Grade 12 · Physics
Use conservation of linear momentum
Linear momentum is the vector quantity p = mv. For Grade 12 Physical Sciences collisions, total momentum before equals total momentum after when the net external force on the system is zero. Choose one positive direction and retain velocity signs. Momentum conservation does not mean kinetic energy is conserved; objects sticking together undergo a perfectly inelastic collision.
Original explanation prepared with AI assistance.
Lees hierdie verduideliking in AfrikaansWhat you will practise
Apply signed momentum conservation to a sticking collision.
The method
Define the system, choose a positive direction and equate the signed total momenta before and after.
Worked example
A 2 kg trolley moving right at 3 m/s sticks to a stationary 1 kg trolley. Find their common velocity, assuming negligible external impulse.
- Choose right as positive.
- Initial momentum is 2(3) + 1(0) = 6 kg·m/s.
- After sticking together the mass is 3 kg: 6 = 3v.
- The common velocity is 2 m/s to the right.
Common mistake
Do not give the moving trolley’s initial velocity to both masses before the collision.
Try it yourself
A 3 kg trolley at 4 m/s right sticks to a stationary 1 kg trolley. Find the final velocity.
Show the explained solution
Initial momentum is 12 kg·m/s. The combined mass is 4 kg, so v = 12/4 = 3 m/s right.
Curriculum and sources
Use the official CAPS subject document and your teacher’s annual plan to confirm the sequence and scope for your class. The link below is an official reference, not evidence of DBE approval of this explanation.
DBE: CAPS for the FET phaseFind your next lesson
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