Forces Flashcards
What is a vector quantity?
A quantity that has a size and a direction e.g. force, velocity
What is a scalar quantity?
Quantities with size but no direction e.g. speed, distance
What is a contact force?
When two objects have to be touching for a force to act e.g. friction
What is a non contact force?
Where objects do not need to be touching for the force to act e.g. magnetic forces
How does gravity effect anything near a planet or star?
On the surface of the planet, it makes all things accelerate towards the ground it gives everything a weight
What is weight?
The force acting on an object due to gravity
What is a centre of mass?
Where the force is acting at a single point on the object; normally the centre
How do you find the weight of an object?
Mass x gravitational field strength
How is a free body diagram used?
The size of the arrows shows the relative magnitude of the forces and the direction show the direction of the force acting on the object
What is resultant force?
If there are more than one forces acting on an object (with the same effect) the overall effect on the object is called resultant force
What happens when a force moves an object?
Energy is transferred and work is done on the object
How can an object be elastically deformed?
If it can go back to its original shape and length when the force is removed
How can an object be inelastically deformed?
If it doesn’t return to the original shape and length after the force has been removed
How does an elastically deformed object affect the energy transferred?
When work is done on the object, energy is transferred to the elastic potential energy stored. If the object is elastically deformed, ALL of the energy goes to the elastic potential energy store
What is the relationship between extension and force?
They are directly proportional
Why does force and extension being directly proportional stop?
There is a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally; this is known as the limit of proportionality
What is pressure?
The force per unit area
What is density?
A measure of compactness of a substance
What happens when the depth of a liquid increases?
The number of particles above that point increases which means that the weight of these particles adds to the pressure felt at that point
What happens when an object is submerged in a fluid?
The pressure of the fluid exerts a force on it from every direction
What is the upthrust of an object submerged in water?
This is a resultant force upwards and is equal to the weight of the object submerged
What happens if an object floats?
This means that the weight of the object is the same as the upthrust
What does an object float upon depend on?
The density
What happens to an object that is less dense than the fluid it is placed in?
This means that it weighs less than the equivalent volume of fluid so it displaces a volume of fluid that is equal to its weight before it is completely submerged
What happens to an object that is denser than the fluid it is placed in?
It is unable to displace enough fluid to equal its weight, so its weight is always larger than the upthrust and it will sink
Why does atmospheric pressure decrease with height?
Atmospheric pressure is created on a surface by molecules colliding with that surface, so when altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface
What is speed?
How fast you are going
What is velocity?
Speed in a given direction
What is happening to an object moving in a circle at constant speed?
It has a constantly changing velocity as the direction is always changing
What is the average speed of a person walking?
1.5 m/s
What is the average speed of a person running?
3 m/s
What is the average speed of a person cycling?
6 m/s
What is the average speed of a car?
25 m/s
What is the average speed of a train?
55 m/s
What is the average speed of a plane?
250 m/s
What is uniform acceleration?
Constant acceleration e.g. acceleration due to gravity is uniform for objects in free fall
What do different distance time graphs tell you about a journey?
Gradient = speed, flat = stationary, straight uphill = steady speed, curves = acceleration or deceleration
What do different velocity time graphs tel you about a journey?
Gradient = acceleration, flat = steady speed, uphill = acceleration, downhill = deceleration, curve = changing acceleration, area under section of graph =distance travelled in time interval
How does a driver travel at a steady speed to counteract friction?
They need to balance the frictional forces as friction always acts in the opposite direction to movement
How do you reduce drag?
By keeping the object streamlined as it allows fluids to flow over it easily
How do you increase the top speed of a vehicle?
Reduce drag by altering the shape and increasing the power of the engine
What is terminal velocity?
A steady velocity that an object falling reaches after the velocity begins to decrease
How would a velocity-time graph look for a falling object?
A steep gradient (for the great acceleration) which then levels out and becomes flat, as the object reaches it’s terminal velocity (as the forces are balanced and the object is no longer accelerating)
What factors affect terminal velocity?
Shape and area, as resistance can play a part in this
What is Newton’s first law?
If the resultant force on a stationary object is zero, the object will remain stationary but if the resultant force on a moving object is zero, it will keep moving at the same speed but if there is a resultant force on an object non-zero, the velocity will change
What changes in velocity can an object experience when a resultant force is acting upon it?
Starting, stopping, changing direction, slowing down and speeding up
What is Newton’s second law?
The larger the resultant force acting upon an object, the larger the acceleration (directly proportional) and acceleration is also inversely proportional to the mass of the object
What is inertia?
The tendency to keep moving at the same velocity or to continue in the same state of motion
What does an objects inertia mass measure?
How difficult it is to change the velocity of an object
What is Newton’s third law?
When two objects interact, the forces they exert on each other are both equal and opposite
What is the total stopping distance?
The sum of the braking distance and the thinking distance
What is the thinking distance?
The distance the vehicle travels during the driver’s reaction time
What is the braking distance?
The distance the vehicle travels after the brakes are applied, before coming to a complete stop
What factors affect thinking distance?
How fact you are going and your reaction time (could be affected by tiredness or drugs etc.)
What factors affect braking distance?
How fast you’re doing, how good your brakes are, how good your tires are and how good the grip is (can be affected by weather conditions)
How can reaction time be tested?
By doing the ruler drop test; the longer the distance caught on, the longer the reaction time
What is the typical human reaction time?
Between 0.2 s and 0.9 s
How can you make the results of an experiment more reliable?
Do repeats, then calculate the mean
Why do the brakes heat up when used?
The brakes rely on friction between the brakes and the wheels, causing work to be done, causing energy transfer from the kinetic energy stores of the wheels to the thermal energy stores of the brakes
What is momentum?
A property of moving objects; the greater the mass and velocity of an object, the greater the momentum it has
What is conservation of momentum?
In a closed system, the total momentum before an event is the same as the total momentum after the event
What happens to momentum in an explosion?
Before the explosion, the momentum is zero and after the explosion, pieces fly in different directions so that the individual momentums of each piece cancel each other out, making the overall momentum zero
Why are cars designed to slow people down over a longer period of time in a crash?
The longer it takes for a change in momentum, the smaller the rate of change in momentum, and so the smaller the force
What safety features do cars have and how do they help?
Crumple zones crumple on impact so increase the time taken to stop, seat belts increase the time taken for the wearer to stop and airbags, when inflated, slows you down more gradually
How do bike helmets help safety?
They contain a crushable layer of foam which helps lengthen the time it takes for your head to stop in a crash, reducing the impact on your brain
How do crash mats and cushioned playground flooring help safety?
The increase the time taken for you to stop when you fall on them as they are made from soft, compressible materials
