless acted upon by a resultant force. Newton’s third law is that for every action, there is an equal and opposite reaction.
Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs. There are some safety features which do exactly this:
Seat Belts
Designed to slightly stretch, thus increasing the time for the passenger’s momentum to equal zero, reducing force on them in a collision as F = ma
They also stop passengers from colliding with the interior of a vehicle, keeping them fixed in their seats
Airbags
deployed at the front on the dashboard and steering wheel when a collision occurs
act as soft cushions to prevent injury on passengers when thrown forwards as F = ma
Crumple Zones
crush or crumple in a controlled way during collisions
increase the time at which the vehicle comes to rest, lowering impact force on passengers as F = ma
Crash mats
Thick and soft to offer shock absorption of force created by a person landing
the body is in contact with the mat for a longer time than if it was not
increases contact time over which momentum is reduced
creating a smaller impact force and a lower chance of injury as F = ma
However, all of the safety features just reduce injury. They don’t completely prevent it.
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