5.5 Forces and motion Flashcards
Distance and displacement
Distance is a measure of how far an object travels. It is a scalar quantity.
Displacement is a measure of how far something is from its starting position, along with its direction. It is a vector quantity
Speed
The speed of an object is the distance it travels over a certain period of time.
Speed is a scalar quantity.
V = S/t
Typical speeds
Walking 1.5m/s
Running 3m/s
Cycling 6m/s
Circular motion
Velocity is a vector quantity, and the velocity of an object is its speed in a given direction.
When an object travels along a circular path, its velocity is always changing
The speed of the object moving in a circle might be constant - that is, it is travelling the same distance every second.
However, the direction of travel is always changing as the object moves along the circular path.
Acceleration
Acceleration is defined as the rate of change of velocity.
a = Δv / t
a = acceleration in metres per second squared (m/s2)
Δv = change in velocity in metres per second (m/s)
t = time taken in seconds (s)
Uniform acceleration
v^2 = u^2 + 2as
s = distance travelled in metres (m)
u = initial speed in metres per second (m/s)
v = final speed in metres per second (m/s)
a = acceleration in metres per second squared (m/s2)
Terminal velocity
At terminal velocity, the object moves at a steady speed in a constant direction because the resultant force acting on it is zero.
Newton’s 1st law
If the resultant force of an object is zero, it will:
If stationary remain stationary.
If moving, keep moving at a steady speed in the same direction. (same velocity).
Newton’s 2nd law
The acceleration of an object is proportional to the resultant force acting on the object and inversely proportional to the mass of the object.
F = ma
Newton’s 3rd law
When two objects interact the forces they exert on each other are equal and opposite.
Inertia
The tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force.
If an object is at rest, it will tend to remain at rest.
If an object is moving at a constant velocity (constant speed in a straight line), it will continue to do so.
Inertial mass
An object with a high mass has more inertia than an object with a lower mass.
It is difficult to move an object with a high mass (and high inertia), and once it is moving, the object’s motion is hard to stop.
m = F/a
m = inertial mass in kilograms (kg)
F = force in newtons (N)
a = acceleration in metres per second squared (m/s2)
Thinking distance
The thinking distance is the distance travelled in the thinking time it takes to think and react to a hazard on the road.
Thinking distance = reaction time x speed
Braking distance
How far the vehicle travels before coming to a complete stop whilst the breaking force is applied.
Stopping distance
Stopping distance is the distance it takes a vehicle to stop.
Stopping distance = thinking distance + breaking distance
Factors affecting thinking distance
Tired drivers will react more slowly in an emergency.
Distractions will cause a driver to react more slowly in an emergency.
Examples include mobile phones or small children.
Drivers under the influence of drugs or alcohol will react more slowly in an emergency.
Factors affecting braking distance
Initial car speed - The faster a car is travelling, the further it will travel before it comes to a stop.
Road conditions - Wet or icy conditions will increase the braking distance.
The condition of the car - If a car’s brakes or tyres are in poor condition, then the braking distance will increase.
Work done when braking
When we push the brake pedal, brake pads are pressed onto the wheels. This contact causes friction. This causes work to be done. The work done between the brakes and the wheels converts energy from kinetic energy in the wheels to thermal energy in the brakes. The temperature of the brakes then increases.
When a car comes to a stop, the work done by the brakes must equal the initial kinetic energy of the car.
Work done = initial kinetic energy.
Fd = 1/2 mv^2
Practical 7 (investigating the effect of force on acceleration)
IV - Force
DV - acceleration
CV - mass
Procedure - Measure intervals along the bench and mark with pencil or chalk. Attach the pulley to the end of the bench.
Tie the string to the toy car and pass it over the pulley. Attach the 1.0 N weight hanger to the other end. Ensure the string is horizontal and aligned with the car.
Hold the car at the starting point. Release the car and start the stopwatch. Press lap mode at each interval and at 1 m.
Note the times for each interval and calculate the average.
Repeat with decreasing weights on the hanger.
Practical 7 (investigating the effect of mass on acceleration)
IV - mass
DV - acceleration
CV - force
Procedure - Measure intervals along the bench and mark with pencil or chalk. Attach the pulley at the end of the bench.
Put a 200 g mass on the toy car and tie the string to it. Pass the string over the pulley and attach the weight hanger.
Ensure the string is horizontal and in line with the car.
Select a weight for the hanger to provide a constant force that gently accelerates the car.
Hold the car at the starting point. Release the car and start the stopwatch simultaneously. Press lap mode at each interval and at 1.0 m.
Note the times and calculate the average.
Repeat steps with increasing masses on the car
