9702 · 3.3
Linear momentum and its conservation flashcards
Revision flashcards for Cambridge 9702 Linear momentum and its conservation (syllabus 3.3). Flip, recall, then mark a real past-paper question.
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What is the formula for calculating linear momentum?
Linear momentum ($p$) is calculated as the product of mass ($m$) and velocity ($v$), so $p = mv$.
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Is linear momentum a scalar or vector quantity?
Linear momentum is a vector quantity, meaning it has both magnitude and direction.
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How is the direction of momentum determined?
The direction of an object's momentum is always the same as the direction of its velocity.
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How does force relate to an object's momentum?
Force is defined as the rate of change of an object's linear momentum ($F = \frac{\Delta p}{\Delta t}$).
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When is the total momentum of a system conserved?
The total momentum of a closed system is conserved if no net external forces act upon it.
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What is the primary difference between an elastic and an inelastic collision regarding energy?
In an elastic collision, total kinetic energy is conserved, while in an inelastic collision, total kinetic energy is not conserved.
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What happens to the kinetic energy that is 'lost' in an inelastic collision?
The 'lost' kinetic energy is converted into other forms, such as heat, sound, or energy for deformation.
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In a perfectly elastic collision, what happens to the relative speed of approach and separation?
The relative speed at which objects approach each other before impact equals their relative speed of separation after impact.
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What does a 'closed system' imply in the context of momentum conservation?
A closed system means no mass or energy enters or leaves, and importantly, no net external forces act on the system.
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Which quantity is always conserved in both elastic and inelastic collisions (in a closed system)?
Linear momentum is always conserved in both elastic and inelastic collisions, provided the system is closed.
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What is impulse, and what are its two main formulas?
Impulse is the change in momentum of an object. It can be calculated as the product of the average force and the time duration ($J = F \Delta t$) or as the change in momentum ($J = \Delta p = mv - mu$).
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What are the units of impulse?
The units of impulse are Newton-seconds (N s), which are equivalent to the units of momentum, kilogram-metres per second (kg m/s).
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An object at rest explodes into two fragments. What can be said about their total momentum after the explosion?
The total momentum of the fragments must be zero, the same as the initial momentum. This means the momenta of the two fragments are equal in magnitude and opposite in direction ($p_1 = -p_2$).
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How do you calculate the kinetic energy lost in a perfectly inelastic collision where two objects stick together?
First, calculate the initial total KE: $KE_i = \frac{1}{2}m_1u_1^2 + \frac{1}{2}m_2u_2^2$. Then, find the final velocity V using momentum conservation and calculate the final KE: $KE_f = \frac{1}{2}(m_1+m_2)V^2$. The loss is $KE_{lost} = KE_i - KE_f$.