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# Momentum for Edexcel GCSE Physics

1. Momentum
2. Conservation of momentum
3. Elastic and inelastic collisions

All moving objects have momentum. Momentum can be defined as the tendency of an object to keep moving in the same direction and it is a vector quantity. It can be calculated by multiplying mass and velocity, and its unit is kgm/s.

Total momentum is conserved in collisions and explosions unless an external force is exerted on the system. This is called the law of conservation of momentum. The law states that for an isolated system, provided that there are no external forces acting and no energy is provided, momentum will remain constant. For many collision and explosion questions this law results in equating total momentum before and total momentum after.

An explosion is when a rapid change occurs to a stationary system causing objects to move apart. A collision can be elastic or inelastic. Kinetic energy is proportional to the mass of an object and the square of the speed of it. An elastic collision is one in which kinetic energy is conserved, whereas in an inelastic collision, there is no conservation of kinetic energy. Momentum is conserved in both cases as long as a situation meets the conditions for the conservation of momentum.

# 1

Two particles are moving towards each other when they collide. The collision causes both particles to stop. Explain whether the collision is elastic or inelastic.

An elastic collision is defined as one in which no kinetic energy is lost. Before the collision, the two particles are moving, meaning that there is kinetic energy. After the collision, both particles are at rest, so there is no kinetic energy. Kinetic energy is not conserved in the collision, therefore it is an inelastic one.

Inelastic since kinetic energy is not conserved.

# 2

A car is travelling in a straight line at a constant velocity of 11 m/s. Given that the momentum of the car is 15125 kgm/s, find the mass of the car.

p = m ร v
15125 kgm/s = mass ร 11 m/s
mass = 15125 รท 11 = 1375 kg

1375 kg

# 3

Two go-karts of equal mass are travelling towards each other and collide. The first go-kart is travelling towards the right at 4 m/s and the second is travelling towards the left at 2.75 m/s. Given that the first go-kart changes direction after the collision and has a speed of 2 m/s, find the velocity of the second go-kart after the collision.

By the law of conservation of momentum, total momentum before = total momentum after.
mass ร 4 m/s โ mass ร 2.75 m/s = mass ร velocity โ mass ร 2 m/s
Since the masses of the two go-karts are equal, we can cancel mass out of the equation.
velocity = 4 โ 2.75 + 2 = 3.25 m/s to the right

3.25 m/s to the right

# 4

A snooker ball of mass 0.17 kg is travelling towards another stationary snooker ball of mass 0.15 kg at 10 m/s. After the collision, the first snooker ball slows down to 3.5 m/s. Calculate the velocity of the second snooker ball after the collision.

By the law of conservation of momentum, total momentum before = total momentum after.
0.17 kg ร 10 m/s = 0.17 kg ร 3.5 m/s + 0.15 kg ร velocity
velocity = (0.17 kg ร 10 m/s โ 0.17 kg ร 3.5 m/s) รท 0.15 kg = 7.37 m/s (3 s. f.)

7.37 m/s

# 5

A ball is moving towards another stationary ball with a kinetic energy of 4.34 J. The two balls collide, stick and move together. After the collision the first ball has 1.22 J of kinetic energy, while the kinetic energy of the second ball is 1.13 J. Determine if the collision is elastic or inelastic and provide reasoning.

kinetic energy after = 1.22 + 1.13 J = 2.35 J < 4.34 J
An elastic collision is one in which no kinetic energy is lost. Since the kinetic energy before is greater than the kinetic energy after, this collision is inelastic.

Inelastic since kinetic energy is not conserved.

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