Momentum and Impulse 2011

Pre-Quiz If you got punched in the face which would be better? – A: Holding your head as stationary as possible – B: Moving your head forward into the punch – C: Moving your head back with the punch Which has more momentum? – A: a truck at rest – B: a rollerskate moving with velocity of 10m/s – C: a rollerskate at rest – D: a truck moving with velocity of 10m/s About eight years ago, I got into a car accident when a 1990 Toyota Corolla ran a red light and smashed into the side of my 1973 Pontiac Bonneville. The Corolla was crumpled up to its windshield by the impact, but the large steel Bonneville only had a dent about a foot wide on the side of it. Which car is actually, generally, safer in a head on collision? – A: The Corolla, which is designed to crumple in a collision. – B. The Bonneville, made of strong steel that will not crumple in an accident.

What is Momentum? Momentum is Inertia in Motion. Recall that Inertia is the tendency of an object to not want to change its state of motion. What factors might make it harder to change the motion of a moving object? – It has a lot of mass – It is moving very fast

Calculating Momentum Since increasing mass and increasing velocity both increase the momentum of an object, we can see that momentum must be directly proportional to both mass and velocity. What would a formula for momentum look like? Thus, to quantify this resistance to change in the state of an object’s motion, we have defined Momentum as the product of the two factors that increase the object’s resistance: Momentum = mass x velocity Also written: p=mv (for some reason, p is used as the variable for momentum, because physicists like to make things confusing like that.)

How do we change momentum? If an object has a lot of momentum, then it either takes more force or more time to change the objects state of motion. This means that the amount that momentum changes must be directly proportional to the force of impact and the time of impact. What would this formula look like? Δp=Ft or Δ(mv)=Ft or m(Δv)=Ft We call the Ft part of the equation the impulse, so: – Impulse = Ft – Impulse is also defined at the change in momentum.

Relationship between Impulse and Newton’s 2 nd law Newton’s Second Law states what? – F=ma How do we define acceleration? – a=Δv/t How do I get to the impulse formula from here? – Take a minute to see if you can figure out how to get the impulse formula from the above two formulas…

Why did the egg that landed on the cushion survive? Was there a difference in the impulse experienced by each egg? – No, both had the same velocity before hitting the surfaces and they have about the same mass, so the impulses they experienced were nearly identical. Were the forces experience by each egg different? – Yes, why? What other quantity was different for the eggs? Time of impact!!! Because the cushion moves with the egg on impact, the time of impact is increased. This decreases the force of impact.

How does impulse help us minimize injury in a fight? When you get punched in the face, what should you do to minimize the amount of force your face absorbs? – When the fist hits your face, it has a set amount of momentum it can impart to your face, mv. – According to the impulse formula, mv=Ft. – Momentum, mv, is a constant, so if we want to make F as small as possible, do we want t to be bigger or smaller? – How can we accomplish this mid-punch? Moving with the punch will increase the time of impact and decrease the force of impact.

Why is follow-through important? When you hit a baseball or golf ball, what is your goal? – To impart as much momentum to the object you are striking as possible. If you only have a finite amount of strength with which to apply force to the object, how can you further increase the amount of momentum you give the object? – By increasing the amount of contact time with the object. Remember Δ(mv)=Ft. If F is a constant, then you want to increase t to get the maximum Δ(mv). This will result in the object having the greatest possible velocity after you hit it, making it go farther.

Impulse and Car Safety In a collision, you want the car and yourself to slow down as slowly as possible, to minimize the amount of force you experience. What devices or design components of a car help by increasing the stopping time in a collision? – Crumple Zones – Air Bags – Seat Belts – Padding on the dashboard – Cushioned seats and headrests – Collapsible Steering Column – Frontal Protection Systems (to help minimize injury to pedestrians involved in vehicular collisions)

Bouncing and Impulse When a karate master strikes a wooden board, how does he manage to break it? – He lets his hand bounce. This increases the force he imparts. Why? – Let’s say his hand, which has a mass of 0.4kg is travelling 5m/s just before impacting the board. If he doesn’t bounce it, what is the impulse imparted by his hand? – Now, let’s say his hand bounces back after hitting the board, so that after the impact it is travelling 5m/s in the opposite direction. How much impulse is imparted by his hand now? If the time of contact is the same in both of these scenarios, which one will apply more force to the board?

Post Quiz If you got punched in the face which would be better? – A: Holding your head as stationary as possible – B: Moving your head forward into the punch – C: Moving your head back with the punch Which has more momentum? – A: a truck at rest – B: a rollerskate moving with velocity of 10m/s – C: a rollerskate at rest – D: a truck moving with velocity of 10m/s About eight years ago, I got into a car accident when a 1990 Toyota Corolla ran a red light and smashed into the side of my 1973 Pontiac Bonneville. The Corolla was crumpled up to its windshield by the impact, but the large steel Bonneville only had a dent about a foot wide on the side of it. Which car is actually, generally, safer in a head on collision? – A: The Corolla, which is designed to crumple in a collision. – B. The Bonneville, made of strong steel that will not crumple in an accident.