Motion

Based on the reading… What is momentum? What affects momentum? What are the units for momentum?

Momentum Momentum is the mass of a object times its velocity. The units for momentum are kilogrammeters per second (kg·m/s).

Momentum Stopping a fast-moving object is harder than stopping a slow-moving one. Is this always true? When is it not true?

Momentum depends on what two things?

How do we get the units for momentum? Units for Mass: — kg Units for velocity: — m/s When we multiply (mass * velocity) we also multiply the units and we get: — kg  m/s

Momentum Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other. The effect of the force is not always the same.

Calculating momentum Momentum is calculated with velocity instead of speed because the direction of momentum is always important.

Inertia vs. Momentum Inertia: — based on mass — has inertia whether moving or not Momentum: — Based on mass and velocity — must be moving to have momentum Before you ask, yes, you need to write this slide down.

Impulse The change in momentum is often referred to as impulse. We can use this relationship to solve equations using changes in mass or velocity.

Impulse The tennis ball’s momentum changes after it is hit by the racquet with a force of 80 N for 0.1 seconds. Both the change of momentum and the impulse are +8 N·s.

Momentum and the third law If we combine Newton’s third law with the relationship between force and momentum, the result is a powerful new tool for understanding motion. If you stand on a skateboard and throw a 5 kg ball, with a velocity of 4 m/s, you apply a force to the ball. That force changes the momentum of the ball. If the ball gains +20 kg·m/s of forward momentum, you must gain –20 kg·m/s of backward momentum assuming there is no friction.

Momentum The result of throwing a 5- kg ball at a speed of 4 m/sec is that person and the skateboard with a total mass of 40 kg move backward at a speed of m/sec (if you ignore friction). We use positive and negative numbers to show opposite directions.

Law of momentum conservation The law says the total momentum in a system of interacting objects cannot change as long as all forces act only between the objects in the system.

Unit 1: Motion Chapter 4: Conservation Laws 4.1 Newton’s Third Law and Momentum 4.2 Energy and the Conservation of Energy 4.3 Collisions

Collisions There are two main types of collisions: elastic inelastic. When an elastic collision occurs, objects bounce off each other with no loss in the total kinetic energy of the system.

Kinetic Energy in Elastic Collision KE before = KE after

Collisions In an inelastic collision objects change shape or stick together, total kinetic energy of the system decreases. An egg hitting the floor is one example of an inelastic collision.

Collisions When two billiard balls collide, it looks like they bounce without a loss of kinetic energy. But the sound of the collision tells you a small amount of kinetic energy is being changed into sound energy.

Elastic or Inelastic? Egg fell to the floor and broke.

Elastic or Inelastic? Dribbling a basketball

Elastic or inelastic? A tennis ball being hit by a racket

Elastic or Inelastic? Being tackled in football and the tackler is still running (with you) after the collision.

Elastic or Inelastic? Dropping your phone. #crackedscreen

Elastic or Inelastic? Bowling ball hitting the pins

Elastic or Inelastic? Drumstick hitting a drum

Momentum As long as there are no outside forces (such as friction), momentum is conserved in both elastic and inelastic collisions. Conservation of momentum makes it possible to determine the motion of objects before or after colliding.

Forces in collisions Collisions create forces because the colliding objects’ motion changes. Since most collisions take place quickly, the forces change rapidly and are hard to measure directly. The total change in momentum is equal to the force multiplied by the time during which the force acts.

Car crash safety Air bags work together with seat belts to make cars safer. An air bag inflates when the force applied to the front of a car reaches a specific level. Automakers use crash test dummies to study the effects of collisions on passengers. Crash test dummies contain electronic sensors to measure the forces exerted at various places on the body.

Forensic Engineering We usually think of engineering as a science focused on designing and constructing things—like bridges, computers, automobiles, or sneakers. However, there is one branch of engineering that focuses on how things fail, collapse, or crash.