Applications of Newton’s Laws
Answers to Even-numbered Conceptual Questions
Spinning the wheels is likely to decrease the force exerted by the Jeep.
The reason is that
the force exerted by the spinning wheels is kinetic friction, and the coefficient of kinetic
friction is generally less than the coefficient of static friction.
The spinning wheels look
better in the movie, however.
The maximum acceleration is determined by the normal force exerted on the drive wheels.
If the engine of the car is in the front, and the drive wheels are in the rear, the normal
force is less than it would be with front-wheel drive.
During braking, however, all four
wheels participate – including the wheels that sit under the engine.
Friction is beneficial whenever you want to start, stop, or turn a car.
It is also beneficial
when you tune a guitar or tie your shoes.
This is possible because if you spin the bucket rapidly enough, the force needed to
produce circular motion is greater than the force of gravity.
In this case, a force in
addition to gravity must act at the top of the circle to keep the water moving in its circular
This force is provided by the bottom of the bucket.
Therefore, the bottom of the
bucket pushes against the water, and the water pushes back against the bucket – this keeps
the water from falling out of the bucket.
At the equator, you are moving in a circular path.
Therefore, part of the force of gravity
acting on you is providing your centripetal acceleration; the rest shows up as a reduced
weight on the scale.
At the poles, the scale reads your full weight.
(In addition, the Earth
bulges at the equator, due to the same effects just discussed.
Therefore, you are farther
from the center of the Earth when you are at the equator, and this too diminishes your
weight, as we shall see in Chapter 12.)
Equilibrium simply means that the net force acting on an object is zero.
an object moving with constant velocity can be considered to be in equilibrium.
frame of reference moving with the same velocity, the object would be at rest and would
have zero net force acting on it – which is the way we usually think of equilibrium.
In principle, the reading on the scale would be greater than 900 N if the Earth were to stop
See the discussion for Question 10 for further details.
Astronauts feel weightless because they are in constant free fall as they orbit, just as you
would feel weightless inside an elevator that drops downward in free fall.
A passenger on this ride feels pushed against the wall, which means that the wall, in turn,
exerts a normal force on the passenger.
If the corresponding force of static friction is
greater than the passenger’s weight, the passenger stays put as the floor is lowered away.