TOPIC 5 - FORCES Flashcards
What is a vector quantity?
Vector quantities have magnitude and a direction.
Give some examples of vector quantities.
force, velocity, displacement, acceleration, momentum etc
What is a scalar quantity?
Only have magnitude but no direction.
Give some examples of scalar quantities.
speed, distance, mass, temperature, time etc
How are vectors usually represented?
By an arrow - the length of the arrow shows the magnitude and the direction shows the direction of the quantity.
What is a force?
A push or pull on an object that is caused by interacting it with something. All forces are either contact or non-contact.
Give examples of contact forces.
friction, air resistance, tension in ropes, normal contact force etc
Give some examples of non-contact forces?
Magnetic force, gravitational force, electrostatic force etc
What is an interaction pair?
A pair of forces that are equal and opposite and act on two interacting objects.
What is gravitational force?
The force of attraction between masses.
It makes all things fall towards the ground, on the surface of a planet
It gives everything weight
What is mass?
Mass is the amount of ‘stuff’ in an object. This will have the same value anywhere in the universe.
What is weight?
The force acting on an object due to gravity (the pull of the gravitational force on the object). Close to Earth, this force is caused by the gravitational field around the Earth. Gravitational field strength varies with location. It is stronger the closer you are to the mass causing the field, and stronger for larger masses. Therefore, the weight of an object changes with location.
What is weight measured in?
Weight is a force measured in Newtons. It is measured using a calibrated spring balance.
What is the relationship between weight and mass?
They are directly proportional.
What is the resultant force?
The resultant force is the overall force on a point or object. If you have a number of forces acting at a single point, you can replace them with a single force (so long as the single force has the same effect as all the original forces together.
If all the forces act along the same line, the overall effect is found by adding those going in the same direction, and subtracting any going in the opposite direction.
When is work done?
When a resultant force moves an object through a distance, energy is transferred and work is done on the object.
The force does work to move the object and energy is transferred from one store to another.
What is the equation for work done?
Work done = force x distance (moved along the line of action of the force) (m)
How is joules converted into newton metres?
1 joule = 1 Nm
One joule of work is done when a force of one newton causes an object to move a distance of one metre.
When is an object in equilibrium?
An object is in equilibrium if the forces on it are balanced. That means that if all the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium.
How can you draw the forces if the force line is at an awkward angle?
Split it into a horizontal and a vertical line.
What are elastic objects?
An object that can be elastically deformed.
What does it mean if an object is inelastically deformed?
An object has been inelastically deformed if it doesn’t return to its original shape and length after the force has been removed.
What is the relationship between extension and force?
Extension and force are directly proportional.
Force = spring constant x extension
When will extension and force no longer be directly proportional?
There is a maximum force above which the graph curves, showing that extension is no longer proportional to force. This is known as the LIMIT OF PROPORTIONALITY
What is the equation that can be used for find elastic potential energy in a spring?
Elastic potential energy = 1/2 x spring constant x extension*2.
This applies as long as the spring is not stretched past its limit of proportionality, then this is the equation to use for the work done in stretching, or compressing.
What is a moment?
The turning effect of a force.
What is the equation to work out the moment of an object?
moment of a force = force x distance (m) (the perpendicular distance from the pivot to the line of action of the force.
How do you get the maximum moment (or turning effect)?
You need to push at right angles to the spanner.
Pushing at any other angle would mean a smaller distance, and so a smaller moment.
How do levers make it easier to do work?
Levers increase the distance from the pivot at which force is applied. Since M= Fd, this means that less force is needed to get the same moment. This means that levers make it easier to do do work, eg lift a load or turn a nut.
What are gears?
Gears are circular discs with teeth around their edges. Their teeth interlock so that turning one causes another to turn, in the opposite direction.
They are used to transmit the rotational effect of a force from one place to another.
What is pressure?
Force per unit area. This means that particles exert a force.
What is the normal?
Force exerted at right angles.
What is the formula to calculate the pressure at the surface of a fluid?
Pressure = force normal to a surface / area of that surface
What is pressure measured in?
Pascals (Pa)
Why does pressure increase when a liquid is more dense?
The more dense a liquid is, the more particles it has in a certain space. This means that more particles are able to collide, so the pressure is higher.
Why does liquid pressure increase with depth?
As the depth of the liquid increases, the number of particles above that point increases. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth.
What is the equation to calculate the pressure at a certain depth due to the column of liquid above?
Pressure = height of the column of liquid (depth - m) x gravitational field strength (N/kg) x density of the liquid (the symbol is called rho (p)).
What is a fluid?
Either a gas or a liquid. Fluids are substances that can flow because their particles are able to move around.
Why is the force exerted on the bottom of the object larger than the force acting on top of the object? In water
Pressure increases with depth
This causes a resultant force known as UPTHRUST
What is upthrust?
A force is exerted on the bottom of the object that is larger than the force acting on top of the object because pressure increases with depth. This creates a resultant force known as upthrust.
The upthrust is equal to the weight of fluid that has been displaced by the object.
When will an object float?
An object floats if its weight = upthrust. This is because the forces balance.
It also depends on its density.
How does density affect whether an object will float or not?
An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of fluid. This means that it displaces a volume of fluid that is equal to its weight before it is completely submerged. At this point, the object’s weight is equal to the upthrust, so the object floats.
An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust, so it sinks.
Why does atmospheric pressure decrease with height?
Atmospheric pressure is created on a surface by air molecules colliding with the surface. As the altitude increases, atmospheric pressure decreases. This is because as the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface. There are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.
What is the displacement?
A vector quantity. It measures the distance and direction in a straight line from an object’s starting point to it’s finishing point eg the plane flew five metres north. The direction could be to a relative point or a bearing.
If you were to walk 5 metres n then 5 metres s, the displacement is 0m but you walked a distance of 10m.
Why does an object moving in a circle at a constant speed have a constantly changing velocity?
Velocity is a vector, whereas speed is a scalar quantity. As the direction of the object is always changing, so is the velocity.
What is acceleration?
Acceleration is the change in velocity in a certain amount of time.
What is the equation used to calculate acceleration?
Acceleration = change in velocity / time
What is deceleration?
Negative acceleration. (1)
What is uniform acceleration?
Constant acceleration. Eg acceleration due to gravity is uniform for objects in free fall.
What is the equation for uniform acceleration?
v2 - u2 = 2as
v - final velocity (m/s)
u - initial velocity
a - acceleration
s - distance
On a distance-time graph what does the gradient represent?
gradient = speed
On a distance-time graph what do flat sections represent?
Stationary - stopped
On a distance-time graph what do curves represent?
Acceleration or deceleration
How do you calculate the speed at a point if an object is changing speed? On a distance time graph?
Finding the gradient of the tangent to the curve at that point.
What do flat sections represent on a velocity time graph?
Flat sections represent travelling at a steady speed.
How do you work out the distance travelled on a velocity-time graph?
The area under any section of the graph is equal to the distance travelled in that time interval.
What is friction?
Friction slows things down. It acts in the opposite direction to the movement. to travel at a steady speed, the driving force needs to balance the frictional forces.
You get friction between two surfaces in contact, or when an object passes through a fluid (drag)
What is drag?
Drag is the resistance you get in a fluid (a gas or liquid). Air resistance is a type of drag.
How do you reduce drag?
Making objects streamline.
When will frictional forces increase?
Frictional forces from fluids increase with speed.
What is terminal velocity?
The object will fall at a steady speed when it reaches its maximum speed, or terminal velocity. At this point, the resultant force is zero because the frictional force is equal to the accelerating force.
What does terminal velocity depend on?
Terminal velocity depends on shape and area. It is determined by its drag in comparison to its weight. The frictional force depends on shape and area.
Why would a rock and a feather that are dropped simultaneously hit the ground at the same time when on the moon?
Because there is no air resistance.
Why does a human skydiver fall faster without the parachute?
There is a smaller surface area. With the parachute open, there is much more air resistance - so will fall at 15mph, in comparison to 120 mph.
What is Newton’s First Law?
If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is 0, it will carry on moving at the same velocity (same speed and direction).
What will a non-zero resultant force produce?
Acceleration or deceleration.
What form can acceleration take?
Starting Stopping Speeding up Slowing down Changing direction.
What is acceleration proportional to?
Acceleration is proportional to the resultant force.
The larger the resultant force acting on an object, the more the object accelerates.
What is acceleration inversely proportional to?
Acceleration is inversely proportional to the mass. So an object with a larger mass will accelerate less than one with a smaller mass.
What formula is used to describe Newton’s Second Law?
Resultant force (N) = Acceleration (m/s*2) x Mass (kg).
What is inertia?
Inertia is the tendency for motion to remain unchanged.
What is Newton’s First Law?
Until acted upon by a resultant force, objects stay at rset and objects moving at a steady speed will stay moving at that speed. This tendency to continue in the same state of motion is called inertia.
What is inertial mass?
An object’s inertial mass measures how difficult it is to change the velocity of an object. It can be found using Newton’s Second Law F=ma.
What is Newtons’s Third Law?
Whenever two objects interact, the forces they exert on each other are equal and opposite.
What is the equation for stopping distance?
Stopping distance = thinking distance + braking distance.
What is the thinking distance?
The thinking distance is how far the car travels during the driver’s reaction time. (the time between the driver seeing the hazard and applying the brakes).
What is the braking distance?
The distance taken to stop under the braking force.
What factors affect your thinking distance?
Your speed - the faster you are going the further you will travel during the time you take to react.
Your reaction time - the longer your reaction time, the longer your thinking distance.
What factors affect your braking distance?
Your speed - for a given braking force, the faster a vehicle travels, the longer it takes to stop.
The weather or road surface - if it is wet then there is less grip and so less friction, which can lead to skidding.
The condition of your tyres - if the tyres are bold then they cannot get rid of water in wet conditions. This leads to them skidding on top of water.
How good your brakes are - if the brakes are worn or faulty, then they will not be able to apply as much force.
How do brakes work?
When the brake pedal is pushed, this causes brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature.
How does speed affect braking?
The faster a vehicle is going, the more energy it has in its kinetic stores, so the more work needs to be done to stop it. This means that a greater braking force is needed to make it stop within a certain distance.
What is a typical reaction time?
Between 0.2 and 0.9 seconds.
What is the equation for momentum?
Momentum = mass x velocity
p = mv
The greater the mass of an object, or the greater its velocity, the more momentum the object has.
What quantity is momentum?
A vector - it has size and direction.
What is momentum measured in?
kg m/s.
What is momentum before equal to?
Momentum after.
What is conservation of momentum?
In a closed system, the total momentum before an event (eg a collision) is the same as after the event.
Give an example of momentum?
A moving car hits the back of a parked car. The crash causes the two cars to lock together, and they continue moving in the direction that the original moving car was travelling, but at a lower velocity.
Before - the momentum was equal to mass of moving car x its velocity.
After - The mass of the moving object has increased, but its momentum is equal to the momentum before the collision. So, an increase in mass causes a decrease in velocity.
What do forces cause?
A change in momentum. The force causing the change is equal to the rate of change of momentum. A larger force means a faster change of momentum.
What is the equation for force?
Force = change in momentum / change in time.
What safety features do cars have?
Crumple zones crumple on impact, increasing the time taken for the car to stop.
Seat belts stretch slightly, increasing the time taken for the wearer to stop.
Air bags inflate before you hit the dashboard. The compressing air inside it slows you down more gradually than if you had just hit the hard dashboard.
