physics chapter 5 conceptual questions

the drivers of two identical cars heading toward each other apple the brakes at the same instant. The skid marks of one of the cars is twice as long as the skid marks of the other vehicle. Assuming that the brakes of both cars apply the same force, what conclusions can you draw about the motion of the cars?

The car which skid twice the distance had twice the kinetic energy before the brakes were applied.

a car traveling at 50.0 km/h skids a distance of 35 m after its brakes lock. estimate how far it will skid if its brakes lock when its initial speed is 100.0 km/h. what happens to the cars kinetic energy as it comes to rest?

0-100.0^2-2X250000/7Xd d=140 m There is force of friction between the brakes and the rim of the wheel. As the car slows down its energy is dissipated as heat and thus both the brakes and rim of the wheel heat up. Thus, the kinetic energy of the car is converted into heat during this process.

two identical objects move with speeds of 5.0 m/s ad 25.0 m/s. what is the ratio of their kinetic energy?

25 is KE2 over KE1

a person drops a ball from the top of a building while another person on the ground observes the balls motion. Each observer chooses his or her own location as the level for zero potential energy. Will they calculate the same values for: A the potential energy associated with the ball? B the change in potential energy associated with the ball? C the balls kinetic energy?

A No, they will calculate different values for the potential energy associated with the ball. The reason is that the value of gravitational potential energy depends upon the choice of the zero level B Yes, they will calculate the same values for the change in potential energy associated with the ball. C Yes, they will calculate the same values for the ball's kinetic energy.

each of the following objects possesses energy. which forms of energy are mechanical, which are nonmechanical, and which are a combination? A glowing embers enter in a campfire B a strong wind C a swinging pendulum D a person sitting on a mattress E a rocket being launched into space

A nonmechanical B kinetic or mechanical C potential and kinetic energy D potential energy or mechanical E kinetic and potential

discuss whether any work is being done by each of the following and if so whether the work is positive or negative A. a chicken scratching the ground B. a person reading a sign C. a crane lifting a bucket of concrete D. the for of gravity on the bucket in c

A. The chicken scratching the ground does positive work on the dirt. B. A person reading a sign does no work on the sign. C. The crane does positive work on the concrete. D. Gravity does negative work on the bucket of concrete - the crane must overcome the force of gravity to lift the concrete.

discuss the energy transformation that occurs during the pole vault event shown in the photograph below. Disregard rotational motion and air resistance

At the first stage, during the approach, the pole vaulter gains kinetic energy. When the stick makes contact with ground, it starts bending and the kinetic energy of the person is converted into elastic potential energy into the pole In the next stage the pole starts to become straight and the person gains height. In this process the elastic potential energy of the pole is converted into the gravitational potential energy. At the maximum height all the stick's elastic energy is used up to increase the person's gravitational potential energy. Finally, after the athlete jumps over the pole, he starts falling and gains speed. During this time his gravitational potential energy is converted into kinetic energy.

can the kinetic energy of an object be negative?

No, kinetic energy of an object cannot be negative.

can the speed of an object change if the net work done on it is zero

No. The speed cannot change unless some work is done on an object, even if that work is friction or air resistance.

furniture movers wish to load a truck using a ramp from the ground into the rear of the truck. One of the movers claims that less work would be required if the ramps length were increased. Is this claim valid? explain

The claim is not valid - the same amount of work will be required. The force required will be lower, but that force must be applied over a longer distance.

discuss the work done and change in mechanical energy as an athlete does the following A lifts a weight B holds the weight up in a fixed position C lowers the weight slowly

When the athlete lifts a weights, he/she applies force against gravity to pull the weight. Since the motion of the weight is parallel to the direction of applied force the athlete does positive work on the weight. the potential energy of weight increases. if the mass of the weight is m and the height lifted is h then the increase in mechanical is mgh Since the work done on the weight is zero, the change in mechanical energy of the weight is zero. work done by the athlete is negative. change in mechanical energy is -mgh

when a punter kicks a football, is he doing work in the ball while his tow is in contact with it? is he doing work on the ball after the ball loses contact with his tow? are any forces doing work on the ball while the ball in in flight?

While his toe is in contact with the football, he is increasing its speed and hence delivering kinetic energy to the football by applying force. Since, the direction of force and the motion of the ball is in the same direction, he is doing work on the ball. After the ball loses contact, the punter is no longer applying force on it and so he is not doing work anymore. Yes, there is always frictional force because of its collision with the air molecules. Since the frictional force acts opposite to the direction of motion, it does negative work on the ball.

explain why more energy is needed to walk up stairs than to walk horizontally at the same speed.

While walking horizontally, you need to only overcome force of friction, since your potential energy doesn't change. Overcoming the frictional force requires much less energy. On the other hand, while walking up stairs you are going against gravity so that your potential energy is increasing. Your body has to work harder to provide this extra potential energy. This is why more energy is needed to walk up stairs than to walk horizontally at the same speed.

can the gravitational potential energy associated with on object be negative? explain

Yes, gravitational potential energy associated with an object can be negative. The value of gravitational potential energy depends on the reference for the zero level. suppose you consider the level of water in the pond to be the zero level for gravitational potential energy. Now, if you throw a stone into it, as it falls through the water, its potential energy becomes negative. In practice, the free space is considered to be the zero level for gravitational potential energy, so that any object on the surface of the earth has a negative potential energy

a satellite is in a circular orbit above earths surface. Why is the work done on the satellite by the gravitational force zero? what does the work-kinetic energy theorem predict about the satellites speed

earth applies a force on the satellite towards its center but the motion of the satellite is always tangential to the force of gravity. thus, the angle between the force and displacement of the satellite is 90 degrees. Thus the work done on the satellite is W=Fdcos90= 0 The work-kinetic energy theorem states that the change in kinetic energy of a system is equal to the total work done the system. Since the work done on the satellite is zero, the kinetic energy of the satellite cannot change. Thus the speed of the satellite will remain constant.

a weight is connected to a spring that is suspended vertically from the ceiling. if the weight is displaced downward from its equilibrium position and released, it will oscillate up and down. how many forms of potential energy are involved? if air resistance and friction are disregarded will the total mechanical energy be conserved? explain

elastic and gravitational energy. yes. if a weight, connected to a spring that is suspended vertically from the ceiling is displaced downward from its equilibrium position, elastic potential energy will be stored in the spring due to its elongation. when released the weight will oscillate up and down and thus its height will change in an oscillatory fashion. this process will involve change in gravitational potential energy. this there are two forms of potential energy involved elastic and gravitational if we neglect air resistance and friction the total mechanical energy will be conserved. at the stretched condition, the spring will have some elastic potential energy. when released the spring will pull the weight and this elastic energy will convert to the weights kinetic energy. as the weight goes further up its kinetic energy will be again converted to potential energy. some part of that energy will be stored inn the spring and the rest part will be stores in the weight as gravitational potential energy. after reaching the maximum height the reverse conversion will take place since there is no dissipation of energy the total mechanical energy will be conserved

a strong cord suspends a bowling ball from the center of a lecture balls ceiling, forming a pendulum. The ball is puled to the tip of a lecturers nose at the front of the room and is then released. If the lecturer remains stationary, explain why the lectuer is not struck by the ball in its return swing. Would this person be safe if the ball were given a slight push from its starting position at the persons nose?

suppose when left along the equilibrium point of the ball is point A and the ball is pulled up to the point B. If we set the zero level for gravitational potential energy to be the level of A then at B the ball will have some potential energy. when the ball is released it will move towards A and its potential energy will convert to kinetic energy. after crossing the point A it will move to the other side to point C which is exactly at the same height a that of point bB. this is because all the kinetic energy at point A has now been converted to potential energy and is no more energy to more further. the same will happen in the return path and this the ball cant go beyond the point B. This is why the lecturer is not struck by the ball on its return swing. practically there will be some loss of energy of the ball due to friction and thus the ball will come to a smaller height and thus cant even reach point B on its return swing. if the ball is given a slight push at point B then it will have both the kinetic and potential energy to start with so that its total energy will be higher then just the potential energy. so the ball will move to a height higher than C where its potential energy will be same as the total initial energy. clearly on the return swing it will reach a point somewhere above B and the lecturer will be hurt. however the ball might not reach b of the loss due to friction is quite large

how can the work-kinetic energy theorem explain why the force of sliding friction reduced the kinetic energy of a particle?

suppose, the initial kinetic energy of a body is KEi and the final kinetic energy is KEf. if the net work done on the body is Wnet then the work kinetic energy theorem states KEi+Wnet=KEf now the force of friction always acts on a direction opposite to the motion of a body. thus, the work done by the force of friction is always negative. so KEf-KEi=Wnet<0 or KEf<KEi this implies that the final kinetic energy is less than the initial kinetic energy

advertisements for a toy ball once states that it would rebound to a height greater than the height from which it was dropped. is this possible?

this is impossible lets assume that the ball is dropped from a height h. if the mass of the ball is m, then the gravitational potential energy stored in it will be mgh. when the ball is dropped its potential energy will gradually decrease and the kinetic energy will increase. at the moment the ball touches the ground all of its potential energy will be converted to kinetic energy. after that the reverse will happen. as the ball bounces back its kinetic energy will decrease and the potential energy will increase. at the height h the ball will gain its initial potential energy. now it has no extra energy to go further up

a ball is thrown straight up. at what position is its kinetic energy at its maximum? at what position is gravitational potential energy at its maximum?

when a ball is thrown straight up, its speed is gradually decreasing from its initial value and finally it comes to a halt. then the ball starts falling down. if we set the zero level for gravitational potential energy to be the point from where the ball is thrown, then initially it has only kinetic energy. as the ball goes up its potential energy increases and kinetic energy decreases. thus the ball has maximum kinetic energy at the position from where it is thrown At the point of maximum height all the kinetic energy of the ball converts into potential energy and the ball stops momentarily. Thus it has maximum gravitational potential energy at the position where it comes to a halt. After reaching the maximum height, the ball starts falling and again gains kinetic energy by loosing potential energy. If we ignore the effect of air friction then it will gain the same kinetic energy as it had in the beginning. Thus at the same position from where it was thrown, it will have maximum kinetic energy.

a pendulum swings back and forth. Does the tension force in the string do work on the pendulum bob? Does the force of gravity do work on the bob? explain

work is done only when the force has a component along the direction of displacement. the tension force is always perpendicular to the displacement of the bob. Because the tension force acts along the rope, and the rope i the radius of the circle on which the bob is moving. And we know that the radius is perpendicular to the tangent. Therefore, tension has no component of force along the displacement of the bob. Hence the work done by tension on the bob is zero