# Ask-a-Physicist by Topic

## Force & Motion In general relativity, gravity is described as a distortion of space time. Most vulgarized books use the simplified image of a 2D plane being bent downwards by a mass, so that any matter traveling in the area would have to follow the bending of the plane, which would then explain why things are attracted to one another.

Keeping the same simplified metaphor, could we imagine something that would bend the plane upwards, thus causing objects to be repelled? Would such a thing be considered to have negative mass? Is the concept theoretically possible?

Find out the answer » My friend and I are having a debate on wind turbines or anything that travels in a circular motion. One of us says that the very tip of the turbine blade is traveling faster than the inner part that is close to the axis. We both agree that it has the same rpm, but what part travels at a faster mph?
Find out the answer » I understand what happens and why, when I drop a tennis ball that is resting on top of a basketball. What I am trying to figure out is, why do I not get the same results with a ping pong ball on top of a basket ball. Any ideas?

Find out the answer » If a person stepped on a home bathroom scale, and weighed 170 lbs, he would weigh a different weight on this same scale when on other planets due to the force of gravity.

If, however, a person stepped on a professional medical scale — the kind with weights attached that you slide (as opposed to a bathroom scale) and weighed 170 pounds on Earth, would the weight of the person be the same on the other planets as it is on Earth (170 lbs)? This has become a bone of contention in an otherwise ideal marriage.

Find out the answer » If air is heated and its starts to go towards the space, what is the reason for this? Do you think the air molecules will get accelerated and because of this they will resist Earth’s gravitational force?

Find out the answer » I've read that orbiting objects like the space station stay in orbit because they are falling at the same rate the Earth is curving away underneath them.

What I don’t understand is their downward velocity should be increasing because it is caused by gravity / acceleration due to gravity. Shouldn’t it be “falling” with a greater velocity the longer it travels? Does it reach a terminal velocity like that of a skydiver? If so Why?

Find out the answer » Under acceleration, a helium-filled balloon inside of a car will jump forward in the direction of acceleration. I have been searching for the reason why with no definitive results. This is the dilemma: An acquaintance of mine, who has a degree from Johns Hopkins is attempting to argue for some magical force that drives the balloon forward, also that it has something to do with gravity.

Find out the answer » I’m not a physics guy, but with all the discussion around the disaster in Oklahoma recently it had me pondering a terminal velocity question. From what I understand, any object with mass would possess a terminal velocity.

Since air has mass, what would be the terminal velocity of air? Specifically, how fast can a tornado/hurricane/natural disaster hurl air before it reaches a terminal velocity?

Find out the answer » My 4 year old would like to know: "If marbles are made of glass then why don't they break when you throw them?
Find out the answer » In figure skating, if you are doing a spin on ice and you leave your arms spread out it creates resistance. If you pull the arms in toward the body, you create less resistance so you spin more easily. How do I express this with mathematical equations in physics?

Find out the answer » If somehow there was a tunnel straight through the center of the earth and we dropped a capsule through it, what will happen to the capsule? Would the tunnel shoot the capsule into space?

Find out the answer » When we talk about an object on an incline, the force of gravity has two components, one vertical and one parallel to the incline, as you know, while the normal force is matched in magnitude by the vertical component of gravity. When we talk about a banked road, however, the normal force becomes the force whose both components are considered, while gravity becomes the force that is matched by the vertical component of normal force. In a way, they almost reverse their roles. Why is that?

Find out the answer » If a helicopter hovers in a fixed position for 24 hours will the earth rotate around it?

Find out the answer » If an object is sitting on the ground, not moving, is it accelerating towards the earth? I ask because gravity is an acceleration and gravity is always acting on all objects, but acceleration measures the change in speed of an object. So if it is not speeding up, but gravity is acting on it, is it accelerating?
Find out the answer » What are the scientific reasons for spin or curve on a soccer ball? What forces are affecting this?

Find out the answer » Why does a stationary body above the surface of the earth not change its position after some time while the earth is moving? Suppose if we place an object above the surface of the earth, would it be at the same position after some time and why?
Find out the answer » Is it true that pressure exerted by a human foot is more than than exerted by that of an elephant?
Find out the answer » Question: I want to ask why water is shaped like a ball when it is in space without gravity?
Find out the answer » If someone was 137 and 5 foot 7 in. how many newtons would it take them to escape the gravity of earth?
Find out the answer » Is it possible to have an instantaneous zero speed but still have an acceleration? Please give an example. - J
Find out the answer » Would a vertical pipe 50 miles long, one end at or near sea level and the other end in space, suck air into space? It seems as though the vacuum of space should create a flow through the pipe since the pressure at each end is different. - P
Find out the answer » It was my understanding that the velocity of an object falling through air has to do with gravity and wind resistance only. Does the object's velocity have anything to do with its weight or mass? - JVN, Brisbane, Australia
Find out the answer » How does pressure affect humans? Why do our ears pop at high altitudes? — KC, Chino Hills, CA
Find out the answer » On earth, two objects of different masses fall at the same rate. However, in space, larger masses exert greater gravitational attraction than smaller masses. This seems inconsistent. What am I confusing? - DD, Miami, FL
Find out the answer » How does the placement of a car's center of gravity affect it? - SM, York Beach, Maine
Find out the answer » Why does a steel ball bounced on a steel surface bounce higher than any other ball on the same surface? - JL, California
Find out the answer » Why if you drop a ball from say 2 meters does it bounce higher than a ball dropped from 1 meter? - E
Find out the answer » How do water-bottle rocket work? I know about Newton's third law and everything, but I don't know how it all goes together in a bottle rocket. - Z, Vancouver, WA
Find out the answer » I have a question about a tennis racquet: when I hold the racquet like a frying pan (plane of the head horizontal) and give it one flip, it changes from smooth up to rough up (or vice versa). When I flip it with the plane of the head vertical, nothing peculiar happens. In both cases, I try not to impart any rotation about the long axis. - GF, Shawnee, Kansas
Find out the answer » I can't understand the instruction stated on my tennis racquet about the relation between string tension and string power. It says "higher tension for more control and less power and vice versa". How can it happen? – AT, Hong Kong
Find out the answer » Why can a human being sense some accelerations, such as in a car or airplane, but not others, such as free fall? - MDJ, Ozark, MO
Find out the answer » In his book "Understanding Physics," Isaac Asimov writes "As the earth rotates about its axis, the surface of the earth is continually undergoing an acceleration inward toward the center of the earth (just as the moon does in revolving around the earth." Does that mean that the acceleration on the surface (g = 9.8 m/s2) is directly related to the centripetal acceleration of an object traveling in a circle (a = v2/r)? - DW, Raleigh, NC
Find out the answer » How do I explain the difference between the following two situations to my junior high son: (1) The horizontal velocity of a bullet fired horizontally from a gun has no effect on how long the bullet takes to hit the earth, i.e., how long it takes gravity to bring the bullet down to earth. (2) The horizontal velocity of the Space Shuttle orbiting the earth does affect how long it takes for the Space Shuttle to hit the earth. The velocity of the Space Shuttle makes it keep missing the earth in its free fall. – MMH, Ohio
Find out the answer » As a downhill cyclist, I enjoy riding "drop-offs," i.e. riding off the edge of drops in the ground and landing at the bottom. It seems common sense to me that the faster I go when I hit the edge, the easier the landing. However, some of the physics I have learned in school suggests that my horizontal velocity would not matter, as my vertical velocity would be the same in any case. Is there something I am forgetting? - IE, Oban, Argyll, Scotland
Find out the answer » Why does a basketball bounce higher when it has more air in it? - SR, Charlotte, North Carolina
Find out the answer » If a truck full of parrots hits a bump in the road and all the parrots begin flying around inside the truck, does the truck's weight stay the same or not? HJ, Thisted, Denmark
Find out the answer » Why is it easy to stay on a bike while moving, but impossible once it stops? - AS, Switzerland
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