Physics I

Practice Problems

Newton's Second Law

 

  1. In an actual experiment on applying the same force to different masses, how would you know it was the "same force"?

 

  1. The acceleration of an object is IO m/sec2 north. In a second experiment, the force is divided in half and turned to the east, and the mass is reduced to one third. What is the object's acceleration?

 

  1. A rocket sled has a mass of 4,440 kg and is propelled by a solid- propellant rocket motor of 890,000-N thrust, which burns for 3.9 sec.

 

    1. What is the sled's average acceleration and maximum speed?

 

    1. This sled has a maximum acceleration of 30 g (= 30 ag). How can that be, considering the data given?

 

    1. If the sled travels a distance of 1,530 m while attaining a top speed of 860 m/sec, what is its average acceleration? How did it attain that high a speed?

 

  1. A 75-kg person stands in an elevator. What force does the floor exert on the person when the elevator starts accelerating downward at 1.5 m/sec2?

 

  1. (a) A replica of the standard kilogram is constructed in Paris and then sent to the National Bureau of Standard near Washington, D.C. Assuming that this secondary standard is not damaged in transit, what is

 

1.      Its mass in Washington?

 

2.  Its weight in Paris and in Washington (in Paris, ag = 9.81 m/sec2; in Washington, ag = 9.80 m/sec2)?

 

(b) What is the change in your own weight as you go from Paris to Washington?

 

  1. Quite apart from pushing down on the ground owing to a runner’s own weight, the sole of a runner’s shoe pushes on the earth in a horizontal direction and the earth pushes with an equal and opposite force on the sole of the shoe. A reasonable value for the average acceleration of a runner is 5 m/sec2, and a reasonable value for the runner's mass would be 60 kg. The mass of the earth is approximately 60 x 1023 kg.

 

    1. What acceleration of the earth would the runner cause?

 

b. If the acceleration lasts for 2 sec, what speed will the runner have reached?

 

c. What speed will the earth have reached?

 

  1. A boy of mass 70 kg and a girl of mass 40 kg are on ice skates holding opposite ends of a 10-m rope. The boy pulls on the rope towards himself with a force of 80 N. Assuming that there is virtually no friction between the skates and the ice surface, what is the girl's acceleration? According to Newton's third law, what is the force on the boy? What is his acceleration?

 

  1. Consider the system consisting of a 1.0-kg ball and the earth. The ball is dropped from a short distance above the ground and falls freely.  Assuming that the mass of the earth is 6.0 x 1024 kg, calculate the acceleration of the earth in this interaction.

 

  1. (a) Two forces act on an object of 5 kg mass. One force is 20 N right and the other is 5 N left. How far will the object move in 10 sec?

 

(b) What is the net force on an object of 4 kg mass if its speed is changed from 40 m/sec to 80 m/sec in 10 sec?

 

(c) A 6-kg block is pulled along the floor whose frictional force is estimated at 3 N. Forces of 18 N right and 15 N left are exerted on the block simultaneously. What is the acceleration of the block?