Dynamics | 9702 Physics Summary Notes

• 4. Dynamics | 9702 Physics Summary Notes

4.1 Newton’s Laws of Motion

·  First law: if a body is at rest it remains at rest or if it is in motion it moves with a uniform velocity until it is acted on by resultant force or torque

·  Second law: the rate of change of momentum of a body is proportional to the resultant force and occurs in the direction of force; F=ma

·  Third law: if a body A exerts a force on a body B, then body B exerts an equal but opposite force on body A, forming an action-reaction pair

4.2 Mass and Weight

Mass	Weight
·  Measured in kilograms ·  Scalar quantity ·  Constant throughout the universe	·  Measured in Newtons ·  Vector quantity ·  Not constant ·  W=mg

·  Mass: is a measure of the amount of matter in a body, & is the property of a body which resists change in motion.

·  Weight: is the force of gravitational attraction (exerted by the Earth) on a body.

4.3 Momentum

· Linear momentum: product of mass and velocity

p = mv

· Force: rate of change of momentum

F = (mv - mu) / t

· Principle of conservation of linear momentum: when bodies in a system interact, total momentum remains constant provided no external force acts on the system.

m_Au_A + m_Bu_B = m_Av_A + m_Bv_B

4.4 Elastic Collisions

· Total momentum conserved

· Total kinetic energy is conserved

Example: Two identical spheres collide elastically. Initially, X is moving with speed v and Y is stationary. What happens after the collision? 

X stops and Y moves with speed v

Relative velocity before collision = Relative velocity after collision

u_A - u_B = v_B - v_A

4.5 Inelastic Collisions

relative speed of approach > relative speed of separation

o Total momentum is conserved

· Perfectly inelastic collision: only momentum is conserved, and the particles stick together after collision (i.e. move with the same velocity)

· In inelastic collisions, total energy is conserved but Ek may be converted into other forms of energy e.g. heat