Why does a child in a wagon seem to fall backward when you give the wagon a sharp pull forward?
The child tends to remain at rest unless a force acts on her. The force is applied to the wagon, not the child, and so the wagon accelerates out from under the child, making it look like the child falls backwards relative to the wagon.if the child is stan
A box rest on the frictionless bed of a truck. The truck driver starts the truck and accelerates forward. The box immediately starts to side forward the rear of the truck bed. Discuss the motion of the box, in terms of newtons laws, as seen (a) by Mary st
(A)Mary sees the box stays stationary with respect to the ground. There is no horizontal force on the box since the truck bed is smooth, and so the box cannot accelerate. Thus Mary would describe the motion of the box in terms of newtons first law-there i
If the acceleration of an object is zero, are no forces acting on it? Explain.
If the acceleration of an object is zero, then by newtons second law, the net force must be zero. There can be forces acting on the object as long as the vector sum of the forces is zero.
Only one force acts on an object. Can the object have zero acceleration? Can it have zero velocity? Explain.
It's only one for his acts on the object, then the net force cannot be zero. Does the object cannot have zero acceleration, by newtons second law. The object can have zero velocity for an instant. For example, an object thrown straight up under the influe
When a golf ball is dropped to the pavement, it bounces back up. (a) is a force needed to make it bounce back up? (b) if so, what exerts the force?
(A)A force is needed to bounce the ball back up, because the ball changes direction, and so accelerates. If the ball accelerates, there must be a force.
(B)the pavement exerts the force on the golf ball.
If you walk along a log floating on a lake, why does the log move in the opposite direction?
When you try to walk east, you push on the ground with a westward force. When you push westward on the massive earth, the earth moves imperceptibly, but by newtons third law there is in eastern force on you, which propels you forward. When walking on the
Why might your foot hurt if you can kick a heavy desk or a wall?
By Newtons third law, the desk or wall exerts a force on your foot equal in magnitude to the floors with which you hit the desk or wall. If you get the desk or wall with a large force, then there will be a large force on your foot, causing pain. Only the
When you are running and want to stop quickly, you must decelerate quickly. (A) what is the origin of the force that causes you to stop? (B) estimate the maximum rate of deceleration of a person running at top speed to come to rest.
(A)when you are running the stopping force is a force of friction between your feet in the ground. You push forward with your feet on the ground, and thus the ground pushes backwards on you, slowing your speed.
(B)A fast person can run about 10 m/s, perha
A stone hangs by a fine thread from a ceiling, and a section of the same thread dangles from the bottom of the stone. If a person gives a sharp pull on the dangling thread, where is the thread likely to break: below the stone or above it? What if the pers
Because of the stones inertia, the upper string does not immediately experience the large force. The bottom string must have more tension in it, and it will break first. The tension in the upper string is going to be larger than the tension in the lower s
The force of gravity on a 2 kg rock is twice as great as that on a 1 kg rock. Why then doesn't the heavier rock fall faster?
The acceleration of both rocks is found by dividing their weight by their mass. The 2 kg rock has a force of gravity on it that is twice as great as the force of gravity on the 1 kg rock, but also twice as great a mass as the 1 kg rock, so the acceleratio
Would a spring scale carried to the moon give accurate results if the scale had been calibrated (a) in pounds, or (b) in kilograms?
Only the pounds reading would be correct. The weight of the object is smaller by a factor of six, and the spring will be pulled to only 1/6 of the distance that it was pulled on the earth. The mass itself doesn't change when moving to the moon, and so a m
You pull a box with a constant force across a frictionless table using an attached rope held horizontally. If you now pulled the rope with the same force at an angle to the horizontal, does the acceleration of the box remain the same, increase, or decreas
When you pull the rope at an angle, only the horizontal component of the pulling force will be accelerating the box across the table. This is a smaller horizontal force than originally used, and so the horizontal acceleration of the box will decrease.
When an object falls freely under the influence of gravity there is a net force mg exerted on it by the earth. Yet by newtons third law the object exerts an equal and opposite force on the earth. Why doesn't the earth move?
The acceleration of the earth would be found using newtons third law and Newton's second law. Aearth=gm/M. The acceleration of the earth is about 10-24, which is tiny and undetectable.
(A)Compare the effort or force needed to lift a 10 kg object when you're on the moon with the force needed to lift it on earth. (B)Compare the force needed to throw a 2 kg object horizontally with a given speed on the moon and on earth.
(A)To lift the object on the earth requires a force the same size as its weight on earth, Fearth=mgearth=98N. To lift the object on the moon requires a force the same size as its weight on the moon Fmoon=mgmoon=mgmoon/6=16N.
(B)The horizontal accelerating
According to newtons third law, each team in a tug-of-war pulls with equal force on the other team. What, then, determines which team will win?
In a tug-of-war, the team that pushes hardest against the ground wins. It is true that both teams have the same force on them due to the tension in the rope. But the winning team pushes harder against the ground and thus the ground pushes harder on the wi
A person exerts an upward force of 40N to hold a bag of groceries. Describe the "reaction" force by stating (a)its magnitude, (b)its direction, (c)on what object it is exerted, and (d)by what object it is exerted.
(A)the magnitude is 40N
(B)the direction is downward
(C)it is exerted on the person
(D)it is exerted by the bag of groceries
When you standstill on the ground, how large a force does the ground exert on you? Why doesn't this force make you rise up into the air?
If you are at rest, the net force on you is zero. Hence the ground exerts a force on you exactly equal to your weight. The two forces acting on you sum to zero, and so you don't accelerate. If you squat down then push with a larger force against the groun
Whiplash sometimes results from an automobile accident when the victim's car is struck violently from the rear. Explain why the head of the victim seems to be thrown backwards in this situation. Is it really?
In a whiplash situation, the car is violently pushed forward. Since the victim's back is against the seat of the car, the back moves forward with the car. But the head has no direct horizontal force to push it, and so it "lags behind". The victim's body i
A heavy crate rest on the bed of a flatbed truck. When the truck accelerates, the crate remains where it is on the truck, so it, too, accelerates. What force causes the crate to accelerate?
The truck bed exerts a force of static friction on the crate, causing the crate to accelerate.
A block is given a push so that is slides up a ramp. After the block reaches its highest point, it slides back down but the magnitude of its acceleration is less on the descent then on the ascent. Why?
On the way up, there are two forces on the block that are parallel to each other causing the deceleration-the component of weight parallel to the plane, and the force of friction on the block. Since the forces are parallel to each other, both pointing dow
What would your bathroom scale read if you weighed yourself on an inclined plane? Assume the mechanism functions properly, even at an angle.
Assume your weight is W. If you weighed yourself on an incline plane that is inclined at angle theta, the bathroom scale would read the magnitude of the normal force between you and the plane, which would be Wcostheta.
Sometimes people say that water is removed from clothes in a spin dryer by centrifugal force throwing the water outward. What is wrong with this statement?
The problem with this statement is that there is nothing to cause an outward force, and so the water removed from the clothes is not thrown outward. Rather, the spinning drum pushes inward on the clothes and water. But where there are holes in the drum, t
Will the acceleration of a car be the same when the car travels around a sharp curve at a constant 60 km/h as when it travels around a gentle curve at the same speed? Explain.
The centripetal acceleration for an object moving in circular motion is inversely proportional to the radius of the curve, given a constant speed(a=v2/R). So for a gentle curve (which means a large radius), the acceleration is smaller, while for a sharp c
Suppose a car moves at constant speed along a hilly road. Where does the car exert the greatest and least forces on the road: (a)at the top of the hill, (b)at a dip between two hills, (c)on a level stretch near the bottom of a hill?
The force that the car exerts on the road is newtons third law reaction to their normal force of the road on the car, and sowe can answer this question in terms of normal force. The car exerts the greatest force on the road at the dip between two hills. T
Describe all the forces acting on a child riding a horse on a merry-go-round. Which of these forces provides the centripetal acceleration of the child?
There are at least three distinct major forces on the child. The force of gravity is acting downward on the child. There is a normal force from the seat of the horse acting upward on the child. There must be friction between the seat of the horse and the
A bucket of water can be whirled in a vertical circle without the water spilling out, even at the top of the circle when the bucket is upside down. Explain.
For the water to remain in the bucket, there must be a centripetal force forcing the water to move in a circle along with the bucket. That centripetal force gets larger with the tangential velocity of the water, since Fr=mv2/r. The centripetal force at th
How many "accelerators" do you have in your car? There are at least three controls in the car which can be used to cause the car to accelerate. What are they? What acceleration do they produce?
The three major accelerators are the accelerator pedal, the brake pedal, and the steering wheel. The accelerator pedal can be used to increase speed or to decrease speed in combination with friction. The brake pedal can be used to decrease speed by depres
A child on a sled comes flying over the crest of a small hill. His sled does not leave the ground, but he feels the normal force between his chest and the sled decrease as he goes over the hill. Explain this decrease using Newton's second law.
When the child is on a level surface, the normal force between his chest and the sled is equal to the child's weight, and thus he has no vertical acceleration. When he goes over the hill, the normal force on him will be reduced. Since the child is moving
Why do bicycle riders lean inward when rounding a curve at high speed?
When a bicycle rider leans inward, the bike tire pushes down the ground at an angle. The road surface then pushes back on the tire both vertically (to provide the normal force which counteracts gravity) and horizontally toward the center of the curve (to
Why do airplanes bank when they turn? How would you compute the banking angle given its speed in radius of the turn?
Airplanes bank when they turn because in order to turn there must be a force that will be exerted towards the center of a circle. By tilting the wings, do you lift force on the wings has a non-vertical component which points toward the center of the curve
A girl is whirling a ball on a string around her head in a horizontal plane. She wants to let go at precisely the right time so that the ball will hit a target on the other side of the yard. When should she let go of the string?
She should let go of the string when the ball is at a position where the tangent to the circle at the balls location, when extended, passes through the targets position. That tangent line indicates the direction of the velocity at that instant, and if the
Does an Apple exert a gravitational force on the earth? If so, how large a force? Consider an apple (a) attached to a tree, and (b) falling.
The apple does exert a gravitational force on the earth. By Newton's third law, the force on the earth due to the apple is the magnitude as the force on the apple due to the earth - the weight of the apple. The force is also independent of the state of mo
If the earth's mass were double what it is, in what ways would the Moon's orbit be different?
The gravitational force on the moon given by GMearthMmoon/R2, where R is the radius of the moon's orbit. This is a radial force, and so can be expressed as MmoonVmoon2/R. This can be changed using the relationship Vmoon=2piR/T, where T is the orbital peri
Which pulls harder gravitationally, the earth on the moon, or the moon on the earth? Which accelerates more?
The gravitational pullis the same in each case, by newton's third law. The magnitude of that pull is given by F=GMearthMmoon/r2earth-moon. To find the acceleration of each body, the gravitational pulling force is divided by the mass of the body. Since the
The sun's gravitational pull on the earth is much larger than the moon's. Yet the moon's is mainly responsible for the tides. Explain. (hint: consider the difference in gravitational pull from one side of the earth to the other.)
The difference in force on the two sides of the earth form from the gravitational pull of either the sun or the moon is the primary cause of the tides. That difference in the force comes about from the fact that the two sides of the earth are a different
Will an object weigh more at the equator or at the poles? What two effects are at work? Do they oppose each other?
And object weighs more at the polls, due to two effects which complement (not oppose) each other. First of all, the Earth Who is slightly flattened at the poles and expanded at the equator, relative to a perfect sphere. Thus The mass at the poles is sligh
The gravitational force on the moon due to the earth is only about half of the force on the moon due to the sun. Why isn't the moon pulled away from the earth?
The moon is not pulled away from the earth because both the moon and the earth are experiencing the same radio acceleration due to the sun. They both have the same period around the sun because they are both, on average, the same distance away from the su
Is the centripetal acceleration of mars in its orbit around the sun larger or smaller than the centripetal acceleration of the earth?
The centripetal acceleration of Mars is smaller than the earth. The acceleration of each planet can be found by dividing force on each planet by the planet's mass. The resulting acceleration is inversely proportional to the square of the distance of the p
Would it require less speed to launch a satellite towards the east or toward the west? Consider the earths rotation direction
In order to orbit, a satellite must reach an orbital speed relative to the center of the earth. Since the satellite is already moving eastward when launch due to the rotation speed at the surface of the earth, it requires less additional speed to launch i
When will your apparent weight be the greatest, as measured by a scale in a moving elevator: when the elevator accelerates downward, accelerates upward, is in freefall, moves upward at a constant speed? In which case would your weight be the least? When w
The apparent weight (the normal force) would be largest when the elevator is accelerating upward. From the freebody diagram, with up as positive, we have Fn-mg=ma->Fn=ma+mg. With a positive acceleration, the normal force is greater than your weight. The a
What keeps a satellite up in its orbit around the earth?
A satellite remains in orbit due to the combination of gravitational force on the satellite directed towards the center of the orbit and the tangential speed of the satellite. First, the proper tangential speed had to be established by some other force th
Astronauts who spend long periods outer space could be adversely affected by weightlessness. One way to simulate gravity is to shape the spaceships like a cylindrical shell that rotates, with the astronauts walking on the inside surface. Explain how this
The passengers are standing on the floor.
(a) if a passenger held an object beside their waist and then released it, the object would move in a straight line, tangential to the circle in which the passenger's waist was moving when the object was released.
Explain how a runner experiences "free fall" or "apparent weightlessness" between steps.
When the runner has both feet off the ground, the only force on the runner is gravity-there is no normal force from the ground on the runner. This lack of normal force is interpreted as "free fall" and "apparent weightlessness".
The earth moves faster in its orbit around the sun in January than in July. Is the earth closer to the sun in January, or in July? Explain.
By Kepler's second law, the earth moves faster around the sun when it is nearest the sun. Kepler's second law says that an imaginary line drawn from the sun to the earth sweeps out equal areas in equal times. So when the earth is close to the sun, it must
The mass of Pluto was not known until it was discovered to have a moon. Explain how this discovery enabled an estimate of Pluto's mass.
Let the mass of Pluto be M, the mass of the moon be m, the radius of the Means orbit be R, and the period of the moon's orbit be T. Then newtons second law for the moon orbiting Pluto will be F=GmM/R2. If that means orbit is a circle, then the form of the