They're all right. Just different ways of looking it.
...I think...
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The only correct answer is E. When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.
This is because of Newton's second and third laws. Newton's second law states that the sum of external forces acting on an object is proportional to the product of the object's mass and the object's acceleration: F = ma. In order for you to accelerate upwards, you must exert a force greater than your weight (otherwise you would not accelerate upwards.)
Newton's third law states that for every action, there is an equal and opposite reaction (in other words, for each force, there is an equal and opposite force.) Therefore, the force that the earth exerts on you must be equal to the force you exert on the earth.
Well put elzoido238. I always like to discuss this one with the 13-14 year-olds that I teach. It totally blows their minds, but they can understand it just fine.
Feb 14, 2010
0
#5
About 288 N ? [I assumed g= 9.81 m/s^2]
A bit rusty on this sort of thing!
An amusing variation on the problem. A 160 kg piano is taken out of a first floor apartment by sliding it down a ladder at 10.4 degrees to the vertical. A man at the bottom of the ladder tries to keep the piano still by pushing horizontally. With what force does he need to push? (you can ignore friction and assume the ladder is absolutely fixed and rigid).
elzoido238 wrote:
The only correct answer is E. When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.
This is because of Newton's second and third laws. Newton's second law states that the sum of external forces acting on an object is proportional to the product of the object's mass and the object's acceleration: F = ma. In order for you to accelerate upwards, you must exert a force greater than your weight (otherwise you would not accelerate upwards.)
Newton's third law states that for every action, there is an equal and opposite reaction (in other words, for each force, there is an equal and opposite force.) Therefore, the force that the earth exerts on you must be equal to the force you exert on the earth.
i.e, jump
Lebeast wrote:
Perfect answer, elzoido238. Thank you.
New Question;
A man pushes on a piano with mass 160 so that it slides at constant velocity down a ramp that is inclined at 10.4 above the horizontal floor. Neglect any friction acting on the piano. Calculate the magnitude of the force applied by the man if he pushes parallel to the floor.
he would have to apply a negative force to keep the velocity constant, but since we want the magnitude this does not matter. Since there is no change in velocity, there must be no net force on the piano. The force on the piano due to gravity along the surface of the plane is sin(10.4)*160 kg*9.81 m/s^2 = 283N.
However, the man is pushing parallel to the floor, so his force is 1/cos(10.4) times this amount. So the man's force is 1/cos(10.4)*sin(10.4)*160 kg*9.81 m/s^2 = tan(10.4)*160 kg*9.81m/s^2 = 288 N
As for the piano sliding down from the apartment, he still needs only 288 N to keep it still. However, if he tried to stop the piano... SPLAT
Perfect answer, elzoido238. Thank you.
New Question;
A man pushes on a piano with mass 160
so that it slides at constant velocity down a ramp that is inclined at 10.4
above the horizontal floor. Neglect any friction acting on the piano. Calculate the magnitude of the force applied by the man if he pushes parallel to the floor.