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Basic Physics of Rowing
Transcript of Basic Physics of Rowing
- Use oars with bigger blades
- Don't wash out at the end of the stroke. Components of Mass:
•Boat & coxswain= 20-30%
•Oars <5% If a crew rows 1 minute at 4 m/s, and then 1 minute at 6 m/s, the total distance they cover is 60 x 4 + 60 x 6 = 600 m. The momentum you put into the water will be the equal & opposite to the momentum the boat gets. In the recovery phase of the stroke we see a clear example of conservation of momentum. The oars are not in the water and the crew is sliding forward towards the stern of the boat, yet the boat surges forward. Before Stroke, p=0
After Stroke:p = MbVb - MwVw = 0
(Netwon's 2nd Law- total momentum can't change) For example: If a boat is to accelerate from rest to Vb=1m/s and it weighs 100kg (boat + crew), then either Mw=10kg water accelerated at Vw= 10m/s or Mw= 20kg to Vw= 5m/s or any combo that gives MwVw=100kg m/s. Momentum is a key factor for optimal boat speed. Momentum of the boat implements two of Newton's Laws:
- Newton's Second Law: (F=ma) the total momentum of a system cannot change
-Newton's Third Law, that describes the "action-reaction" principle. Propulsion: to push forward or drive an object forward p=mv For example, something with a large "p" (big mass and high velocity) takes a big force to get it up to its speed and back down. The lighter and slower an object is, the less of "p" is has. Oar gives momentum to the waterboat accelerates in the opposite direction of the oar in order for the oar to conserve momentum. Effective: move large amount of water slowly blades have large surface area to maximize water moved. The sliding of seats in the boat complicates things because the center of mass of rowers is variable; when oars are not in the water, and rowers move toward the stern, the boat surges forward (during recovery) The resistance in rowing is called Drag Bodies moving in fluids slow down b/c of DRAG:
-the forces that move in the opposite direction of fluid velocity.
-a transfer of momentum from the body to the fluid: the surrounding fluid speeds up as the body slows down, so total momentum still remains constant.This means the velocity of the boat determines drag.
-Direct relationship between velocity and drag: As velocity increases, drag increases. 80% of drag in shells is skin drag, while
Air resistance is variable, usually small but can rise to 10% if there is a big head wind. Skin Drag is proportional to the square of the velocity
(V2= square of the velocity) & (a=constant)
P required (=force x velocity) is P=av^3
This means that to double the boat speed, you need to supply 2^3 = 8 times more power.
If you double the power, you only go 1.26 (=21/3) times as fast. This is why rowing firm pressure doesn't get you caught up to a boat rowing lightly that quickly Kinetic Energy Energy that a body possesses by virtue of being in motion. 10 Kg: 10m/s
20Kg 5m/s This kinetic energy represents mechanical work performed by the rower, but in the first case they have to do a lot more work (2x) to achieve similar speed -The center of mass: mass weight average of positions of center of mass of each component. Individual component center of mass can move relative to each other. Center of mass of whole system cannot change momentum or velocity unless external forces are applied (via water) When crew comes toward stern at a negative velocity of the crew relative to the velocity of the boat, the boat must move toward the bow at a different relative velocity to conserve momentum (because it’s a closed system) If Crew= 80% of total mass
Mc= (4/5)(Mc + Mb) Mc= 4Mb so Vb= 4Vc
Say Vc= .2 m/s (towards stern), then Vb= 4(.08)= .8m/s (towards bow)
Bow appears to surge. This is what it looks like—when the crew moves 1m toward the stern, the boat moves 80 cm toward the crew Say they row 2 minutes at a constant speed of 5 m/s. Distance will be the same, however the amount of energy it took is different... The average power is reduced, so less energy is used to cover the same distance in the same time --> energy-efficient to keep the same pace rather than start fast and get slower OR vice-versa Since skin drag resistance depends on the hull speed. If the hull spends half of each stroke at 4m/s and half at 6m/s it is less efficient that keeping the speed constant at 5 m/s. It's undesirable to have too much variation in hull speed the boat bobs up and down excessively). We love you Chica... The Basic Physics of Rowing The gearing of a lever: the ratio of the distance moved by the load to the distance moved by the effort, which is also the same as the ratio of the distances of the Load and the Effort from the Fulcrum. This ratio determines how light or heavy the water feels. Which, according to a rower's strength & ability will produce varying results Levers allow you to transmit more force ...Because you row too!!! center of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero. Scientists are constantly searching for the most aerodynamic shell that uses the least amount of mass; however, there is a boat limit that must be made in order to race at high-level competitions. http://www.concept2.com/oars/oar-options/length *five, count them five, of your physics students