Topic 3 Notes
Newton’s 3rd Law
When body A exerts a force on body B, force on body A is equal in magnitude to force on body
B from A. Both forces are in opposite directions and of same kind.
Action reaction forces:
- act on different bodies
- be of the same type
- equal magnitude
- act in opposite direction
Newton’s 1st Law
An object continues to be in a state of rest or in motion with constant velocity, unless acted
upon by a resultant force.
Mass: Property of a body which resists change in motion
Weight: Force acting on it due to gravitational field.
Linear momentum
The product of a body’s mass and velocity
Linear
Momentum
𝑝 = 𝑚𝑣
p is the linear momentum (kg m s-1)
m is the mass of the object (m)
v is the velocity of the object (m s-1)
Newton’s 2nd Law
The rate of change of momentum of a body is directly proportional to the resultant force acting
on it, and it takes place in the direction of the resultant force.
Newton’s
2nd Law
𝐹𝑛𝑒𝑡 =𝑑𝑝
𝑑𝑡 =𝑚𝑎
Fnet is the resultant force (N)
p is the linear momentum (kg m s-1)
t is the time taken for the object to change momentum (s)
m is the mass of the object (m)
a is the acceleration of the object (m s-2)
Weightlessness = no contact force but there is weight. Weighing scales do not always
measure true weight but the contact force acting on the weighing scale.
Free Fall
As an object falls, its vertical acceleration decreases over time. Initially, its acceleration is 9.81
m s-2. As it increases in velocity, the air resistance will increase with it, but the acceleration
decreases. When it reaches terminal velocity, it will fall at constant speed where W=Fdrag. No
acceleration.
Flowing Mass and Newton’s 2nd Law
