Further Mechanics Mistakes Collection
Work done and energy
Mistakes
• Don’t use the force concept to think (i.e. Why for a slope there is no work done by the component of
force parallel to the slope)
- In force, the GPE is not included too (assume they have same effect)
• μ can be greater than 1 (very rare)
• The inextensible of the string implies that the acceleration of car = the acceleration of the
trailer
• Take 𝑔 = 9.8 not 9.81
• Reference about A level math revision (work energy and power) last question, we can find the
position of the particle when it is at C (above or below), if vertical displacement above, still flying so
before won’t touch, if vertical displacement below, before had touch already (or else it can’t go
down```)
Elastic Springs
Mistakes
• When asked why when particle first comes to rest, it is only at rest for an instant
◦ If 1 mark just say compressed and other stuff
◦ However, if the question consist more marks, you have to write more
- you have to calculate the compression force at that instant, then compare it to the friction
at the limiting equilibrium
• If the string that is used in vertical is not light (has mass), you have to include the GPE of the string/
the string also have KE
• The place for maximum velocity is at the equilibrium position (not the natural length position), as
only that place there is no net force, other places have force would still further affect the velocity
𝜆 𝜆
• Bear in mind about the change in EPE, it is (𝑥12 − 𝑥22 ), 𝑛𝑜𝑡 (𝑥1 − 𝑥2 )2
2𝐿 2𝐿
• It is possible that even for string, although unlike spring it won’t immediately compress after passing
the natural length, but if it pulls to far, it will have tension in the opposite direction
2D Collisions
Mistakes
• The change in momentum (Impulse) is equal to the change in momentum parallel to the line of
change
- The perpendicular of line of change remains constant and will be cancelled out
• To calculate the change in momentum, we take 𝑚𝑣 − 𝑚𝑢 of 𝐵 not 𝑚𝑣(𝐵) − 𝑚𝑣(𝐴)
- Think about the change in momentum of A is equal to the change in momentum of B
(Newton's 3rd law)
• When calculating 𝑒, you may have to assume one of them is larger (because 𝑒 has to be positive,
must be 𝐿𝑎𝑟𝑔𝑒 − 𝑆𝑚𝑎𝑙𝑙 )
• When doing dot product, remember to multiple the results with the unit vector too*
• When comparing the velocity (by inequality), draw arrows out and observe the direction
- Multiply by -1 when suitable
• Take absolute value is not changing the direction, just finding the magnitude, so you can compare
speed by speed
Work done and energy
Mistakes
• Don’t use the force concept to think (i.e. Why for a slope there is no work done by the component of
force parallel to the slope)
- In force, the GPE is not included too (assume they have same effect)
• μ can be greater than 1 (very rare)
• The inextensible of the string implies that the acceleration of car = the acceleration of the
trailer
• Take 𝑔 = 9.8 not 9.81
• Reference about A level math revision (work energy and power) last question, we can find the
position of the particle when it is at C (above or below), if vertical displacement above, still flying so
before won’t touch, if vertical displacement below, before had touch already (or else it can’t go
down```)
Elastic Springs
Mistakes
• When asked why when particle first comes to rest, it is only at rest for an instant
◦ If 1 mark just say compressed and other stuff
◦ However, if the question consist more marks, you have to write more
- you have to calculate the compression force at that instant, then compare it to the friction
at the limiting equilibrium
• If the string that is used in vertical is not light (has mass), you have to include the GPE of the string/
the string also have KE
• The place for maximum velocity is at the equilibrium position (not the natural length position), as
only that place there is no net force, other places have force would still further affect the velocity
𝜆 𝜆
• Bear in mind about the change in EPE, it is (𝑥12 − 𝑥22 ), 𝑛𝑜𝑡 (𝑥1 − 𝑥2 )2
2𝐿 2𝐿
• It is possible that even for string, although unlike spring it won’t immediately compress after passing
the natural length, but if it pulls to far, it will have tension in the opposite direction
2D Collisions
Mistakes
• The change in momentum (Impulse) is equal to the change in momentum parallel to the line of
change
- The perpendicular of line of change remains constant and will be cancelled out
• To calculate the change in momentum, we take 𝑚𝑣 − 𝑚𝑢 of 𝐵 not 𝑚𝑣(𝐵) − 𝑚𝑣(𝐴)
- Think about the change in momentum of A is equal to the change in momentum of B
(Newton's 3rd law)
• When calculating 𝑒, you may have to assume one of them is larger (because 𝑒 has to be positive,
must be 𝐿𝑎𝑟𝑔𝑒 − 𝑆𝑚𝑎𝑙𝑙 )
• When doing dot product, remember to multiple the results with the unit vector too*
• When comparing the velocity (by inequality), draw arrows out and observe the direction
- Multiply by -1 when suitable
• Take absolute value is not changing the direction, just finding the magnitude, so you can compare
speed by speed
