Newton(theory questions) Flashcards
What properties do forces have that allow us to classify them as vectors?
Forces are directional and have magnitude.
A woman was transporting an open box of cupcakes to a school party. The car in front of her
stopped suddenly; she applied her brakes immediately. She was wearing her seat belt and
suffered no physical harm (just a great deal of embarrassment), but the cupcakes flew into the
dashboard and became “smushcakes.” Explain what happened.
The cupcake velocity before the braking action was the same as that of the car. Therefore, the
cupcakes were unrestricted bodies in motion, and when the car suddenly stopped, the cupcakes
kept moving forward according to Newton’s first law.
How much does a 70-kg astronaut weight in space, far from any celestial body? What is her
mass at this location?
The astronaut is truly weightless in the location described, because there is no large body (planet
or star) nearby to exert a gravitational force. Her mass is 70 kg regardless of where she is
located.
A rock is thrown straight up. At the top of the trajectory, the velocity is momentarily zero.
Does this imply that the force acting on the object is zero? Explain your answer.
No. If the force were zero at this point, then there would be nothing to change the object’s
momentary zero velocity. Since we do not observe the object hanging motionless in the air, the
force could not be zero.
When you stand on Earth, your feet push against it with a force equal to your weight. Why
doesn’t Earth accelerate away from you?
The force you exert (a contact force equal in magnitude to your weight) is small. Earth is
extremely massive by comparison. Thus, the acceleration of Earth would be incredibly small. To
see this, use Newton’s second law to calculate the acceleration you would cause if your weight is
600.0 N and the mass of Earth is
6.00 x 10^24 kg
Identify the action and reaction forces in the following situations: (a) Earth attracts the Moon,
(b) a boy kicks a football, (c) a rocket accelerates upward, (d) a car accelerates forward, (e) a
high jumper leaps, and (f) a bullet is shot from a gun.
a. action: Earth pulls on the Moon, reaction: Moon pulls on Earth; b. action: foot applies force to
ball, reaction: ball applies force to foot; c. action: rocket pushes on gas, reaction: gas pushes back
on rocket; d. action: car tires push backward on road, reaction: road pushes forward on tires; e.
action: jumper pushes down on ground, reaction: ground pushes up on jumper; f. action: gun
pushes forward on bullet, reaction: bullet pushes backward on gun.
(a) Why does an ordinary rifle recoil (kick backward) when fired? (b) The barrel of a
recoilless rifle is open at both ends. Describe how Newton’s third law applies when one is fired.
(c) Can you safely stand close behind one when it is fired?
a. The rifle (the shell supported by the rifle) exerts a force to expel the bullet; the reaction to this
force is the force that the bullet exerts on the rifle (shell) in opposite direction. b. In a recoilless
rifle, the shell is not secured in the rifle; hence, as the bullet is pushed to move forward, the shell
is pushed to eject from the opposite end of the barrel. c. It is not safe to stand behind a recoilless
rifle.
A particle is moving to the right. (a) Can the force on it to be acting to the left? If yes, what
would happen? (b) Can that force be acting downward? If yes, why?
a. Yes, the force can be acting to the left; the particle would experience acceleration opposite to
the motion and lose speed. B. Yes, the force can be acting downward because its weight acts
downward even as it moves to the right.
If a book is located on a table, how many forces should be shown in a free-body diagram of
the book? Describe them
two forces of different types: weight acting downward and normal force acting upward
If the sprinter from the previous problem accelerates at that rate for 20.00 m and then
maintains that velocity for the remainder of a 100.00-m dash, what will her time be for the race?
9.259 s
What is the acceleration opposite to the motion of the rocket sled if it comes to rest in 1.10 s
from a speed of 1000.0 km/h? (Such acceleration opposite to the motion caused one test subject
to black out and have temporary blindness.)
253 m/s^2
A particle of mass 2.0 kg is acted on by a single force F = 18i N (a) What is the particle’s
acceleration? (b) If the particle starts at rest, how far does it travel in the first 5.0 s?
110 m
A body of mass 2.00 kg is pushed straight upward by a 25.0 N vertical force. What is its acceleration?
2.70 m/s^2
A body with a mass of 10.0 kg is assumed to be in Earth’s gravitational field with
g = 9.80 m/s . What is the net force on the body if there are no other external forces acting on
the object?
98 N
A baseball catcher is performing a stunt for a television commercial. He will catch a baseball
(mass 145 g) dropped from a height of 60.0 m above his mitt. His mitt stops the ball in 0.0100 s.
What is the force exerted by his mitt on the ball?
499 N
VRAE: https://stemlearn.sun.ac.za/pluginfile.php/143020/mod_resource/content/1/UniversityPhysicsVolume1-Ch05.pdf
To simulate the apparent weightlessness of space orbit, astronauts are trained in the hold of a
cargo aircraft that is accelerating downward at g. Why do they appear to be weightless, as
measured by standing on a bathroom scale, in this accelerated frame of reference? Is there any
difference between their apparent weightlessness in orbit and in the aircraft?
The scale is in free fall along with the astronauts, so the reading on the scale would be 0. There is
no difference in the apparent weightlessness; in the aircraft and in orbit, free fall is occurring.
When you learn to drive, you discover that you need to let up slightly on the brake pedal as
you come to a stop or the car will stop with a jerk. Explain this in terms of the relationship
between static and kinetic friction.
If you do not let up on the brake pedal, the car’s wheels will lock so that they are not rolling;
sliding friction is now involved and the sudden change (due to the larger force of static friction)
causes the jerk.
A physics major is cooking breakfast when she notices that the frictional force between her
steel spatula and Teflon frying pan is only 0.200 N. Knowing the coefficient of kinetic friction
between the two materials, she quickly calculates the normal force. What is it?
5.00 N
Define centripetal force. Can any type of force (for example, tension, gravitational force,
friction, and so on) be a centripetal force? Can any combination of forces be a centripetal force?
Centripetal force is defined as any net force causing uniform circular motion. The centripetal
force is not a new kind of force. The label “centripetal” refers to any force that keeps something
turning in a circle. That force could be tension, gravity, friction, electrical attraction, the normal
force, or any other force. Any combination of these could be the source of centripetal force, for
example, the centripetal force at the top of the path of a tetherball swung through a vertical circle
is the result of both tension and gravity.
Race car drivers routinely cut corners, as shown below (Path 2). Explain how this allows the
curve to be taken at the greatest speed.
The driver who cuts the corner (on Path 2) has a more gradual curve, with a larger radius. That
one will be the better racing line. If the driver goes too fast around a corner using a racing line,
he will still slide off the track; the key is to stay at the maximum value of static friction. So, the
driver wants maximum possible speed and maximum friction.
Consider the equation for centripetal force:
2
c
v F m
r = where v is speed and r is the radius of
curvature. So by decreasing the curvature (1/r) of the path that the car takes, we reduce the
amount of force the tires have to exert on the road, meaning we can now increase the speed, v.
Looking at this from the point of view of the driver on Path 1, we can reason this way: the
sharper the turn, the smaller the turning circle; the smaller the turning circle, the larger is the
required centripetal force. If this centripetal force is not exerted, the result is a skid.
Suppose a mass is moving in a circular path on a frictionless table as shown below. In Earth’s
frame of reference, there is no centrifugal force pulling the mass away from the center of
rotation, yet there is a force stretching the string attaching the mass to the nail. Using concepts
related to centripetal force and Newton’s third law, explain what force stretches the string,
identifying its physical origin.
There must be a centripetal force to maintain the circular motion; this is provided by the nail at
the center. Newton’s third law explains the phenomenon. The action force is the force of the
string on the mass; the reaction force is the force of the mass on the string. This reaction force
causes the string to stretch.
Suppose a child is riding on a merry-go-round at a distance about halfway between its center
and edge. She has a lunch box resting on wax paper, so that there is very little friction between it
and the merry-go-round. Which path shown below will the lunch box take when she lets go? The
lunch box leaves a trail in the dust on the merry-go-round. Is that trail straight, curved to the left,
or curved to the right? Explain your answer.
If there is no friction, then there is no centripetal force. This means that the lunch box will move
along a path tangent to the circle, and thus follows path B. The dust trail will be straight. This is a
result of Newton’s first law of motion.
What causes water to be removed from clothes in a spin-dryer?
The barrel of the dryer provides a centripetal force on the clothes (including the water droplets)
to keep them moving in a circular path. As a water droplet comes to one of the holes in the
barrel, it will move in a path tangent to the circle.
A car rounds a curve and encounters a patch of ice with a very low coefficient of kinetic
fiction. The car slides off the road. Describe the path of the car as it leaves the road.
Since the radial friction with the tires supplies the centripetal force, and friction is nearly 0 when
the car encounters the ice, the car will obey Newton’s first law and go off the road in a straight
line path, tangent to the curve. A common misconception is that the car will follow a curved path
off the road.
Two friends are having a conversation. Anna says a satellite in orbit is in free fall because the
satellite keeps falling toward Earth. Tom says a satellite in orbit is not in free fall because the
acceleration due to gravity is not 2 9.80 m/s . Who do you agree with and why?
Anna is correct. The satellite is freely falling toward Earth due to gravity, even though gravity is
weaker at the altitude of the satellite, and g is not 2 9.80 m/s . Free fall does not depend on the
value of g; that is, you could experience free fall on Mars if you jumped off Olympus Mons (the
tallest volcano in the solar system).
Athletes such as swimmers and bicyclists wear body suits in competition. Formulate a list of
pros and cons of such suits.
The pros of wearing body suits include: (1) the body suit reduces the drag force on the swimmer
and the athlete can move more easily; (2) the tightness of the suit reduces the surface area of the
athlete, and even though this is a small amount, it can make a difference in performance time.
The cons of wearing body suits are: (1) The tightness of the suits can induce cramping and
breathing problems. (2) Heat will be retained and thus the athlete could overheat during a long
period of use.
As cars travel, oil and gasoline leaks onto the road surface. If a light rain falls, what does this
do to the control of the car? Does a heavy rain make any difference?
The oil is less dense than the water and so rises to the top when a light rain falls and collects on
the road. This creates a dangerous situation in which friction is greatly lowered, and so a car can
lose control. In a heavy rain, the oil is dispersed and does not affect the motion of cars as much.
A 35.0-kg dolphin accelerates opposite to the motion from 12.0 to 7.50 m/s in 2.30 s to join
another dolphin in play. What average force was exerted to slow the first dolphin if it was
moving horizontally? (The gravitational force is balanced by the buoyant force of the water.)
–68.5 N
A large rocket has a mass of kg 1000.2 6 × at takeoff, and its engines produce a thrust of
N 1050.3 7 × . (a) Find its initial acceleration if it takes off vertically. (b) How long does it take to
reach a velocity of 120 km/h straight up, assuming constant mass and thrust?
a. 2 7.70 m/s ; b. 4.33 s
A student’s backpack, full of textbooks, is hung from a spring scale attached to the ceiling of
an elevator. When the elevator is accelerating downward at 2 3.8 m/s , the scale reads 60 N. (a)
What is the mass of the backpack? (b) What does the scale read if the elevator moves upward
while speeding up at a rate 2 3.8 m/s ? (c) What does the scale read if the elevator moves upward
at constant velocity? (d) If the elevator had no brakes and the cable supporting it were to break
loose so that the elevator could fall freely, what would the spring scale read?
a. 10 kg; b. 140 N; c. 98 N; d. 0
A roller coaster car starts from rest at the top of a track 30.0 m long and inclined at 20.0° to
the horizontal. Assume that friction can be ignored. (a) What is the acceleration of the car? (b)
How much time elapses before it reaches the bottom of the track?
a. 2 3.35 m/s ; b. 4.2 s
(a) If half of the weight of a small 1.00 x10^3 -kg × utility truck is supported by its two drive
wheels, what is the maximum acceleration it can achieve on dry concrete? (b) Will a metal
cabinet lying on the wooden bed of the truck slip if it accelerates at this rate? (c) Solve both
problems assuming the truck has four-wheel drive.
a. 2 4.9 m/s ; b. The cabinet will not slip. c. The cabinet will slip.
Calculate the maximum acceleration of a car that is heading down a 6.00° slope (one that
makes an angle of 6.00° with the horizontal) under the following road conditions. You may
assume that the weight of the car is evenly distributed on all four tires and that the coefficient of
static friction is involved—that is, the tires are not allowed to slip during the acceleration
opposite to the motion. (Ignore rolling.) Calculate for a car: (a) On dry concrete. (b) On wet
concrete. (c) On ice, assuming that 100.0 µs = , the same as for shoes on ice.
a.
2 10.8 m/s ; b. 2 7.85 m/s ; c. 2 2.00 m/s
(a) A 22.0-kg child is riding a playground merry-go-round that is rotating at 40.0 rev/min.
What centripetal force is exerted if he is 1.25 m from its center? (b) What centripetal force is
exerted if the merry-go-round rotates at 3.00 rev/min and he is 8.00 m from its center? (c)
Compare each force with his weight.
a. 483 N; b. 17.4 N; c. 2.24, 0.0807
A machine at a post office sends packages out a chute and down a ramp to be loaded into
delivery vehicles. (a) Calculate the acceleration of a box heading down a 0.10 ° slope, assuming
the coefficient of friction for a parcel on waxed wood is 0.100. (b) Find the angle of the slope
down which this box could move at a constant velocity. You can neglect air resistance in both
parts.
a. 2 0.737 m/s ; b. 5.71°
What is the ideal banking angle for a gentle turn of 1.20-km radius on a highway with a 105
km/h speed limit (about 65 mi/h), assuming everyone travels at the limit?
4.14°
(a) What is the radius of a bobsled turn banked at 75.0° and taken at 30.0 m/s, assuming it is
ideally banked? (b) Calculate the centripetal acceleration. (c) Does this acceleration seem large
to you?
a. 24.6 m; b. 2 36.6 m/s ; c. 3.73 times g
If a car takes a banked curve at less than the ideal speed, friction is needed to keep it from
sliding toward the inside of the curve (a problem on icy mountain roads). (a) Calculate the ideal
speed to take a 100.0 m radius curve banked at 15.0° . (b) What is the minimum coefficient of
friction needed for a frightened driver to take the same curve at 20.0 km/h?
a. 16.2 m/s; b. 0.234
Railroad tracks follow a circular curve of radius 500.0 m and are banked at an angle of 5.0° .
For trains of what speed are these tracks designed?
20.7 m/s
The terminal velocity of a person falling in air depends upon the weight and the area of the
person facing the fluid. Find the terminal velocity (in meters per second and kilometers per hour)
of an 80.0-kg skydiver falling in a pike (headfirst) position with a surface area of 140.0 .
115 m/s or 414 km/h
A car rounds an unbanked curve of radius 65 m. If the coefficient of static friction between
the road and car is 0.70, what is the maximum speed at which the car can traverse the curve
without slipping?
21 m/s
A 560-g squirrel with a surface area of 2 930 cm falls from a 5.0-m tree to the ground.
Estimate its terminal velocity. (Use a drag coefficient for a skydiver falling feet first.) What will
be the velocity of a 56-kg person hitting the ground, assuming no drag contribution in such a
short distance?
v. . =11 8 m/s; v 9 9 m/s
DOEN VRAE: https://stemlearn.sun.ac.za/pluginfile.php/143021/mod_resource/content/1/UniversityPhysicsVolume1-Ch06.pdf
