science help...energy!!?

Question

1. Why does an automobile moving at constant velocity have to keep burning fuel to maintain motion?


2. The potential energy acquired in driving to the top of a mountain is the same whether the road goes straight up or zigzags in gradual slopes. Why then do most mountain roads wind up gradually rather than rising straight up with a steep slope?

3. A light truck and a heavy truck are traveling the same speed. It is easier for the light truck to stop. Explain why.


4. A light truck and a heavy truck are moving with the same momentum. Which requires the most work to stop? Explain.


5. When a rifle with a long barrel is fired, the force of expanding gases acts on the bullet for a longer distance. What effect does this have on the velocity of the emerging bullet?

Answer 1

1. To overcome the forces of rolling friction and air resistance.

2. A steep slope would require both high power to climb and a large coefficient of static friction.

3. Not necessarily true in the world of theoretical physics. But in reality the friction force provided by the brakes is a smaller percentage of the weight of the truck than for the car.

4. The heavier truck has more kinetic energy, thus requires more work to change its kinetic energy to zero.

5. The force acts for a longer distance; more work is done and more kinetic energy (that is, velocity) given to the bullet.

Answer 2

1. There are forces at work that require adding energy to the car to keep the speed constant. SOME of the things that are slowing down the car are wind resistance, friction at the tires (if you don't have friction, then the tires won't push the car along) heat loss, sound loss, and so on. Bearings aren't frictionless, some gasoline goes out the exhaust pipe unburned. You've got to keep your foot on the gas to maintain your speed.

2. The energy is the same, but it takes less power if you spend the energy more slowly. For example, is it easier to RUN up a stair case, or WALK? It takes the same energy, but less power. By making the mountain road wind, cars with small engines can still make it to the top, rather than needing a big V8. If they went straight up, cars and trucks could bog down to a stop if their engines were under-powered.

3. The light truck has less kinetic energy than the heavy truck. (KE = .5 x M x V x V) Since the two trucks have the same (V) velocity, the energy is dependent only upon how much they weigh (M.)

4. Momentum is M x V (mass times velocity.) Since they have different masses, they MUST have different velocities so that they have the same momentum. Using the Kinetic Energy forumula in 3, above, the velocity component is SQUARED, but the mass component is not. To maintain equal momenta, the light truck must be traveling much faster than the heavy truck. It will have much more kinetic energy than the heavy truck. (The heavy truck, being heavier than the light truck, will have less velocity if it is to have the same momentum.)

5. Work is force x distance. A long barrel does more work on the bullet than a short barrel. In this case, the work done on the bullet will be the same as the kinetic energy of the bullet. Referring back to the KE forumula in 3, the bullet must travel more slowly with a short barrel since the mass remains the same. (This assumes no frictional losses in the barrel. You can't make a barrel 10 miles long and expect the bullet to come out at the speed of light. In reality, there is friction between the barrel and the bulllet, wind resistance, etc.)

Answer 3

1) because the other forces, friction, air resistance will begin to slow it if the forward velocity is removed

2) It is easier for the car to drive up lots of less steep slopes rather then 1 big slope that it would struggle up in 1st gear and have huge amounts of forces against it due to gravity, so theres less force on the less steep slope making it easier.

3) The heavy truck has a larger momentum P=mv and as its mass is larger this increases P the momentum therefore it takes longer for it to stop.

4)The Heavy truck takes longer as it can be found it has more kinetic energy by using KE = 1/2mv^2

5) It makes the velocity of the bullet much faster,its just like keeping your foot down longer in a car

Answer 4

1. Resistance from wind, rollng friction

2. Most cars and trucks don't have the power to go up such a steep slope. Going down a very steep slope is dangerous.

3. Light truch has less momentum, therefore it takes less force to stop it.

4. Same

5. It is higher because the bullet is accelerating from the force for a longer time.

Answer 5

1. Friction, from both the air and the road.
2. Probably because the slope is to steep.
3. Momentum.
4. Neither. If they have the same momentum then, by definition, they require the same energy to stop.
5. It is higher.

Answer 6

1. wind resistence and friction

2. not sure this state ments true but. It caused for grdual inclination plus its much easier to pave, try parking on 60 degree slope.

3. yes it requires less energy to stop because theirs less weight to push agaisnt to slow

4. sounds like a repeat

5. it would move faster

Answer 7

1 to combat friction
2 safer
3 less mass to affect inertia
4 the same
5 ?