Chapter 5: Laws of Motion Revision Notes
- Dynamics is a branch of physics that studies the motion of a body while taking into account the cause, i.e. the force that causes the motion.
- Force: A force is an external force that produces or tries to produce motion in a still body, stops or tries to stop a moving body, or changes or tries to change the direction of motion of a moving body.
- Inertia is the inherent property of a body that prevents it from changing its state of motion. The inertia increases as the body weight increases, and the inertia decreases as the body weight decreases.
- The law of inertia states that a body’s state of rest, uniform motion (i.e., a motion with constant velocity), or motion direction cannot be changed by itself.
Newton’s Laws of Motion
- Unless an external force is applied to a body, it will remain at rest or continue to move at a uniform velocity.
- The ‘Law of Inertia’ is another name for the first law of motion. It establishes the concepts of inertia, force, and inertial frame of reference.
- To describe and understand particle motion, there is always a need for a ‘frame of reference,’ and the simplest ‘frame of reference’ used are known as inertial frames.
- A referent frame, e, is known as an inertial frame because all accelerations of any particle within it are caused by the action of ‘real forces’ on that particle.
- The frame of reference used when discussing accelerations caused by “fictitious” or “pseudo” forces is non-inertial.
- When an external force is applied to a body of constant mass, it causes an acceleration that is proportional to the force and inversely proportional to the mass of the body.
“There is an equal and opposite reaction force to every action.” When a body A applies a force to another body B, B responds by applying an equal and opposite force to A.
The product of a body’s mass and velocity is defined as the linear momentum of the body.
Impulsive forces are those that act for a short period of time. The product of force and the short time interval over which it acts is defined as the impulse.
Law of Conservation of Momentum:
An isolated system of particles’ total momentum is conserved. In other words, the system’s total momentum remains constant when no external force is applied.
Commonly Used Forces
(i) Weight of a Body: It’s the force that pulls a body towards the center of the earth. The weight of the body is Mg in a vertically downward direction if M is the mass of the body and g is the acceleration due to gravity.
(ii) Normal Force: When two bodies come into contact, a contact force is created, which is normal to the plane of contact if the surface is smooth. This force is referred to as normal force.
(iii) String tension: Assume a block is suspended from a string. The block’s weight is acting vertically downward but it is not moving, so it is balanced by a force due to the string. ‘Tension in string’ is the name given to this force. In a stretched string, tension is a force. Its path is along the string, away from the subject’s body.
- Consider two bodies with masses m1 and m2 tied together at the ends of an inextensible string that passes through a light, friction-free pulley.