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Unformatted text preview: ConcepTest ConcepTest 6.1 Kinetic Energy
Car #1 has twice the mass of car #2, but they both have the same kinetic energy. How do their speeds compare? 1) 2 v1 = v2 2) √ 2 v1 = v2 3) 4 v1 = v2 4) v1 = v2 5) v1 = √ 2 v2 ConcepTest ConcepTest 6.1 Kinetic Energy
Car #1 has twice the mass of car #2, but they both have the same kinetic energy. How do their speeds compare? 1) 2 v1 = v2 2) √ 2 v1 = v2 3) 4 v1 = v2 4) v1 = v2 5) v1 = √ 2 v2 Since the kinetic energy is 1/2 mv2, and the mass of car #1 is mv greater, then car #2 must be moving faster. If the ratio of m1/m2 is 2, then the ratio of v2 values must also be 2 This means that 2. the ratio of v2/v1 must be the square root of 2 2. ConcepTest ConcepTest 6.2 Free Fall
Two stones one twice the Two stones, one twice the mass mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? compared to the light one? 1) quarter as much 2) half as much 3) the same 3) the same 4) twice as much 5) four times as much ConcepTest ConcepTest 6.2 Free Fall
Two stones one twice the Two stones, one twice the mass mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? compared to the light one? 1) quarter as much 2) half as much 3) the same 3) the same 4) twice as much 5) four times as much Consider the work done by gravity to make the stone fall distance d: ΔKE = Wnet = F d cosθ ΔKE = mg d Thus, the stone with the greater mass has the greater KE which is twice KE, which is twice as big for the heavy stone. big for the heavy stone FollowFollowup: How do the initial values of gravitational PE compare? ConcepTest ConcepTest 6.3 Slowing Down
If car traveling 60 km/hr can If a car traveling 60 km/hr can brake brake to a stop within 20 m, what m, is its stopping distance if it is traveling 120 km/hr? Assume km/hr? that the braking force is the same in both cases same in both cases. 1) 20 m 2) 30 m 3) 40 3) 40 m 4) 60 m 5) 80 m ConcepTest ConcepTest 6.3 Slowing Down
If car traveling 60 km/hr can If a car traveling 60 km/hr can brake brake to a stop within 20 m, what m, is its stopping distance if it is traveling 120 km/hr? Assume km/hr? that the braking force is the same in both cases same in both cases. 1) 20 m 2) 30 m 3) 40 3) 40 m 4) 60 m 5) 80 m F d = Wnet = ΔKE = 0 – 1/2 mv2 thus: F F d = 1/2 mv2 Therefore, if the speed doubles doubles, the stopping distance gets four times larger. times larger ConcepTest ConcepTest 6.4 Speeding Up
A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph Which takes more accelerates to 60 mph. Which takes more energy, the 0→30 mph, or the 30→60 mph? 1) 0 → 30 mph 2) 30 → 60 mph 30 3) both the same ConcepTest ConcepTest 6.4 Speeding Up
A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph Which takes more accelerates to 60 mph. Which takes more energy, the 0→30 mph, or the 30→60 mph? 1) 0 → 30 mph 2) 30 → 60 mph 30 3) both the same The change in KE (1/2 mv2 ) involves the velocity squared. velocity So in the first case, we have: 1/2 m (302  02) = 1/2 m (900) 1/2 In the second case, we have: 1/2 m (602  302) = 1/2 m (2700) 1/2 Thus, the bigger energy change occurs in the second case case. FollowFollowup: How much energy is required to stop the 60mph car? 60 ConcepTest ConcepTest 6.5 Work and Energy I
Two blocks of mass m1 and m2 (m1 > m2) blocks of mass slide on a frictionless floor and have the same kinetic energy same kinetic energy when they hit a long they hit long rough stretch (μ > 0), which slows them down to stop Which one goes farther? down to a stop. Which one goes farther? 1) m1 2) m2 3) they will go the same distance m1 m2 ConcepTest ConcepTest 6.5 Work and Energy I
Two blocks of mass m1 and m2 (m1 > m2) blocks of mass slide on a frictionless floor and have the same kinetic energy same kinetic energy when they hit a long they hit long rough stretch (μ > 0), which slows them down to stop Which one goes farther? down to a stop. Which one goes farther? 1) m1 2) m2 3) they will go the same distance With the same ΔKE both blocks KE, must have the same work done to them by friction. The friction force is less for m2 so stopping di distance must be greater er. m1 m2 FollowFollowup: Which block has the greater magnitude of acceleration? ...
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 Fall '09
 ORDONEZ
 Energy, Kinetic Energy, Mass

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