Section 1 momentum and collisions Chapter 6 Section 1 momentum and collisions

Momentum and impulse What is momentum? Momentum is a commonly used term in sports. A team that has the momentum is on the move and is going to take some effort to stop. A team that has a lot of momentum is really on the move and is going to be hard to stop. Momentum is a physics term; it refers to the quantity of motion that an object has. A sports team that is on the move has the momentum. If an object is in motion (on the move) then it has momentum.

Momentum Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion. The amount of momentum that an object has is dependent upon two variables: how much stuff is moving and how fast the stuff is moving. Momentum depends upon the variables mass and velocity. In terms of an equation, the momentum of an object is equal to the mass of the object times the velocity of the object. Momentum = mass • velocity In physics, the symbol for the quantity momentum is the lower case "p". Thus, the above equation can be rewritten as p = m • v where m is the mass and v is the velocity. The equation illustrates that momentum is directly proportional to an object's mass and directly proportional to the object's velocity. The units for momentum would be mass units times velocity units. The standard metric unit of momentum is the kg•m/s

Example#1 Determine the momentum of a ... a. 60-kg halfback moving eastward at 9 m/s. b. 1000-kg car moving northward at 20 m/s. c. 40-kg freshman moving southward at 2 m/s. A. p = m*v = 60 kg*9 m/s p = 540 kg•m/s, east   B. p = m*v = 1000 kg*20 m/s p = 20 000 kg•m/s, north C. p = m*v = 40 kg*2 m/s p = 80 kg•m/s, south

Example#2 A 2250 kg pick up truck has a velocity of 25 m/s to the east. What is the momentum of the truck?

Student guided practice Do problems 1 and 2

Impulse The product of average force and the time it is exerted is called the impulse of force. From Newton's second law impulse of force can be extracted and found to be equal to the change in momentum of an object provided the mass is constant:

Impulse-momentum theorem both sides of the above equation are multiplied by the quantity t, a new equation results. This equation represents one of two primary principles to be used in the analysis of collisions during this unit. To truly understand the equation, it is important to understand its meaning in words. In words, it could be said that the force times the time equals the mass times the change in velocity. In physics, the quantity Force • time is known as impulse. And since the quantity m•v is the momentum, the quantity m•Δv must be the change in momentum. The equation really says that the Impulse = Change in momentum   One focus of this unit is to understand the physics of collisions. The physics of collisions are governed by the laws of momentum; and the first law that we discuss in this unit is expressed in the above equation. The equation is known as the impulse-momentum change equation. The law can be expressed this way: In a collision, an object experiences a force for a specific amount of time that results in a change in momentum. The result of the force acting for the given amount of time is that the object's mass either speeds up or slows down (or changes direction). The impulse experienced by the object equals the change in momentum of the object. In equation form, F • t = m • Δ v.

Example#3 Jennifer, who has a mass of 50.0 kg, is riding at 35.0 m/s in her red sports car when she must suddenly slam on the brakes to avoid hitting a deer crossing the road. She strikes the air bag, that brings her body to a stop in 0.500 s. What average force does the seat belt exert on her? Solution = (mass * velocity change)/time F = (50 * 35) / 0.500 F = 3500 N

Example#4 A hockey player applies an average force of 80.0 N to a 0.25 kg hockey puck for a time of 0.10 seconds. Determine the impulse experienced by the hockey puck. Impulse = F*t = 80 N * 0.1 s Impulse = 8 N*s

Example#5 A 1400 kg car moving westward with a velocity of 15m/s collides with a utility pole and is brought to rest in .30 sec. Find the force exerted on the car during the collision

Homework!!! Do problems 1-4 in your book page 193

Closure Today we learned about momentum and impulse. Next class we are going to learn about conservation of momentum