Sana 10.01.2019 Hajmi 6.71 Mb.

• ## Building on the concept of inertia, we ask the question, “How hard is it to stop an object?”

• We call this property of an object that is related to both its mass and its velocity, the momentum of the object.

• ## p = mv

• The bold-faced symbols above, momentum and velocity, are vectors with identical direction.
• Look at the equation. Can you guess why the units are defined this way?

• ## For example, two ways of changing an object’s momentum by 10 kilogram-meters per second are:

• exert a net force of 5 newtons on the object for 2 seconds, or
• exert 100 newtons for 0.1 second.

• ## Because our bones break when forces are large, the particular combination of force and time interval is important.

• Sometimes, as in a gymnasium, a wood floor may be enough of a cushion; in a car, the dashboards are made from foam rubber. Bumpers and air bags further increase the vehicle’s (and the passengers’) Δt.

• ## Sometimes accidents can fill in for the lack of a parachute:

• During World War II, a Royal Air Force airman jumped in just his uniform from a flaming bomber flying at 18,000 ft. His terminal speed (Ch. 3, p. 46) was 120 mph, but tree branches and then 1½ feet of snow cushioned his fall.
• The result: minor scratches and bruises.
• The record for surviving a fall without a parachute is held by Vesna Vulovic. She was serving as a hostess on a Yugoslavian DC-9 that blew up at 33,330 ft in 1972.

• ## Imagine standing on a giant skateboard, at rest.

• The total momentum of you and the skateboard must be zero, because everything is at rest.

• ## The force you exert on the skateboard is, by Newton’s third law, equal and opposite to the force the skateboard exerts on you.

• Because you and the skateboard each experience the same force for the same time interval, you must each experience the same-size impulse and, therefore, the same-size change in momentum.

• ## At one instant of time add up all the momenta of all of the particles in the system.

• Vector addition, of course, but let’s not worry much about that.

• ## The Law of Conservation of Linear Momentum:

• The total linear momentum of a system does not change if there is no net external force.

• ## Although the same effect occurs when we disembark from an ocean liner, the large mass of the ocean liner reduces the speed given it by our stepping off.

• A large mass requires a small change in velocity to undergo the same change in momentum.

• ## If a rifle of mass 4.5 kg fires 10-gram bullets at a speed of 900 m/s (2000 mph), what will be the speed of its recoil?

• The magnitude of the momentum p of the bullet is:
• ## p = mv = (0.01 kg)(900 m/s) = 9 kg·m/s

• Because the total momentum of the bullet and rifle was initially zero, conservation of momentum requires that the rifle recoil with an equal momentum in the opposite direction. If the mass of the rifle is M, the speed V of its recoil is given by

• ## Interacting objects don’t need to be initially at rest for conservation of momentum to be valid. Suppose a ball moving to the left with a certain momentum crashes head-on with an identical ball moving to the right with the same-size momentum.

• Before the collision, the two momenta are equal in size but opposite in direction, and because they are vectors, they add to zero.
• After the collision the balls move apart with equal momenta in opposite directions.

• ## A question on the final exam asks, “What do we mean when we claim that the total momentum is conserved during a collision?” The following two answers are given:

• Answer 1: Total momentum of the system stays the same before and after the collision.
• Answer 2: Total momentum of the system is zero before and after the collision.

• ## Conservation of momentum tells us that the total momentum must be the same before and after the collision.

• Before the collision, one car is moving at 10 meters per second.
• After the collision, five identical cars are moving with a common final speed.
• Because the amount of mass that is moving has increased by a factor of five, the speed must decrease by a factor of five. The cars will have a final speed of 2 meters per second.

• ## From an analysis of the length of the skid marks made after the collision and the type of surface, the total momentum of the two cars just after the collision can be calculated.

• (We will see how to do this in Chapter 7; for now, assume we know their total momentum.)

• ## Let’s use conservation of momentum to analyze such a collision.

• For simplicity assume that each car has a mass of 1000 kg (a metric ton) and that the cars traveled along a straight line.
• Further assume that we have determined that the speed of the two cars locked together was 10 m/s (about 22 mph) after the crash.