Knowt
FORCES AND DOING WORK EDEXCEL
Forces
A force is a push or pull; can cause an object to stop, move, speed up, slow down or change direction
Finding the net force of forces acting on the same straight line:
To find the resultant (net) force, forces in the same direction are added up; those in the opposite are subtracted
The resultant force is where the bigger force is
When the resultant force is 0, all forces acting on an object are balanced so it's in equilibrium
Newton's laws
1st Law
An object stays at rest or continues moving in the same straight line and velocity unless acted upon by a resultant force.
2nd Law
If a resultant force acts on an object, it'll cause it to accelerate
This acceleration could also just be a change in direction without any change in speed
e.g. in the circular motion of the moon's orbit around the Earth, the moon's speed doesn't change but its direction does --> acceleration
Force = mass x acceleration
a ∝ F
Acceleration is largest when mass is small and force is large
Force and acceleration are in the same direction
3rd Law
Every action has an equal and opposite reaction
Circular motion
An object moving in a circular path is circular motion
An object in circular motion has constant speed but varying direction
The force that acts on an object in a circular motion to cause acceleration is **centripetal force**
It acts towards the centre and keeps the body moving in a circle
A greater force is needed if:
the object's mass and the circle's radius (in which it's moving) are constant but the velocity increases
the circle's radius is small but mass and speed are constant
a greater mass but constant radius of circle and speed
Centre of gravity
The point at which the whole weight of a body appears to act
For regular objects, the centre of gravity is in the middle/geometric centre
The mass of a body is equally distributed on all sides about the centre of mass
__Plumb line experiment__
Make 3 holes in a lamina
Suspend it from a point
Hang a plumb line from that point and mark the point where it stops; draw a line from there
Repeat this from multiple points
The point where all the lines intersect is the centre of gravity
Stability
The lower the centre of gravity, the more stable the object is
The greater the base area, the more stable the object is
The vertical line from the centre of gravity mustn't fall outside the object's base or it'll topple
Friction
Friction opposes an object's motion, causing it to slow down
When friction occurs, energy transfers in the form of heat
This raises the temperature as work is done against friction
Air resistance is a type of friction that slows down anything travelling in air
Solid friction: force between two surfaces that impede/hinder motion and produce heating
Extension in springs
Extension is the change in length of a body
Extension (x) = final length - initial length
if final length > initial length, extension will be +ve
initial length > final length, extension will be -ve (compression)
Deformation and Hooke's law
Hooke's law: the force applied on a body is directly proportional to its extension within the limit of proportionality.
F = kx (Force = spring constant x extension)
F ∝ x, thus a force-extension graph will have a straight line passing through the origin
**Elasticity:** the property of a body to return to its original shape
**Deformation:** the change in shape or size of an object
**Elastic deformation:** upon removing force, an object returns to its original shape
**Inelastic/plastic deformation:** upon removing force, an object permanently deforms
**limit of proportionality:** the point till which the object maintains proportionality.
extending an object beyond its limit of proportionality will make it permanently change its shape (plastic deformation)
a curve in the graph means the limit of proportionality has been crossed
a straight line means it is obeying Hooke's law
Spring constant:
spring constant (k) is the force per unit extension
force required to extend 1 cm of a spring
the higher the spring constant, the stiffer the material
k = F/x
unit: N/m
Moments
Pivot: a joint about which rotation happens
Moment: turning effect of a force about a point
SI unit: Newton metres (Nm)
Moment = force x perpendicular distance from pivot
moment ∝ perpendicular distance
the perpendicular distance must be at a right angle to the direction of force
Everyday examples: scissors, wheelbarrow, lever, spanners
**Principle of moments:** For an object in equilibrium, sum of clockwise moments = sum of anticlockwise moments so the net (resultant) moment becomes 0
The sum of forces in one direction must equal the sum of forces in the opp. direction
Pressure
Pressure is the force acting per unit area
Pressure = Force/Area
The greater the force the greater the pressure
The greater the area the lesser the pressure
The unit of pressure is Pascals or Newton/m3
Liquid Pressure
A liquid exerts pressure on an object immersed in it
The pressure is equal throughout a liquid in all directions
The pressure depends on the depth and density of the liquid
Liquid pressure = Liquid Density x gravity x depth
The volume of the liquid or the shape of the container do not affect the pressure
Work Done
Work is done when an object moves in the direction of the force applied
The greater the force or the distance travelled, the greater the work done
Energy is transferred whenever work is done
Work done = Energy transferred
Work = force x distance travelled
The unit of work is Joules, or Newton Meter
Power
Power is the rate of transfer of energy
Power is the rate of doing work
Power = Energy transferred/time
The unit of power is Watt or Joules per second
Energy:
Energy: the capacity of something to do work
Law of Conservation:
Energy cannot be created nor destroyed
It can only change from one form to another
No matter how much it may change its form, the total energy will remain constant: e.g
falling object in a vacuum: gravitational potential energy ⇾ kinetic energy
a gas cooker: from chemical energy ⇾ internal heat
in an LED: electrical energy to light
Types of Energy:
1- Kinetic Energy:
The energy of a moving object
suppose a box is resting on the ground. Means it has no kinetic energy
but once you applied a force and it started moving, means it does have kinetic energy
2- Gravitational Potential:
The energy something gains when you lift it up, and loses when it falls
3- Elastic:
Also called strain
The energy of a stretched spring
4- Chemical:
Energy in a chemical substance
5- Nuclear:
Energy in an atom's nucleus
6- Internal:
Also called thermal or heat energy. It is the energy in something due to its temperature or state.
Keywords:
The collection of a matter is called a system. When the system changes, energy is transferred either between objects or between different forms e.g GPE to kinetic energy.
Outside world: It is all of the matter outside the system. An open system can lose or gain energy as it interacts with the outside world.
Energy transfers:
There are four ways through which energy can be transferred.
1- Forces (mechanical):
When a force acts on a body, the energy transfers between two forms.
2- Heating:
Conduction, convection and thermal radiation cause internal energy to transfer.
3- Electrical currents:
A power source gives energy to a circuit's component through electricity.
4- Waves:
Light and sound have energy, so transfer it between places
when energy transfers from one form to another, we lose some of the energy.
this lost energy spreads out in the environment, or becomes dissipated as light, heat or sound
Gravitational Potential Energy:
Energy of an object due to its height in a gravitational field
If an object is lifted up it will gain GPE
If it falls, it will lose GPE
GPE = mass x gravitational field strength x height
Kinetic Energy:
The energy of an object due to its speed
Formula for KE: 1/2 x mass x speed²
Remember that only speed is squared
Energy resources:
FORCES AND DOING WORK EDEXCEL
Forces
A force is a push or pull; can cause an object to stop, move, speed up, slow down or change direction
Finding the net force of forces acting on the same straight line:
To find the resultant (net) force, forces in the same direction are added up; those in the opposite are subtracted
The resultant force is where the bigger force is
When the resultant force is 0, all forces acting on an object are balanced so it's in equilibrium
Newton's laws
1st Law
An object stays at rest or continues moving in the same straight line and velocity unless acted upon by a resultant force.
2nd Law
If a resultant force acts on an object, it'll cause it to accelerate
This acceleration could also just be a change in direction without any change in speed
e.g. in the circular motion of the moon's orbit around the Earth, the moon's speed doesn't change but its direction does --> acceleration
Force = mass x acceleration
a ∝ F
Acceleration is largest when mass is small and force is large
Force and acceleration are in the same direction
3rd Law
Every action has an equal and opposite reaction
Circular motion
An object moving in a circular path is circular motion
An object in circular motion has constant speed but varying direction
The force that acts on an object in a circular motion to cause acceleration is **centripetal force**
It acts towards the centre and keeps the body moving in a circle
A greater force is needed if:
the object's mass and the circle's radius (in which it's moving) are constant but the velocity increases
the circle's radius is small but mass and speed are constant
a greater mass but constant radius of circle and speed
Centre of gravity
The point at which the whole weight of a body appears to act
For regular objects, the centre of gravity is in the middle/geometric centre
The mass of a body is equally distributed on all sides about the centre of mass
__Plumb line experiment__
Make 3 holes in a lamina
Suspend it from a point
Hang a plumb line from that point and mark the point where it stops; draw a line from there
Repeat this from multiple points
The point where all the lines intersect is the centre of gravity
Stability
The lower the centre of gravity, the more stable the object is
The greater the base area, the more stable the object is
The vertical line from the centre of gravity mustn't fall outside the object's base or it'll topple
Friction
Friction opposes an object's motion, causing it to slow down
When friction occurs, energy transfers in the form of heat
This raises the temperature as work is done against friction
Air resistance is a type of friction that slows down anything travelling in air
Solid friction: force between two surfaces that impede/hinder motion and produce heating
Extension in springs
Extension is the change in length of a body
Extension (x) = final length - initial length
if final length > initial length, extension will be +ve
initial length > final length, extension will be -ve (compression)
Deformation and Hooke's law
Hooke's law: the force applied on a body is directly proportional to its extension within the limit of proportionality.
F = kx (Force = spring constant x extension)
F ∝ x, thus a force-extension graph will have a straight line passing through the origin
**Elasticity:** the property of a body to return to its original shape
**Deformation:** the change in shape or size of an object
**Elastic deformation:** upon removing force, an object returns to its original shape
**Inelastic/plastic deformation:** upon removing force, an object permanently deforms
**limit of proportionality:** the point till which the object maintains proportionality.
extending an object beyond its limit of proportionality will make it permanently change its shape (plastic deformation)
a curve in the graph means the limit of proportionality has been crossed
a straight line means it is obeying Hooke's law
Spring constant:
spring constant (k) is the force per unit extension
force required to extend 1 cm of a spring
the higher the spring constant, the stiffer the material
k = F/x
unit: N/m
Moments
Pivot: a joint about which rotation happens
Moment: turning effect of a force about a point
SI unit: Newton metres (Nm)
Moment = force x perpendicular distance from pivot
moment ∝ perpendicular distance
the perpendicular distance must be at a right angle to the direction of force
Everyday examples: scissors, wheelbarrow, lever, spanners
**Principle of moments:** For an object in equilibrium, sum of clockwise moments = sum of anticlockwise moments so the net (resultant) moment becomes 0
The sum of forces in one direction must equal the sum of forces in the opp. direction
Pressure
Pressure is the force acting per unit area
Pressure = Force/Area
The greater the force the greater the pressure
The greater the area the lesser the pressure
The unit of pressure is Pascals or Newton/m3
Liquid Pressure
A liquid exerts pressure on an object immersed in it
The pressure is equal throughout a liquid in all directions
The pressure depends on the depth and density of the liquid
Liquid pressure = Liquid Density x gravity x depth
The volume of the liquid or the shape of the container do not affect the pressure
Work Done
Work is done when an object moves in the direction of the force applied
The greater the force or the distance travelled, the greater the work done
Energy is transferred whenever work is done
Work done = Energy transferred
Work = force x distance travelled
The unit of work is Joules, or Newton Meter
Power
Power is the rate of transfer of energy
Power is the rate of doing work
Power = Energy transferred/time
The unit of power is Watt or Joules per second
Energy:
Energy: the capacity of something to do work
Law of Conservation:
Energy cannot be created nor destroyed
It can only change from one form to another
No matter how much it may change its form, the total energy will remain constant: e.g
falling object in a vacuum: gravitational potential energy ⇾ kinetic energy
a gas cooker: from chemical energy ⇾ internal heat
in an LED: electrical energy to light
Types of Energy:
1- Kinetic Energy:
The energy of a moving object
suppose a box is resting on the ground. Means it has no kinetic energy
but once you applied a force and it started moving, means it does have kinetic energy
2- Gravitational Potential:
The energy something gains when you lift it up, and loses when it falls
3- Elastic:
Also called strain
The energy of a stretched spring
4- Chemical:
Energy in a chemical substance
5- Nuclear:
Energy in an atom's nucleus
6- Internal:
Also called thermal or heat energy. It is the energy in something due to its temperature or state.
Keywords:
The collection of a matter is called a system. When the system changes, energy is transferred either between objects or between different forms e.g GPE to kinetic energy.
Outside world: It is all of the matter outside the system. An open system can lose or gain energy as it interacts with the outside world.
Energy transfers:
There are four ways through which energy can be transferred.
1- Forces (mechanical):
When a force acts on a body, the energy transfers between two forms.
2- Heating:
Conduction, convection and thermal radiation cause internal energy to transfer.
3- Electrical currents:
A power source gives energy to a circuit's component through electricity.
4- Waves:
Light and sound have energy, so transfer it between places
when energy transfers from one form to another, we lose some of the energy.
this lost energy spreads out in the environment, or becomes dissipated as light, heat or sound
Gravitational Potential Energy:
Energy of an object due to its height in a gravitational field
If an object is lifted up it will gain GPE
If it falls, it will lose GPE
GPE = mass x gravitational field strength x height
Kinetic Energy:
The energy of an object due to its speed
Formula for KE: 1/2 x mass x speed²
Remember that only speed is squared
Energy resources:
