Physics

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How much work does a supermarket checkout attendant do on a can of soup he pushes 0.310 m horizontally with a force of
5.10 N? Express your answer in joules and kilocalories.
Additional Materials
Reading
J
kcal
Suppose a car travels 108 km at a speed of 35.0 m/s, and uses 2.10 gallons of gasoline. Only 30% of the gasoline goes into
useful work by the force that keeps the car moving at constant speed despite friction. (The energy content of gasoline is 1.3 ✕✕ 108
J per gallon.)
(a) What is the force exerted to keep the car moving at constant speed?
N
(b) If the required force is directly proportional to speed, how many gallons will be used to drive 108 km at a speed of
28.0 m/s?
gallons
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Reading
(a) Calculate the force needed to bring a 1100 kg car to rest from a speed of 90.0 km/h in a distance of 130 m (a fairly typical
distance for a nonpanic stop).
N
(b) Suppose instead the car hits a concrete abutment at full speed and is brought to a stop in 2.00 m. Calculate the force exerted
on the car and compare it with the force found in part (a), i.e. find the ratio of the force in part(b) to the force in part(a).
(force in part (b) / force in part (a))
Additional Materials
Reading
A car’s bumper is designed to withstand a 5.04-km/h (1.4-m/s) collision with an immovable object without damage to the body of
the car. The bumper cushions the shock by absorbing the force over a distance. Calculate the magnitude of the average force on a
bumper that collapses 0.255 m while bringing a 820-kg car to rest from an initial speed of 1.4 m/s.
N
Additional Materials
Reading
(a) How much gravitational potential energy (relative to the ground on which it is built) is stored in an Egyptian pyramid, given its
mass is about 6 ✕✕ 109 kg and its center of mass is 29.0 m above the surrounding ground?
J
(b) What is the ratio of this energy to the daily food intake of a person (1.2 ✕✕ 107 J)?
:1
Additional Materials
Reading
In a downhill ski race surprisingly little advantage is gained by getting a running start. This is because the initial kinetic energy is
small compared with the gain in gravitational potential energy even on small hills. To demonstrate this, find the final speed and
the time taken for a skier who skies 60.0 m along a 35° slope neglecting friction for the following two cases. (Note that this time
difference can be very significant in competitive events so it is still worthwhile to get a running start.)
(a) starting from rest
final speed
m/s
time taken
s
(b) starting with an initial speed of 2.50 m/s
final speed
m/s
time taken
s