A 3 kg cart moves at −4 m/s. Find signed momentum.
Answer and explanation
-12 kg·m/s
p = mv = 3 × (−4) = −12 kg·m/s.
Subjects
Momentum & collisions
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Sample exercises
A 2 kg cart at 6 m/s sticks to a 4 kg stationary cart. External impulse is zero. Find final velocity.
Answer and explanation
2 m/s
Initial momentum is 12; divide by combined mass 6 kg.
When is total momentum conserved for a chosen system?
- Only when objects have equal masses
- Whenever one object stops
- When net external impulse is zero
Answer and explanation
When net external impulse is zero
External impulse determines the change in total system momentum.
Two momenta are +15 and −9 kg·m/s. Find their total.
Answer and explanation
6 kg·m/s
Signed addition gives 15 − 9 = 6 kg·m/s.
Why must westward and eastward velocities not both be inserted as positive in a one-axis collision model?
Answer and explanation
They point in opposite directions and therefore need opposite signs on the same axis.
Credit a consistent direction convention.
In an isolated collision where objects stick, what is generally not conserved?
- Total momentum
- Total energy accounting across all forms
- Kinetic energy
Answer and explanation
Kinetic energy
Some kinetic energy transfers to other internal forms.
Study notes
Momentum in one-dimensional collisions
Use signed velocities and a clearly chosen isolated system.
Grades 9–12Momentum in one-dimensional collisions · Revision sheet
Use signed velocities and a clearly chosen isolated system.
Grades 9–12Impulse and stopping time
Relate changes in momentum to average forces over a time interval.
Grades 9–12Impulse and stopping time · Revision sheet
Relate changes in momentum to average forces over a time interval.
Grades 9–12Signed linear momentum
Use consistent SI units and keep the stated direction convention.
Grades 9–12Objects stick together
For this isolated collision momentum is conserved; kinetic energy generally is not.
Grades 10–12Impulse from velocity change
Use consistent SI units and keep the stated direction convention.
Grades 9–12Average force during contact
Convert milliseconds to seconds before dividing.
Grades 10–12Momentum & collisions · Formula reference
Formula reference
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