Momentum

Defining Momentum

• Momentum is a vector quantity that is the product of an object’s mass and velocity.
• It has both magnitude and direction, meaning it depends on both how fast something is moving and in which direction it’s moving.
• Momentum is measured in kg m/s (kilogram metres per second).

Calculating Momentum

• The formula to calculate momentum is momentum = mass x velocity.
• If two objects collide, their total momentum before collision will be equal to their total momentum after collision, as long as no external forces act upon them.
• This principle is known as the principle of conservation of momentum.

Impulse and Change in Momentum

• Impulse is the change in momentum of an object when a force is applied over time.
• The formula for impulse is impulse = Force x time and it can also be calculated as impulse = change in momentum.
• The units of impulse are also kg m/s, the same as momentum.
• Impulse is a vector quantity, so direction matters when calculating it.

Momentum and Collisions

• In a collision, an object’s momentum can be transferred to another object.
• If no external force is acting, the total momentum before the collision should be equal to the total momentum after the collision. This is known as conservation of momentum.
• In an elastic collision, both momentum and kinetic energy are conserved.
• In an inelastic collision, momentum is conserved but kinetic energy is not.
• An extreme case of an inelastic collision is a perfectly inelastic collision, where two objects stick together and move as one after the collision. Again, momentum is conserved in these collisions.

Momentum and Safety

• The concept of momentum is crucial in understanding safety features such as air bags, seat belts and crumple zones in cars.
• These devices help to increase the time over which a collision occurs, spreading out the change in momentum, and therefore reducing the force felt by passengers. This helps to protect them from injury.