75 Unit 7 Review, Practice, and Assessment

Outcome 1

1) A rubber ball is lifted and dropped. It then bounces back up, but not all the way to the original height. Fill in the blanks in the following statements about a rubber ball using the terms listed.

• chemical potential
• gravitational potential
• elastic potential
• kinetic
• thermal
• positive
• negative
• zero

a) While lifting the ball up at constant speed, ____________energy is being exchanged for ____________ energy. The exchange occurs as you do positive work on the ball and gravity does __________ work. The ball moves at constant speed because the net work is ________.

b) While falling, the force of gravity is doing work and that work is _____________. Air resistance is doing a smaller amount of  ________work. Therefore the net work is __________ and __________energy is increasing. Overall, ____________________energy is being exchanged for _________________ energy.

c) While in contact with the floor and compressing, the normal force from the floor is doing _________ work and gravity is doing_____________ work. The ball is slowing down so net work on the ball must be ___________. Therefore the normal force must be larger than the weight of the ball. Overall, __________________energy and a bit of gravitational potential energy are being exchanged for _______________ energy and ______________energy. (Remembering that the ball does not reach the same height after bouncing).

d) While in contact with the floor and decompressing, the normal force from the floor is doing _________ work and gravity is doing_____________ work. The ball is speeding up so net work must be ___________. Therefore the normal force must be larger than the weight of the ball. Overall, __________________energy is being exchanged for  _______________ energy and ___________energy and a bit of gravitational potential energy.

e) While rising after leaving the floor gravity is the only force doing work and that work is _____________, therefore the net work is __________ and __________energy is decreasing. Overall, ____________________energy is being exchanged for _________________ energy.

Outcome 2

2) In the absence of gravity or friction, a spaceship engine supplies 55,000 N of thrust force to a 1500 kg ship.

a) Use the work-energy principle to determine what distance the rocket will cover before it reaches a speed of 1200 m/s , starting from rest.

3) A car is moving at 55 mph (26 m/s)on flat ground when a hazard is spotted 85 m (278 ft) ahead. (Such as deer, tree-limb, or broke-down car in the road).

a) What distance does the car travel before the brakes are even applied if the driver takes typical 1.5 s braking reaction time? ^[1]

b) Draw a free body diagram of this car, while brakes are applied.

c) What is the normal force on the car?

d) If the car slams on the brakes what is the frictional force? Assume that the car has anti-lock bakes that prevent sliding and the static friction coefficient between tire rubber and dry asphalt is 0.7.

e) Use the work-energy principle to determine the stopping distance.

f) What is the total stopping distance, including distance covered during the reaction time? Does an accident occur?

4) A person “curls” a 5 kg load a distance of 0.1 m. Answer the following questions using the mechanical advantage value for the example bicep/elbow/forearm system  found in the previous unit.

a) What force should the person need to apply?

b) What distance will the person’s biceps muscle need to contract?

c) How much useful much work does the person do?

d) What is the change in gravitational potential energy of the mass?

e) If the body is only 20% mechanically efficient, how much chemical potential energy did the person actually convert?

f) How much thermal energy was generated in the person’s body?

Outcome 3

5) A person uses a pulley system with mechanical advantage of three to lift a 65 kg load a distance of 0.5 m. 

a) What force should the person need to apply?

b) How much gravitational potential energy is gained by the load?

c) How much work should the person need to do?

d) What distance do they need to pull the rope? [Hint: Assuming friction if the pulleys is small enough to ignore, the work input to the pulley system and work output need to be the same, otherwise energy was created within the pulley].

e) If the person actually had to pull that same distance with 650 N due to friction in the pulley system, how much work did they actually input to the system?

f) What is the mechanical efficiency of this pulley system?

g) Remembering that the body is only 20% mechanically efficient, how much chemical potential energy did the person expend?

h) What is the mechanical efficiency of the entire system, including the person?

Outcome 4

i) If the person was able to pull the rope with the 650 N for the required distance in a time of 20 s, what was their mechanical power output?

j) Considering the efficiency of the person, what were their total power output and thermal power output in units of Watts and HP.

k) How does their total power output compare to a 100 W lightbulb? (Give a ratio).