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Flashcards in Forces and Movement Deck (18):

When will an object accelerate?

An object will not change its velocity (accelerate) unless there is an unbalanced force acting on it


What is the relationship between force and acceleration?

Force is proportional to acceleration


How do you work out force?

Force = acceleration x mass
F = ma
N = Kg x m/s squared


What is a negative acceleration?

deceleration (don't forget to put the negative sign if it is deceleration but the question is acceleration)


What happens with falling objects?

-When an object falls through a liquid or a gas the two forces acting on the object is weight (gravitational force) and a frictional force acting upwards (drag/air resistance)
-The size of this resistive force/drag depends on the shape, speed, surface finish of the object and the density of the fluid (streamline and smooth surfaces produce less drag)
-Drag force increases with the speed of the object, as drag increases with speed it affects the way that dropped objects accelerate because the faster they get the greater the force opposing their motion becomes


What happens when objects fall?

1. As objects start to fall their drag force is 0, and so the object begins to accelerate at about 10 m/s squared because the gradient is equal to the acceleration and there is no drag force
2. As the object picks up speed the drag force increase, and so the overall downwards fore is smaller, reducing acceleration.
3. Terminal velocity is reached when drag force equals the weight. The resistive force is equal to the weight subtract the overall force and so since there is no unbalanced force the acceleration is zero. However the object is still falling but at a constant speed.


What happens with parachutes?

1. When the skydiver initially jumps out of the plane she will accelerate for a time and then reach her terminal velocity, however this is still at a high speed.
2. Therefore she opens her parachute, this upsets the equilibrium and the parachute has a much larger surface area, and so has a much larger resistive force than her weight and so the skydiver will decelerate but carry on falling downwards
3. As she slows down, the size of the drag force will decrease, until a new terminal velocity is reached (depends on shape and design of parachute).


How does drag force change in liquids?

The size of the drag force is higher in liquids than in gases, so objects falling through liquids have a much lower terminal velocity than objects falling through air


How can you measure terminal velocity?

By using a light gate and a data logger.


Why does a hammer reach the ground before a feather

1. The hammer has a greater weight than the feather and so needs a greater air resistance to oppose its weight.
2. Therefore the feather will reach terminal velocity before the hammer and so will accelerate for a shorter amount of time than the hammer.
3. The hammer takes longer to reach its terminal velocity (which it may never) as a greater air resistance is needed and so until it reaches terminal velocity it will accelerate
4. Therefore the hammer will more area faster than the feather which is travelling at the constant speed of its terminal velocity, meaning that the hammer will reach the ground before the feather.


What is thinking distance?

The distance travelled between seeing the hazard and applying the breaks
Thinking distance is proportional to speed, twice the speed means twice the thinking distance


What is breaking distance?

The distance travelled between applying the breaks and coming to a complete stop


What is stopping distance?

thinking distance + breaking distance


What affects reaction time?

-use of mobile phones
-NOT depend on how fast you are travelling


When will the stopping distance be larger?

-When the breaking force is smaller, car tyre or brakes are in poor condition, road surface is wet or slippery.
-If you don't press very hard on breaks or at a slower rate, the graph will have a shallower gradient as there is lower deceleration
-Higher speed: twice the speed = four times the breaking distance
-Mass of car larger (e.g. lorry = smaller rate of deceleration, travel further while breaking
-Road condition
-Reaction time


How could you work out the deceleration of a car?

a = F/m (negative tho!)


What are some speed conversions?

miles/hour multiplied by 1.6 gets km/hour and then that multiplied by 0.27 gets m/s


How can you use sycamore seeds to investigate the forces acting on a falling object?

1. Sycamore seeds have a small weight and a large surface area, so they reach terminal velocity really quickly and fall slowly, and this makes them great for investigating falling objects
2. Collect a bunch of sycamore seeds of different sizes and measure the mass and wing lengths of each one
-Use an accurate ruler (or callipers) and repeat each measurement several times to make sure its accurate
-Use all the seeds that have a similar mass but different wing lengths
3. Drop each of the seeds from the same height and use a stopwatch to find out how long each one takes to fall to the ground
-The higher you drop them from the better as it gives a larger measurement and so improves the accuracy of the measurement
4. Repeat the experiment for each seed and find an average time
5. You can then plot a graph for the length of the wings against the time taken to hit the ground. This will tell you if there is a relation between the shape of the sycamore seeds and their terminal velocity
6. You should get something that suggests bigger wings means bigger surface area and so higher drag. Higher drag means lower terminal velocity, and so the seeds fall more slowly