Topic 4 - Dynamics Flashcards

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1
Q

Momentum

A
  • Product of mass and velocity

- Unit : kgms-² / Ns

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2
Q

Newton’s First Law

A

A body will remain its state of rest or uniform velocity unless an external force acts on it.

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3
Q

Inertia

A

Property of a body to resist changes to its initial state of motion.

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4
Q

Newton’s Second Law

A

Rate of change of momentum is directly proportional to the net force acting on it and takes place in the direction of force.

F = ma

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5
Q

Newton’s Third Law

A

When two bodies interact, they exert equal magnitude of forces which acts in opposite direction on one another.

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6
Q

Weight

A

Force of gravity acting in the body

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7
Q

Principle of Conservation of Momentum

A

In a closed system, the total momentum remains constant if no external forces act on it.

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8
Q

Explain how momentum is conserved.

A
  • When two bodies collide, there’s a change in momentum. (N2L)
  • Hence, this give rise to force.
  • Force exerted on the bodies are equal in magnitude and opposite in direction. (N3L)
  • Lost in momentum of body A is equal to the gain of momentum of body B.
  • Total change in momentum is zero.
  • Momentum is conserved.
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9
Q

Elastic collision

A
  • Total kinetic energy is conserved.
  • Relative speed of approach = relative speed of separation

u1 - u2 = v2 - v1

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10
Q

Inelastic collision

A

Total kinetic energy is not conserved.
- lost in the form of other energy (internal energy)

  • Inelastic (separates)
  • Perfectly inelastic (moves together after collision)
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11
Q

When does the momentum is not conserved?

A

• There’s an external force acting.

  • Friction force
  • Gravitational force

• Not a closed system.

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12
Q

Example :

A ball was released from a certain height. The ball then bounces off when it hits the ground. Is the momentum conserved?

A

Collision : between ball and ground

For PCM, the ground must gain momentum as the ball loses momentum.

  • change in momentum if the ground = change in momentum of the ball.
  • Ground exerts a reaction force.
  • momentum is conserved.
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13
Q

Example :

An object is undergoing free fall. Is the momentum conserved?

A

Answer 1 :

  • Momentum is conserved.
  • Lose in momentum of the ball = Gain in momentum of the ground.
  • Acts in opposite direction.
  • Total momentum = 0

Answer 2 :

  • Momentum is not conserved.
  • Presence of external force, gravitational force.
  • Technically, the ball does not collide with other bodies.
  • Momentum is not conserved.
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