Questions to ponder… Why do golfers and bowlers follow through? Why do skydivers bend their knees upon impact? Why does falling on a wood floor hurts less than falling on a cement floor? What’s the best strategy to win the “water balloon toss” or “egg toss”?Why do cars have air bags and padded dashboards? Why is something more likely to break when it is dropped on a carpet than on a hardwood floor?
Impulse and Momentum Each of these situations involves a CHANGE in an object’s momentum! • Most often mass doesn’t change so velocity changes and that is acceleration. •And mass x acceleration = forcenet •Applying a force over a time interval to an object changes the momentum •Force x time interval = Impulse •So, Impulse = F t or • Units: Ns Ft = mv
MOMENTUM To INCREASE MOMENTUM, apply the greatest force possible for as long as possible. •Examples : • pulling a sling shot • drawing an arrow in a bow all the way back • a long cannon for maximum range •hitting a golf ball or a baseball (follow through is important for these !)FORCE TIME
MOMENTUMDecreasing Momentum•Which would it be more safe to hit in a car ? Ft mv•Knowing the physics helps us understand why hitting a softobject is better than hitting a hard one. mv Ft
MOMENTUM• IF the two cars have the same mass, and are going the same speed, are their impulses the same?? • YES! Their momentum changes same amount, so the impulse is the same• Hitting the haystack extends the impact time (the time in which the momentum is brought to zero).• The longer impact time reduces the force of impact and decreases the deceleration.• Whenever it is desired to decrease the force of impact, extend the time of impact !
DECREASING MOMENTUM• If the time of impact is increased by 100 times (say from .01 sec to 1 sec), then the force of impact is reduced by 100 times (say to something survivable). Can you think of any examples of when you would want to decrease momentum?• EXAMPLES• Padded dashboards on cars• Airbags in cars or safety nets in circuses• Moving your hand backward as you catch a fast-moving ball with your bare hand or a boxer moving with a punch.• Flexing your knees when jumping from a higher place to the ground.• Dropping a glass dish onto a carpet instead of a sidewalk.
EXAMPLES OF DECREASING MOMENTUM• Bruiser Bruno on boxing … F = change in t momentum F t = change in momentum• Increased impact time reduces force of impact• Barney Jervais on bungee Jumping … Ft = Δmv applies here. mv = the momentum gained before the cord begins to stretch that we wish to change. Ft = the impulse the cord supplies to reduce the momentum to zero. Because the rubber cord stretches for a long time the average force on the jumper is small.
Example problems1. A 0.50-kg cart (#1) is pulled with a 1.0-N force for 1 second; another 0.50 kg cart (#2) is pulledwith a 2.0 N-force for 0.50 seconds. Which cart (#1 or #2) has the greatest impulse?•Which cart has the same greatest change in momentum?•Greatest acceleration?
Example problem• 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? 3500 N• If Jennifer had not been wearing her seat belt and not had an air bag, then the windshield would have stopped her head in 0.002 s. What average force would the windshield have exerted on her? 875000 N
Example problem: bouncing off a wallA tennis ball is thrown into a wall at a velocity of 5.2 m/s. It bounces off of the wall at a velocity of 4.8m/s. If the wall is in contact with the tennis ball for 0.15 seconds, what is the force that the wall exertson the ball?(hint: are the velocities in the same direction??)
momentumWe have discussed the motion of particles or objects…now we will move on to another way to describe the motion of object(s) What is momentum? How do we use momentum to describe the motion of an object?
Which has more momentum? Can we tell…or do we need more information ??????
Which has more momentum?1. A sedan or truck moving atthe same velocity 2. Two bees of same size 2. The tall guy or the moving at same velocity short guy if they are 3. Two bees of same size traveling at the same moving at different velocities velocity
momentum• Definition: Inertia in motion• It is a vector quantity• The more momentum an object has, the more difficult it is to stop • So more force will need to be applied to stop the object.• We will discuss momentum as a way to describe collisions or explosions between particles.
So What’s Momentum ? Momentum = mass x velocity This can be abbreviated to : p = mv •Units for momentum: kg-m/s•So, could a truck and a rollerskate ever have the same momentum?
Question :• Under what circumstances would a 1 kg roller skate and the 1000 kg truck have the same momentum ?• The roller skate and truck can have the same momentum if the ratio of the speed of the skate to the speed of the truck is the same as the ratio of the mass of the truck to the mass of the skate.• A 1000 kg truck moving at 0.01 m/sec has the same momentum as a 1 kg skate moving at 10 m/sec. Both have a momentum of 10 kg m/sec. ( 1000 x .01 = 1 x 10 = 10 ) 1 kg 10 m/sec 1000 kg .01 m/sec
Sample problem• A 2250 kg pickup truck has a velocity of 25 m/s to the east. What is the momentum of the truck?Meanwhile, a 0.500 g fly is headed to the west right into the windshield of the pickup truck. What speed would the fly need to have in order to have the same momentum as the truck?
Wrap-upHow does momentum relate to what we have learned in physics so far?