Conservation of Linear Momentum

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AP Physics 1 › Conservation of Linear Momentum

Questions 1 - 10
1

Two identical carts on a frictionless track interact via a compressed spring. Initially, both carts are at rest and the spring is compressed between them; the system is the two carts (ignore the spring’s mass). External forces on the system are negligible. After release, cart 1 moves left at $3.0,\text{m/s}$. What is the velocity of cart 2?

$3.0,\text{m/s}$ right

$1.5,\text{m/s}$ right

$3.0,\text{m/s}$ left

$0,\text{m/s}$

Explanation

This problem demonstrates conservation of linear momentum in an explosion-type interaction. When external forces on a system are negligible, the total momentum remains constant throughout any internal interaction. Initially, both carts are at rest, so p_initial = 0. After the spring releases, the system momentum must still be zero: m₁v₁ + m₂v₂ = 0. Since the carts are identical and cart 1 moves left at 3.0 m/s, we have m(-3.0 m/s) + m(v₂) = 0. Choice A (3.0 m/s left) would give the same direction for both carts, violating momentum conservation. Solving: v₂ = +3.0 m/s (rightward). For explosion problems starting from rest, remember that objects must move in opposite directions with momenta that cancel out.

2

A $3.0,\text{kg}$ cart moving right at $2.0,\text{m/s}$ collides and sticks to a $1.0,\text{kg}$ cart initially at rest on a frictionless track. The system is both carts, and external forces are negligible during the collision. What is the final velocity of the combined carts?

$0.50,\text{m/s}$ right

$6.0,\text{m/s}$ right

$2.0,\text{m/s}$ right

$1.5,\text{m/s}$ right

Explanation

This problem demonstrates conservation of linear momentum in a perfectly inelastic collision with one object initially at rest. When external forces are negligible, the total momentum of the system remains constant before and after the collision. Initially, only the 3.0 kg cart moves: p_initial = (3.0 kg)(2.0 m/s) + (1.0 kg)(0 m/s) = 6.0 kg·m/s rightward. After sticking together, the combined mass is 4.0 kg, so p_final = (4.0 kg)v_final. Choice D (6.0 m/s) incorrectly uses the momentum value as velocity. Setting p_initial = p_final: 6.0 = 4.0v_final, giving v_final = 1.5 m/s rightward. To find the final velocity in sticky collisions, divide the initial total momentum by the combined final mass.

3

Two carts on a frictionless track are connected by a compressed spring. Cart 1 has mass $0.40,\text{kg}$ and Cart 2 has mass $0.60,\text{kg}$. The system is both carts, and external forces are negligible. When released, Cart 1 moves left at $3.0,\text{m/s}$. What is Cart 2’s velocity?

$3.0,\text{m/s}$ right

$2.0,\text{m/s}$ left

$4.5,\text{m/s}$ right

$2.0,\text{m/s}$ right

Explanation

This question assesses the conservation of linear momentum in an explosion-like release of a compressed spring. In a system where external forces are negligible, the total linear momentum remains constant and is zero if the system starts at rest. When Cart 1 moves left at 3.0 m/s, its momentum is -1.2 kg·m/s (right as positive). To conserve zero total momentum, Cart 2 must have +1.2 kg·m/s, resulting in 2.0 m/s right. Option B, 3.0 m/s right, could stem from assuming equal speeds without considering mass ratios. A transferable strategy is to assume initial momentum is zero for stationary systems and solve for opposite momenta in explosions or separations.

4

A $1.0,\text{kg}$ cart moving east at $5.0,\text{m/s}$ collides head-on and sticks with a $4.0,\text{kg}$ cart moving west at $1.0,\text{m/s}$. Define the system as both carts; external forces are negligible. What is the final velocity of the combined carts?

$1.0,\text{m/s}$ west

$0.20,\text{m/s}$ east

$1.0,\text{m/s}$ east

$0.20,\text{m/s}$ west

Explanation

This question assesses the conservation of linear momentum in an inelastic head-on collision. In a system where external forces are negligible, the total linear momentum remains constant, with initial equaling final. The initial momentum is 1.0 kg × 5.0 m/s east plus 4.0 kg × (-1.0 m/s) west, totaling +1.0 kg·m/s east. After sticking, the 5.0 kg combined mass has velocity 1.0 / 5.0 = 0.20 m/s east. Option B, 0.20 m/s west, might result from switching the direction signs in the calculation. A transferable strategy is to assign consistent signs for directions and verify the net momentum direction matches the initial calculation.

5

Two carts collide on a frictionless track. Cart X ($2.0,\text{kg}$) moves east at $1.5,\text{m/s}$ and Cart Y ($1.0,\text{kg}$) moves west at $0.50,\text{m/s}$. The system is both carts; external forces are negligible. What is the system’s total momentum after the collision?

$0,\text{kg}\cdot\text{m/s}$

$+3.0,\text{kg}\cdot\text{m/s}$

$+2.5,\text{kg}\cdot\text{m/s}$

$+3.5,\text{kg}\cdot\text{m/s}$

Explanation

This question assesses the conservation of linear momentum in a collision between two carts. In a system where external forces are negligible, such as on a frictionless track, the total linear momentum remains constant before and after the collision. The initial momentum is 2.0 kg × 1.5 m/s east plus 1.0 kg × (-0.50 m/s) west, totaling +2.5 kg·m/s. This value is conserved, so the total momentum after is also +2.5 kg·m/s. Option D, 0 kg·m/s, might be selected if one ignores the directions and assumes cancellation. A transferable strategy is to calculate the net initial momentum considering directions and recognize it persists unchanged in isolated systems.

6

A $0.20,\text{kg}$ puck moving east at $4.0,\text{m/s}$ collides and sticks to a $0.30,\text{kg}$ puck initially at rest on frictionless ice. Treat the two pucks as the system, and assume external forces are negligible during the collision. Afterward, the stuck-together pucks move in the same direction. What is their final speed?

$0.80,\text{m/s}$

$1.6,\text{m/s}$

$2.4,\text{m/s}$

$4.0,\text{m/s}$

Explanation

This problem tests conservation of linear momentum in a perfectly inelastic collision. When external forces are negligible, the total momentum of a system remains constant before and after any interaction. Initially, the system has momentum from only the moving puck: p_initial = (0.20 kg)(4.0 m/s) + (0.30 kg)(0 m/s) = 0.80 kg·m/s eastward. After the collision, both pucks stick together with combined mass 0.50 kg, so p_final = (0.50 kg)v_final must equal 0.80 kg·m/s. Choice A (0.80 m/s) incorrectly uses the momentum value as the velocity. Solving for v_final: v_final = 0.80 kg·m/s ÷ 0.50 kg = 1.6 m/s. To solve momentum conservation problems, always write p_initial = p_final and carefully track the masses before and after the collision.

7

Two skaters push off each other on level ice. Skater A has mass $50,\text{kg}$ and is initially at rest; Skater B has mass $70,\text{kg}$ and is initially at rest. Define the system as both skaters; external forces are negligible. After pushing off, Skater A moves west at $2.8,\text{m/s}$. What is Skater B’s velocity?

$4.0,\text{m/s}$ east

$2.0,\text{m/s}$ east

$2.8,\text{m/s}$ east

$2.0,\text{m/s}$ west

Explanation

This question assesses the conservation of linear momentum during an interaction where two objects push off each other. In a system where external forces are negligible, such as on level ice, the total linear momentum of the system remains constant and is zero since both skaters start at rest. When Skater A moves west at 2.8 m/s, their momentum is -140 kg·m/s (taking east as positive). To conserve momentum, Skater B must have +140 kg·m/s, resulting in a velocity of 2.0 m/s east. Option A, 2.8 m/s east, might be selected if one assumes equal speeds regardless of mass differences. A transferable strategy is to define a consistent direction for positive momentum and ensure the vector sum remains constant for the isolated system.

8

On frictionless ice, a $2.0,\text{kg}$ cart moving east at $3.0,\text{m/s}$ collides and sticks to a $1.0,\text{kg}$ cart initially at rest. Treat the two carts as the system; external forces are negligible during the collision. What is the final velocity of the stuck-together carts?

$2.0,\text{m/s}$ east

$6.0,\text{m/s}$ east

$1.0,\text{m/s}$ east

$3.0,\text{m/s}$ east

Explanation

This question assesses the conservation of linear momentum in an inelastic collision. In a system where external forces are negligible, such as on frictionless ice, the total linear momentum of the system remains constant before and after the collision. The initial momentum is calculated as the 2.0 kg cart's momentum of 6.0 kg·m/s east, since the 1.0 kg cart is at rest. After the carts stick together, their combined mass is 3.0 kg, so the final velocity is found by dividing the conserved momentum by the total mass, yielding 2.0 m/s east. Option A, 1.0 m/s east, might result from incorrectly averaging the velocities instead of conserving momentum. A transferable strategy is to always calculate the total initial momentum of the system and set it equal to the total final momentum to solve for unknown velocities.

9

A $0.20,\text{kg}$ puck slides east at $4.0,\text{m/s}$ and makes a head-on elastic collision with a $0.30,\text{kg}$ puck sliding west at $2.0,\text{m/s}$. Consider the system of both pucks; external forces are negligible. What is the system’s total momentum after the collision?

$+1.4,\text{kg}\cdot\text{m/s}$

$-1.4,\text{kg}\cdot\text{m/s}$

$0,\text{kg}\cdot\text{m/s}$

$+0.20,\text{kg}\cdot\text{m/s}$

Explanation

This question assesses the conservation of linear momentum in an elastic collision. In a system where external forces are negligible, the total linear momentum of the system remains constant, equal before and after the collision. The initial momentum is 0.20 kg × 4.0 m/s east plus 0.30 kg × (-2.0 m/s) west, totaling +0.20 kg·m/s. Since momentum is conserved regardless of the collision type, the total momentum after is also +0.20 kg·m/s. Option B, 0 kg·m/s, might arise from mistakenly assuming the momenta cancel out completely without calculation. A transferable strategy is to compute the initial total momentum vectorially and recognize it must equal the final total for isolated systems.

10

On a frictionless horizontal track, a $0.60,\text{kg}$ cart moving right at $2.0,\text{m/s}$ collides and sticks to a $0.40,\text{kg}$ cart initially at rest. Treat the two carts as the system; external forces on the system are negligible during the collision. After the collision, the carts move together. What is the speed of the combined carts immediately after the collision?

$3.0,\text{m/s}$

$1.2,\text{m/s}$

$2.0,\text{m/s}$

$0.80,\text{m/s}$

Explanation

This question assesses the skill of applying conservation of linear momentum to an inelastic collision between two carts. The total system momentum remains constant because external forces are negligible during the collision on the frictionless track. The initial momentum is (0.60 kg)(2.0 m/s) + (0.40 kg)(0 m/s) = 1.2 kg·m/s to the right. After colliding and sticking, the final momentum is (1.0 kg) v, so v = 1.2 m/s to the right. A common distractor like 2.0 m/s might come from ignoring the mass of the second cart and using only the initial velocity. A transferable strategy is to define a positive direction, calculate the total initial momentum, set it equal to the total final momentum, and solve for unknowns.

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