## 04. Forces

### Types of Forces

• Static friction
• Frictional force between surfaces at rest with respect to each other.
• It increases with increasing applied force up to a maximum value (which is proportional to normal contact force).
• Kinetic friction
• Frictional force acting between surfaces in relative motion.
• Viscous forces
• Resistive force experienced by a solid moving in a fluid.
• Dependent on speed of object v, e.g. $F_D\propto v$ at low speeds and $F_D\propto v^2$ at high speeds.

## 03. Dynamics

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[accordion title="1. Newton's Laws of Motion"]

• Newton's First Law:  a body will remain in its state of rest or uniform motion in a straight line unless acted upon by a resultant force.
• Newton's Second Law the rate of change of momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the resultant force.
• $F =\frac{dp}{dt}$ in general
• $F =ma$ when mass is constant.
• Newton's Third Law:  if body A exerts a force on body B, then body B exerts an equal and opposite force on body A

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[accordion title="2. Linear Momentum"]

• The linear momentum of a body is defined as the product of its mass and its velocity.
• Impulse is the product of the force acting on a body and the time interval during which the force is exerted. It is equal to the change in momentum of the body.
• For constant force, impulse = $\Delta p =F \Delta t$
• In general, impulse = $\Delta p =\int {F .dt}$

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[accordion title="3. Collision Problems"]

• The principle of conservation of momentum states that the total momentum of a system of colliding objects remains constant provided no resultant external force acts on the system.
• Conservation of momentum applies to both elastic and inelastic collisions.
• $m_1u_1+m_2u_2=m_1v_1+m_2v_2$
• Conservation of kinetic energy applies only to elastic collisions.
• $\frac{1}{2}m_1u_1^2+\frac{1}{2}m_2u_2^2=\frac{1}{2}m_1v_1^2+\frac{1}{2}m_2v_2^2$
• Relative speed of approach = Relative speed of separation
• $u_2-u_1=v_1-v_2$

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