Effects of Forces Chapter Notes | Physics for GCSE/IGCSE - Year 11 PDF Download

 Page 1


Forces and Motion
? Force is a push or pull of an object
? S.I unit is Newton
? Force = mass x acceleration / F= ma
? kgm/s2
? Effects of forces
? It can change the speed of an object
? It can change the shape of an object
? It can change the direction of motion of an object
? It can make a moving object stop
? It can make an object move
? Work
? Force causing movement/displacement
? W= mass x gravity(10g) x height/ W=mgh
? Weight of an object is the pull of gravity on it
? Friction opposes the motion
? Upthrust is the upward push of a liquid or gas on an object
? Forces
? Balanced force
¦ Resultant force= 0
¦ 2 equal forces acting in opposite directions
¦ Same state of rest or motion
? Unbalanced force
¦ Resultant force is not 0, object will either accelerate or decelerate
¦ If 2 or more forces are unbalanced, the resultant force is not 0
? Newton first law of motion
? Everybody continues its state of rest or state of motion unless it is compelled by an
external force
? Inertia
¦ ? Rest  tendency of an object to continue the same state of rest
¦ ? Motion  tendency of an object to continue the same state of motion
? The more mass on an object, the greater the inertia
Page 2


Forces and Motion
? Force is a push or pull of an object
? S.I unit is Newton
? Force = mass x acceleration / F= ma
? kgm/s2
? Effects of forces
? It can change the speed of an object
? It can change the shape of an object
? It can change the direction of motion of an object
? It can make a moving object stop
? It can make an object move
? Work
? Force causing movement/displacement
? W= mass x gravity(10g) x height/ W=mgh
? Weight of an object is the pull of gravity on it
? Friction opposes the motion
? Upthrust is the upward push of a liquid or gas on an object
? Forces
? Balanced force
¦ Resultant force= 0
¦ 2 equal forces acting in opposite directions
¦ Same state of rest or motion
? Unbalanced force
¦ Resultant force is not 0, object will either accelerate or decelerate
¦ If 2 or more forces are unbalanced, the resultant force is not 0
? Newton first law of motion
? Everybody continues its state of rest or state of motion unless it is compelled by an
external force
? Inertia
¦ ? Rest  tendency of an object to continue the same state of rest
¦ ? Motion  tendency of an object to continue the same state of motion
? The more mass on an object, the greater the inertia
? Examples
? If the bus accelerates suddenly, the passengers fall backwards
? Bus stops abruptly, the passengers lurch forward
? When the cardboard is flicked, the coin falls into the glass because inertia of the coin
maintains its state at rest and it falls into the glass due to gravity
? Gravity
? The force of attraction on an object to the centre of Earth
? Mass
? The amount of matter contained in an object
? Weight
? Mass x acceleration due to gravity
? W= mg
? W= mass x gravity
? Newton's Second law of motion
? The rate of change of momentum of an object is directly proportional to the applied force
? u= initial velocity
? v= final velocity
? t= time taken
? Momentum
? It is the product of mass and velocity (kgm/s)
? ? Momentum(P1)  mass x u = mu
? ? Momentum(P2)  mass x v = mv
? Change in momentum = mv - mu m(v-u) or
? Rate of change of momentum= m(v-u)/t Impulse/ t or
? Acceleration
? It is the rate of change of velocity
? A = (v-u)/t
? A= force/mass
? Acceleration and mass are inversely proportional
? Impulse
? Product of force and time or change in momentum of a large force acting in short interval
of time
? Impulse = Ft or m(v-u)
? ? S.I unit  Ns
Page 3


Forces and Motion
? Force is a push or pull of an object
? S.I unit is Newton
? Force = mass x acceleration / F= ma
? kgm/s2
? Effects of forces
? It can change the speed of an object
? It can change the shape of an object
? It can change the direction of motion of an object
? It can make a moving object stop
? It can make an object move
? Work
? Force causing movement/displacement
? W= mass x gravity(10g) x height/ W=mgh
? Weight of an object is the pull of gravity on it
? Friction opposes the motion
? Upthrust is the upward push of a liquid or gas on an object
? Forces
? Balanced force
¦ Resultant force= 0
¦ 2 equal forces acting in opposite directions
¦ Same state of rest or motion
? Unbalanced force
¦ Resultant force is not 0, object will either accelerate or decelerate
¦ If 2 or more forces are unbalanced, the resultant force is not 0
? Newton first law of motion
? Everybody continues its state of rest or state of motion unless it is compelled by an
external force
? Inertia
¦ ? Rest  tendency of an object to continue the same state of rest
¦ ? Motion  tendency of an object to continue the same state of motion
? The more mass on an object, the greater the inertia
? Examples
? If the bus accelerates suddenly, the passengers fall backwards
? Bus stops abruptly, the passengers lurch forward
? When the cardboard is flicked, the coin falls into the glass because inertia of the coin
maintains its state at rest and it falls into the glass due to gravity
? Gravity
? The force of attraction on an object to the centre of Earth
? Mass
? The amount of matter contained in an object
? Weight
? Mass x acceleration due to gravity
? W= mg
? W= mass x gravity
? Newton's Second law of motion
? The rate of change of momentum of an object is directly proportional to the applied force
? u= initial velocity
? v= final velocity
? t= time taken
? Momentum
? It is the product of mass and velocity (kgm/s)
? ? Momentum(P1)  mass x u = mu
? ? Momentum(P2)  mass x v = mv
? Change in momentum = mv - mu m(v-u) or
? Rate of change of momentum= m(v-u)/t Impulse/ t or
? Acceleration
? It is the rate of change of velocity
? A = (v-u)/t
? A= force/mass
? Acceleration and mass are inversely proportional
? Impulse
? Product of force and time or change in momentum of a large force acting in short interval
of time
? Impulse = Ft or m(v-u)
? ? S.I unit  Ns
? Law of conservation of momentum
? When 2 objects interact the total momentum before collision is equal to the momentum
after collision
? Momentum after collision is equal to the momentum after collision
? Newton's 3rd Law of motion
? For every action there is an equal and opposite reaction
? Action and reaction are forces that acts on different objects
? Examples
? Hit tennis ball with racquet, the force on the ball due to the racquet is the same as the
force in the racquet but the more in different direction
? When you swim, and push back the water, you move forward the same amount
Scaler and Vector quantities
? Scaler
? Only magnitude
? ? E.g  mass, distance, speed, time etc.
? Vector
? Direction and magnitude
? ? E.g  Acceleration, force, weight etc.
? Rules for vector addition
? Draw arrows end to end
? Choose a scale that gives a large triangle
? Join the start of the first arrow to the end of the last arrow to find the resultant
? Adding forces
? 2 forces acting in same direction
¦ Add them
? 2 forces acting in the opposite direction
¦ Subtract them
? 2 forces perpendicular to each other
¦ Use pythagoras theorem
? Forces that do not act in the same straight line
¦ - Not in Portion -
Page 4


Forces and Motion
? Force is a push or pull of an object
? S.I unit is Newton
? Force = mass x acceleration / F= ma
? kgm/s2
? Effects of forces
? It can change the speed of an object
? It can change the shape of an object
? It can change the direction of motion of an object
? It can make a moving object stop
? It can make an object move
? Work
? Force causing movement/displacement
? W= mass x gravity(10g) x height/ W=mgh
? Weight of an object is the pull of gravity on it
? Friction opposes the motion
? Upthrust is the upward push of a liquid or gas on an object
? Forces
? Balanced force
¦ Resultant force= 0
¦ 2 equal forces acting in opposite directions
¦ Same state of rest or motion
? Unbalanced force
¦ Resultant force is not 0, object will either accelerate or decelerate
¦ If 2 or more forces are unbalanced, the resultant force is not 0
? Newton first law of motion
? Everybody continues its state of rest or state of motion unless it is compelled by an
external force
? Inertia
¦ ? Rest  tendency of an object to continue the same state of rest
¦ ? Motion  tendency of an object to continue the same state of motion
? The more mass on an object, the greater the inertia
? Examples
? If the bus accelerates suddenly, the passengers fall backwards
? Bus stops abruptly, the passengers lurch forward
? When the cardboard is flicked, the coin falls into the glass because inertia of the coin
maintains its state at rest and it falls into the glass due to gravity
? Gravity
? The force of attraction on an object to the centre of Earth
? Mass
? The amount of matter contained in an object
? Weight
? Mass x acceleration due to gravity
? W= mg
? W= mass x gravity
? Newton's Second law of motion
? The rate of change of momentum of an object is directly proportional to the applied force
? u= initial velocity
? v= final velocity
? t= time taken
? Momentum
? It is the product of mass and velocity (kgm/s)
? ? Momentum(P1)  mass x u = mu
? ? Momentum(P2)  mass x v = mv
? Change in momentum = mv - mu m(v-u) or
? Rate of change of momentum= m(v-u)/t Impulse/ t or
? Acceleration
? It is the rate of change of velocity
? A = (v-u)/t
? A= force/mass
? Acceleration and mass are inversely proportional
? Impulse
? Product of force and time or change in momentum of a large force acting in short interval
of time
? Impulse = Ft or m(v-u)
? ? S.I unit  Ns
? Law of conservation of momentum
? When 2 objects interact the total momentum before collision is equal to the momentum
after collision
? Momentum after collision is equal to the momentum after collision
? Newton's 3rd Law of motion
? For every action there is an equal and opposite reaction
? Action and reaction are forces that acts on different objects
? Examples
? Hit tennis ball with racquet, the force on the ball due to the racquet is the same as the
force in the racquet but the more in different direction
? When you swim, and push back the water, you move forward the same amount
Scaler and Vector quantities
? Scaler
? Only magnitude
? ? E.g  mass, distance, speed, time etc.
? Vector
? Direction and magnitude
? ? E.g  Acceleration, force, weight etc.
? Rules for vector addition
? Draw arrows end to end
? Choose a scale that gives a large triangle
? Join the start of the first arrow to the end of the last arrow to find the resultant
? Adding forces
? 2 forces acting in same direction
¦ Add them
? 2 forces acting in the opposite direction
¦ Subtract them
? 2 forces perpendicular to each other
¦ Use pythagoras theorem
? Forces that do not act in the same straight line
¦ - Not in Portion -
Turning effects of motion
? If something is balanced the moments are equal and opposite
? The size of a moment depends on
? The size of the force
? The distance between the force and the pivot
? Moment of force
? The turning effect of force is called the moment of force
or it is the product of force and perpendicular distance
? Moment = force x perpendicular distance
? S.I unit of moment is Nm (newton metre)
? Pivot
? Centre point/ fixed point at which the object will turn
? Turning effect of force is also known as torque or moment
? Principle of moment
? For an object to be in equilibrium, the sum of all clockwise moments about a pivot is equal
to the sum of anticlockwise moments about the same pivot
? There must be no resultant force acting on it
? Centre of mass
? It is a point in space where all the mass of an object can be thought to act
? Method to find centre of mass of an irregular object
? Suspend the shape
? Draw a plumb line
? Suspend the shape from another location
? Mark the line again
? THe intersection of the 2 lines is the centre of mass
? Centre of gravity/ stability
? Point at which the entire weight of an object appears to act
? How to get more stability
? More base width of the object, the more stable
? More mass of body, the more stable
? Lower the centre of gravity, higher the stability
Page 5


Forces and Motion
? Force is a push or pull of an object
? S.I unit is Newton
? Force = mass x acceleration / F= ma
? kgm/s2
? Effects of forces
? It can change the speed of an object
? It can change the shape of an object
? It can change the direction of motion of an object
? It can make a moving object stop
? It can make an object move
? Work
? Force causing movement/displacement
? W= mass x gravity(10g) x height/ W=mgh
? Weight of an object is the pull of gravity on it
? Friction opposes the motion
? Upthrust is the upward push of a liquid or gas on an object
? Forces
? Balanced force
¦ Resultant force= 0
¦ 2 equal forces acting in opposite directions
¦ Same state of rest or motion
? Unbalanced force
¦ Resultant force is not 0, object will either accelerate or decelerate
¦ If 2 or more forces are unbalanced, the resultant force is not 0
? Newton first law of motion
? Everybody continues its state of rest or state of motion unless it is compelled by an
external force
? Inertia
¦ ? Rest  tendency of an object to continue the same state of rest
¦ ? Motion  tendency of an object to continue the same state of motion
? The more mass on an object, the greater the inertia
? Examples
? If the bus accelerates suddenly, the passengers fall backwards
? Bus stops abruptly, the passengers lurch forward
? When the cardboard is flicked, the coin falls into the glass because inertia of the coin
maintains its state at rest and it falls into the glass due to gravity
? Gravity
? The force of attraction on an object to the centre of Earth
? Mass
? The amount of matter contained in an object
? Weight
? Mass x acceleration due to gravity
? W= mg
? W= mass x gravity
? Newton's Second law of motion
? The rate of change of momentum of an object is directly proportional to the applied force
? u= initial velocity
? v= final velocity
? t= time taken
? Momentum
? It is the product of mass and velocity (kgm/s)
? ? Momentum(P1)  mass x u = mu
? ? Momentum(P2)  mass x v = mv
? Change in momentum = mv - mu m(v-u) or
? Rate of change of momentum= m(v-u)/t Impulse/ t or
? Acceleration
? It is the rate of change of velocity
? A = (v-u)/t
? A= force/mass
? Acceleration and mass are inversely proportional
? Impulse
? Product of force and time or change in momentum of a large force acting in short interval
of time
? Impulse = Ft or m(v-u)
? ? S.I unit  Ns
? Law of conservation of momentum
? When 2 objects interact the total momentum before collision is equal to the momentum
after collision
? Momentum after collision is equal to the momentum after collision
? Newton's 3rd Law of motion
? For every action there is an equal and opposite reaction
? Action and reaction are forces that acts on different objects
? Examples
? Hit tennis ball with racquet, the force on the ball due to the racquet is the same as the
force in the racquet but the more in different direction
? When you swim, and push back the water, you move forward the same amount
Scaler and Vector quantities
? Scaler
? Only magnitude
? ? E.g  mass, distance, speed, time etc.
? Vector
? Direction and magnitude
? ? E.g  Acceleration, force, weight etc.
? Rules for vector addition
? Draw arrows end to end
? Choose a scale that gives a large triangle
? Join the start of the first arrow to the end of the last arrow to find the resultant
? Adding forces
? 2 forces acting in same direction
¦ Add them
? 2 forces acting in the opposite direction
¦ Subtract them
? 2 forces perpendicular to each other
¦ Use pythagoras theorem
? Forces that do not act in the same straight line
¦ - Not in Portion -
Turning effects of motion
? If something is balanced the moments are equal and opposite
? The size of a moment depends on
? The size of the force
? The distance between the force and the pivot
? Moment of force
? The turning effect of force is called the moment of force
or it is the product of force and perpendicular distance
? Moment = force x perpendicular distance
? S.I unit of moment is Nm (newton metre)
? Pivot
? Centre point/ fixed point at which the object will turn
? Turning effect of force is also known as torque or moment
? Principle of moment
? For an object to be in equilibrium, the sum of all clockwise moments about a pivot is equal
to the sum of anticlockwise moments about the same pivot
? There must be no resultant force acting on it
? Centre of mass
? It is a point in space where all the mass of an object can be thought to act
? Method to find centre of mass of an irregular object
? Suspend the shape
? Draw a plumb line
? Suspend the shape from another location
? Mark the line again
? THe intersection of the 2 lines is the centre of mass
? Centre of gravity/ stability
? Point at which the entire weight of an object appears to act
? How to get more stability
? More base width of the object, the more stable
? More mass of body, the more stable
? Lower the centre of gravity, higher the stability
? Centre of mass of plane lamina
? Make 3 holes around edge of lamina
? Fix pin horizontally in the clamp
? Using one hole, hang the lamina from the pin, it should be able to turn freely
? Hang string from pin, make 2 points on the lamina along the length of string
? Repeat steps 3 and 4 for the other holes
? Lay the lamina, using a ruler, draw lines joining each pair of points, where the lines cross is
the centre of mass of the lamina