Practice Problems: http://wc.pima.edu/~solson/phy121/notes/notes18.pdf,

http://www.occc.edu/ktapp/gophysics/phys1114/PHYS1114_SP13_SampleExam3_Soln.pdf,

http://www.physics.rutgers.edu/ugrad/203f08/solutions/Week3,

http://www.solvephysics.com/dynamics_part1.shtml

http://www.solvephysics.com/dynamics_part2.shtml

An object is moving at a constant velocity. All but one of the following statements could be true. Which one cannot be true? **A single force acts on the object. If only one force acts on the object, it is the net force; thus, the net force must be nonzero. Consequently, the velocity would change, according to Newton’s first law, and could not be constant.**

Three forces act simultaneously on the object.

This statement could be true if the three forces had magnitudes and directions such that the vector sum of the three were zero. Then the net force would be zero, and the velocity would remain constant, according to Newton’s first law.

No forces act on the object. This statement could be true, because if no forces are acting, the net force is zero, and the velocity does not change, according to Newton’s first law.

Which one of the following is true, according to Newton’s laws of motion? Ignore friction.

Sam (18 years old) and his sister (9 years old) go ice skating. They push off against each other and fly apart. Sam flies off with the greater acceleration. This answer is false. According to the third law, Sam and his sister experience forces of equal magnitudes during the push-off. Since Sam has the greater mass, he flies off with the smaller acceleration, according to the second law.

A sports utility vehicle (SUV) hits a stationary motorcycle. Since it is stationary, the motorcycle sustains a greater force than the SUV does. This answer is false, according to the third law, which states that whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. It does not matter whether one of the bodies is stationary.

Two astronauts on a space walk are throwing a ball back and forth between each other. In this game of catch the distance between them remains constant. This answer is false because in catching and throwing the ball each astronaut applies a force to it, and, according to the third law, the ball applies an oppositely directed force of equal magnitude to each astronaut. These reaction forces accelerate the astronauts away from each other, so that the distance between them increases.

A semitrailer truck crashes all the way through a wall. Since the wall collapses, the wall sustains a greater force than the truck does. This answer is false, according to the third law, which states that whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. It does not matter whether one of the bodies collapses.

**None of the these is true, according to the third law. Answers (a) and (b) are false, according to the third law, which states that whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. It does not matter whether one of the bodies is stationary or whether it collapses. Answer (c) is false because, according to the third law, Sam and his sister experience forces of equal magnitudes during the push-off. Since Sam has the greater mass, he flies off with the smaller acceleration, according to the second law. Answer (d) is false because in catching and throwing the ball each astronaut applies a force to it, and, according to the third law, the ball applies an oppositely directed force of equal magnitude to each astronaut. These reaction forces accelerate the astronauts away from each other, so that the distance between them increases.**

** **