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 or m(v-u)
○ Rate of change of momentum= m(v-u)/t or Impulse/ t
● 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 -
