Applying Conservation of Linear Momentum (Edexcel International A Level Physics)

Trolley A of mass 0.80 kg collides head-on with stationary trolley B whilst travelling at

3.0 m s^–1. Trolley B has twice the mass of trolley A. On impact, the trolleys stick together.

Using the conservation of momentum, calculate the common velocity of both trolleys after the collision.

Answer:

Momentum is a vector quantity, therefore, you should always define a direction to be 'positive' when applying the principle of conservation of momentum. In this worked example, we implicitly took velocity 'to the right' as the positive direction.

Sometimes, however, you might encounter two objects moving towards each other before colliding. If both objects have the same mass m and speed v, then the total momentum (before collision) is zero, because p_total = (mv) + (–mv) = 0. Note the negative sign indicates a body travelling in the opposite direction. 

A red snooker ball, travelling at 2.5 m s–1 collides with a green snooker ball, which is at rest. Both snooker balls have the same mass m. 

The angle of collision is such that the red ball moves off at 28° below the horizontal at 1.8 m s^–1 and the green ball moves off at 55° above the horizontal, with a speed v, as shown.

Determine the size of v.

Answer:

Step 1: Write the conservation of linear momentum for horizontal components

• The question is worded in terms of the horizontal direction, so write: 

Horizontal momentum before = horizontal momentum after

Step 2: Resolve the velocity of each ball to find the horizontal component: 

• Since momentum p = mv, then the horizontal component of momentum p_horiz = mv_horiz

  • Therefore, the horizontal component of the red ball is 1.8 cos 28°

  • The horizontal component of the green ball is v cos 55°

Step 3: Substitute quantities into the conservation of momentum

Horizontal momentum before = horizontal momentum after

mu_red + mu_green = mv_horiz(red) + mv_horiz(green)

m (2.5) + 0 = m (1.8 cos 28°) + m (v cos 55°)

Step 4: Simplify and rearrange to calculate v

2.5 = 1.8 cos 28° +  v cos 55°

2.5 = 1.6 + v cos 55°

0.9 = v cos 55°

v = 1.6 m s^–1

Generally speaking, whenever you see any vector given at an angle to the horizontal or the vertical (e.g. velocity, or momentum), think "resolve"! It's extremely likely you will need to consider the separate components of motion for a projectiles question or for a conservation of momentum question. 

Questions which ask you to use the principle of conservation of linear momentum in 2D are usually worth a lot of marks, so make sure you practise lots of questions involving resolving vectors!