Physics - Forces (Paper 2) 9-1 exam AQA

Force = a push or a pull applied by one object onto another

Force is measured in newtons (N)

contact force = the object are physically touching (friction, air resistance, tension, normal contact force)

non-contact force = objects are physically seperated (gravity, electrostatic, magnetic)

scalar quantity = magnitude (size) only (distance)

vector quantity = magnitude and direction (displacement)

normal contact force = the force when the floor pushes up on you

displacement = vector quantity containing both the direction the force acts in and how great the force is

speed (m/s) = distance (m) / time (s)

average speed (m/s) = total distance (m) / total time (s)

average speed is used because speed is rarely constant throughout a journey

distance time graph = shows the distance travelled in a certain time on a graph

the line will be curved

to calculate the speed at a total point, draw a tangent to that point on the line and calculate the gradient

gradient = difference in y / difference in x

tangent = a line that just touches the curve at one specific point

acceleration = when the velocity increases (positive)

deceleration = when the velocity decreases (negative)

all objects accelerate at the same rate under gravity (when dropped) at 9.8m/s^2

speed increases as an object is dropped

acceleration (m/s^2) = change in velocity (m/s) / time (s)

an object moving in a circle at a constant speed is accelerating

the velocity constantly changes so the acceleration changes 

acceleration refers to the increase of the speed/velocity

velocity refers to the increase of the speed

velocity-time graphs = show the change in velocity over time

flat line = constant velocity

positive gradient = increasing velocity

negative gradient = decreasing velocity

negative velocity = change in direction

when velocity is 0, the object is back at its starting position

steep gradient = large acceleration and a rapid change in velocity

distance/displacement = area under the graph

uniform motion = an object with constant acceleration

(final velocity)^2 - (initial velocity)^2 = 2 x acceleration x displacement

v^2 - u^2 = 2as

without air resistance, a falling object has an acceleration of 9.8m/s^2 due to gravity

when an object is thrown exactly vertically, it starts to decelerate and has an acceleration of -9.8m/s^2 due to gravity

this equation can only be used for an object that is travelling with constant uniform motion in a straight line

mass = amount of a substance in an object measured in kilograms (kg)

weight = the force acting on a mass in a gravitational field measured in newtons (N)

the weight of an object depends on the strength of the gravitational field that it is in

gravity is a non-contact force

anything in a gravitational field is pulled to one single point where the weight is located called the centre of mass

weight (N) = mass (kg) x gravitational field strength (N/kg)

W=mg

mass is constant everywhere in the universe

weight is different in space because there are different gravitational field strengths

when an obejct is hanging, the weight and tension force are equal, so they cancel out and the object does not move or fall

when an object is hanging and then falls, the weight is greater than the tension force, so the object moves downwards with a force equal to the difference between the weight and tension force

resultant force = the force an object moves with in a certain direction

if the forces acting in all directions of the object are equal, the resultant force is 0

Newton's first law of motion: 

• an object at rest will remain at rest unless a greater force acts upon it in one direction
• an object moving will remain moving unless a force acts upon it to balance the forces acting in all directions

normal = perpendicular to a point

resultant force = the sum of all forces acting in one direction subtract the sum of all forces acting in the opposite direction

free-body diagram = shows the magnitude and direction of the different forces acting upon a diagram

use pythagoras and trigonometry to calculate the magnitude and direction of the resultant force

draw the two forces and find the hypotenuse (magnitude) and the angle (direction)

a free-body diagram is drawn using a point (the obejct) and arrows that are different sizes according to the magnitude of the force

all arrows are the same size if the obejct is staionary

an object can accelerate if the forward force is increased to be greater than the opposing forces

resultant force (N) = mass (kg) x acceleration (m/s^2)

F=ma

Newton's second law of motion = law of inertia

inertia = a measure of how difficult it is to change the velocity of an object

a greater mass will require a greater force to be applied to raise its acceleration to the same value of a lighter object

inertial mass = force/acceleration

Risk assessment:

• be careful that masses do not fall or snap the pulley
• too many masses could cause too large an acceleration and the trolley could hit someone

Variables:

• Control = mass of the trolley
• Independent = force applied to the trolley (number of masses on the pulley)
• Dependent = acceleration of the trolley

Connect a trolley on a rail and hang a 10N mass on a pulley. Place a light gate at each end of the rail.

The computer will record the velocity at each light gate and the time taken to get from one to the other. It will calculate the acceleration from this.

Repeat every 10N up to 100N. The acceleration is in direct proportion to the force applied.

When object A applies a force on object B, object B always applies a force on obejct A that is equal in magnitude but opposite in direction

These are called force pairs

Newton's third law of motion = every action has an opposite and equal reaction

The obejcts in force pairs will experience the same force but will not have the same acceleration if they have different masses (F=ma)

The two forces in force pairs are the same type of force

Applies for both contact and non-contact forces

gravitational force applied downwards on a stationary object causes the object to apply a gravitational force upwards on the Earth

momentum (kgm/s) = mass (kg) x velocity (m/s)

p=mv

momentum is a vector

momentum = the tendency for an object to keep moving in the same direction after the forward force has been removed

large force applied = momentum changes quicker

safety features in a car make the momentum take longer to change

longer time = smaller force on the passengers

crumple zone = increase the time between impact and the car stopping so the rate of change of momentum is lower

conservation of momentum = the total momentum before a collision is equal to the total momentum after in a closed system

reaction time = time taken for a driver to react to a stimulus

high speed of the car = reaction time is longer

thinking distance = distance travelled during the reaction time

braking distance = distance travelled from when brakes are applied to when the car stops

stopping distance = thinking distance + braking distance

reaction time/thinking distance increases when the driver is tired, drunk or distracted

braking distance increases when the road is slippy, the car has poor brakes or the car is at a high speed

large deceleration = brake pads overheat and there is a rapid change in passenger momentum so a larger force is applied to them

seat belts and air bags increase the time taken for passenger momentum to change

moment = a turning effect whose size depends on the force and the distance between the pivot and the area where the force is applied to

pivot = the point a moment acts around

moment (Nm) = force (N) x distance (m)

M=Fd

when a beam is balanced on a pivot it is in equalibrium

the sum of clockwise moments about a pivot is the same as the sum of anticlockwise moments about the same pivot

centre of mass = one single point where an objects weight is

an object becomes unbalanced when the pivot and centre of mass are not aligned

lever = moving a large force using a smaller force by a small distance using a moment

to lift an obejct with a lever, the force pushing down on one end gives a clockwise rotational moment greater than the anticlockwise moment

work done (J) = effort force (N) x distance moved (m)

W=Fs

if the effort force is greater than the load force, the distance moved by the effort force must be greater

energy transferred to the load = gravitational potential energy

gears = transmit the rotational effect of a force from one point of a machine to another

if both gears are the same size, they turn at the same speed but a larger gear turns slower with a greater force

the first gear (clockwise) drives the second gear (anticlockwise)

pressure = a force normal to a surface acting in all directions

fluids = gas and liquid

pressure (Pa) = force normal to surface (N) /area of that surface (m^2)

p=F/A

increase in depth of the water = greater pressure because there is an increased weight of water pushing down on the object

upthrust = when a submerged object experiences a greater pressure on the bottom surface than the top surface so a resultant force is produced pushing the object upwards

object density = liquid density then the object will float because the weight is equal to the upthrust

object density > liquid density then the object will sink because the weight is greater than the upthrust

air gives a pressure causing a force normal to any surface

air pushes on the inside and outside of an obejct

removing internal air = external pressure is greater than the internal = object collapses

atmospheric pressure is caused by the weight of the air above an object

increase in height = decrease in atmospheric pressure because there is less air above the object

particles in a gas constantly move around at a high speed and collide with each other and the walls of the container which they are in

collisions with container = pressure

collisions with each other does not make pressure

increase in air denisty = increase in weight of the air = increase in particles in a given space = increase in collisions per second with a surface = increase in air pressure

forces can stretch or squash a spring elastically or inelastically

elastic deformation = a spring returns back to its original shape when all of the forces have been removed

inelastic deformation = a spring does not return back to its original shape when all of the forces have been removed

the force applied to the spring is directly proportional to the extension of the spring

force (N) = spring constant (N/m) x extension (m)

F=ke

limit of proportionality = the maximum force/extension before the spring is inelastically deformed

doing work on a spring transfers energy into elastic potential energy

elastic potential energy (J) = 0.5 x spring constant (N/m) x extension^2 (m)

Ek = 0.5 x ke^2

assuming no energy is dissipated, the decrease in elastic potential energy stores = the increase in gravitational potential energy stores

Risk assessment: 

• Don't add too many weights - can cause the spring to snap and hit you
• Don't place the clamp stand on the edge of the table - can fall on your feet

Variables:

• Independent = force on the spring
• Dependent = extension of the spring

Method:

• Hang a spring and mass holder off a clamp stand and measure the length of the spring
• Add a 1N  to the mass holder and measure/record the extension of the spring
• Continue to add masses and record extensions up to 10N
• The extension should increase as the force applied increases