A person throws an object on a horizontal frictionless plane surface. It is noticed that there are two forces acting on this object-(i) gravitational pull and (ii) normal reaction of the surface. According to the third law of motion, the net resultant force is zero. Which one of the following can be said for the motion of the object?
- A.The object will move with acceleration.
- B.The object will move with deceleration.
- C.The object will move with constant speed, but varying direction.
- D.The object will move with constant velocity.
Correct answer
Explanation
The correct answer is D, The object will move with constant velocity. With no net force acting on it, the object keeps moving at the same speed in the same straight line.
On the frictionless horizontal surface, the downward pull of gravity is exactly balanced by the upward normal reaction, and there is no friction to act along the surface. Newton's first law, the law of inertia, says that a body continues in its state of rest or of uniform motion in a straight line unless an unbalanced external force acts on it. So after the throw, nothing changes the object's speed or direction.
Option A is wrong because acceleration needs a net unbalanced force, which is absent here. Option B is wrong because deceleration also needs a force opposing the motion, such as friction, and the surface is frictionless. Option C is wrong because changing direction at constant speed, as in circular motion, needs a centripetal force towards the centre.
Exam tip: net force zero means zero acceleration, so the velocity stays constant, which is the first law of motion (inertia).
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
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- D.Urea is produced in lung and kidney.
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- B.Objective lens-Condenser-Body tube-Eyepiece
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- D.Eyepiece-Objective lens-Body tube-Mirror
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- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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