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 every second
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
Initially, the upwards air resistance is very small because the skydiver isn’t falling very quickly.
As they speed up, the air resistance increases, eventually growing large enough to balance the downwards weight force.
Once air resistance equals weight, there is no longer any resultant force.
Therefore, the skydiver’s acceleration is zero - they now travel at a constant speed.
This speed is called their terminal velocity.
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.
Every action has an equal and opposite reaction.
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 time it takes for a driver to react to a situation is their reaction time.
During this reaction time, the car carries on moving.
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)
Aim: investigate the effect of varying force on the acceleration of an object of constant mass.
Procedure - Use the metre ruler to measure out intervals on the bench, e.g. every 0.2 m for a total distance of 1 m. Draw straight lines with pencil or chalk across the table at these intervals.
Attach the bench pulley to the end of the bench.
Tie some string to the toy car or trolley. Pass the string over the pulley and attach the mass hanger to the other end of the string.
Make sure the string is horizontal (i.e. parallel to the bench) and is in line with the toy car or trolley.
Hold the toy car or trolley at the start point.
Attach the full set of weights (total = 1.0 N) to the end of the string.
Release the toy car or trolley at the same time as you or a partner starts the stopwatch. Press the stopwatch (in lap mode) at each measured interval on the bench and for the final time at 1.0 m.
Record the results in the table and repeat step 7 to calculate an average time for each interval.
Repeat steps 5-8 for decreasing weights on the weight hanger, e.g. 0.8 N, 0.6 N, 0.4 N, and 0.2 N. Make sure you place the masses that you remove from the weight stack onto the top of the car, using the Blu-tac, each time you decrease the weight.
Practical 7 (investigating the effect of mass on acceleration)
Aim: investigate the effect of varying mass on the acceleration of an object produced by a constant force.
Procedure - Use the metre ruler to measure out intervals on the bench, e.g. every 0.2 m for a total distance of 1 m. Draw straight lines with pencil or chalk across the table at these intervals.
Attach the bench pulley to the end of the bench.
Put a 200 g mass on the car.
Tie some string to the toy car or trolley. Pass the string over the pulley and attach the mass hanger to the other end of the string.
Make sure the string is horizontal (i.e. parallel to the bench) and is in line with the toy car or trolley.
Select a weight to put on the weight hanger that will gently accelerate the car along the bench. This provides the constant force on the car or trolley and will not change.
Hold the car at the start point.
Release the car at the same time as you or a partner start the stopwatch. Press the stopwatch (in lap mode) at each measured interval on the bench and for the final time at 1.0 m.
Record the results in the table and repeat step 7 to calculate an average time for each interval.
Repeat steps 5-8 for increasing mass on the car, e.g. 400 g, 600 g, 800 g and 1000 g.
