Speed
Speed tells us how fast an object is moving. It is calculated by dividing the distance travelled by the time taken to cover that distance and has the unit of m/s:
speed = distance travelled/time taken (v = s/t)
Given the speed (v) and the time (t) of travel, the distance (s) travelled is simple s = vt. Given the distance travelled and the speed, the time taken is t = s/v.
In real journeys the speed is not constant and changes with time. We can plot a speed-time graph and use it to calculate the distance travelled by working out the area under the graph.
Newton's 1st Law of Motion
Newton's 1st law of moton states that "objects will continue to travel in the same direction with the same speed unless an unbalanced force acts on them". We often think that a force is needed to keep an object moving at the same speed. In fact the force is needed to overcome friction which acts against the motion. When the force is greater than the frictional force opposing the motion, the object's speed will increase. If the force is less than the frictional force, the object will slow down. That is, an unbalanced force will change the speed or direction of motion.
Acceleration and Deceleration
Acceleration tells us how fast the speed of an object is changing. It is calculated by dividing the change in speed by the time taken and has the unit of m/s2:
acceleration = change in speed/time taken (a = Δs/Δt)
On a speed-time graph, a slope means changing speed and hence accelerating (positive slope) or decelerating (negative slope). A steeper slope means a larger acceleration/deceleration.
Frictional force always act against the motion to slow down (i.e. to decelerate). Friction force is the resistant force at the moving surfaces of contacting objects. A force is needed to over the friction force for an object to remain at constant speed. Friction force can be reduced by the use of lubricants to reduce the contact between the surfaces. However, sometimes friction force is beneficial as it helps in braking to slow down, and that without friction between a car's wheels and the ground, the wheels would turn but the car could not move forward.
For objects moving through air, there is another force opposing the motion known as air resistance. This is because the air is made up of gases and the molecules of these gases bump into moving objects causing a force against the motion. Air resistance is proportional to speed, frontal area and shape and the shape can be streamlined to allow the air to flow smoothly over the object and reduce the air resistance. An aircraft's wing is called an aerofoil and has been designed to allow air to flow over it smoothly.
In term's of Newton's first law of motion:
to accelearte, a force greater than the forces of friction and air resistance is needed
to remain at constant speed, the force moving the object must balance the forces of friction and air resistance
to decelerate, reduce the force moving the object to less than the forces of friction and air resistance
There are four factors affecting the stopping distance of a vehicle. These are the speed of the vehicle, the reaction time of the driver, the road surface and the condition of the vehicle's brakes and tyres. If the tyre tread pattern is worn, or if the rood is wet or icy, there is less friction between the tyres and the road and it will take much longer to stop the vehicle.
Free Falling and Air Resistance
On Earth gravity makes free falling objects accelerate at about 10m/s2. However, this 'constant'acceleration is only true if an object is falling in a vaccum. In air, a frictional force called air resistance acts against gravity and so reduces the acceleration. Air resistance is proportional to the speed and the faster the motion, the greater the air resistance. When air resistance balances the weight of the falling object, there is no acceleration and the object falls at a constant speed or terminal velocity.
In fact, air resistance is also proportional to the frontal area of the object and an open parachute increases the drag force because the parachute presents a larger frontal area.
Air Resistance (Drag) ∝ Av2Cd (v is speed, A is the 'frontal' area of the object, Cd is the drag coefficient)
The drag coefficient Cd is determined by the shape of the object and it is possible to design a streamlined shape to minimise the value of the drag coefficient.