A: Dynamics Without Friction

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Forces: Problem Set 1

A: Dynamics without Friction

1.  (I) What net force will cause a 3.00 kg mass to accelerate at a rate of 4.00 m/s2?

2.  (I) A 873 kg (1930 lb) dragster, starting from rest attains a speed of 26.3 m/s (58.9 mph) in 0.59 s.

a.  What is the average acceleration of the dragster during this time interval?

b.  What is the magnitude of the average net force on the dragster during this time?

c.  Assume that the driver has a mass of 68 kg. What horizontal force does the seat exert on the driver?

3.  (I) After a day of testing race cars, you decide to take your own 1550 kg car onto the test track. While moving down the track at 10.0 m/s, you uniformly accelerate to 30.0 m/s in 10.0 s. What is the average net force that is applied to the car during this time?

4.  (III) A 65 kg swimmer jumps off a 10.0 m tower

a.  What is the swimmer’s velocity on hitting the water?

b.  The swimmer comes to a stop 2.0 m below the surface of the water. What is the net force exerted by the water on the swimmer?

5.  (II) A 4500 kg helicopter accelerates upward at 2.0 m/s2. What lift force is exerted by the air on the propellers?

6.  (II) A crane holds a wrecking ball that weighs 98,000 N.

a.  What upward force does the crane exert on the wrecking ball?

b.  If the crane begins to move the wrecking ball upward with an acceleration of 2.0 m/s2, what force is the crane exerting on the wrecking ball?

7.  (II) A force of 90.0 N is exerted straight up on a stone that weighs 7.35 N.

a.  What is the mass of the stone?

b.  What is the net force acting on the stone?

c.  What is the acceleration of the stone?

8.  (II) A rocket weighs 7840 N on the earth. The force of propulsion is 10,440 N

a.  What is the mass of the rocket?

b.  What is the net force acting on the rocket?

c.  What is the acceleration of the rocket?

d.  Starting from rest, what is the velocity of the rocket at the end of 10.0 s?

1)  12.0 N / 2)  a) 45 m/s2 (b) 39000 N (c) 3100 N / 3)  3100 N / 4)  a) 14 m/s (b) 3185 N / 5)  53,000 N
6)  (a) 98,000 N (b) 118,000 N
7)  (a) 0.75 kg (b) 82.65 N up (c) 110.2 m/s2 up / 8)  (a) 800. kg (b) 2600 N up
(c) 3.25 m/s2 up (d) 32.5 m/s

B: Statics and State of Equilibrium

9.  (III) Find the tension in each cable that supports the mailbag that weighs 625 N shown below.

10.  (III) What is the mass of the stoplight shown below?

11.  (III) Find weight of the cat burglar shown below if the tension in the horizontal cable is 845 N.

9)  1040 N; 829 N / 10)  8.7 kg / 11)  637 N

C: Dynamics with Friction

12.  (II) A sled of mass 50.0 kg is pulled along snow covered flat ground. The coefficient of static friction is 0.30. The coefficient of kinetic friction is 0.10.

a.  How much does the sled weigh?

b.  What force will be needed to just start the sled moving?

c.  What force is needed to keep the sled moving at a constant velocity?

13.  (II) The engine of a car with a mass of 350 kg applies a force of 14, 875 N. The coefficient of rolling friction between the tires and the pavement is 0.25.

a.  What is the weight of the car?

b.  What is the force of friction opposing the motion of the car?

c.  What is the net force acting on the car?

d.  What is the acceleration of the car?

14.  (II) A 225 kg block accelerates across the floor as the result of an applied force of 1100 N to the right. The kinetic frictional force is 600 N.

a.  What is the net force causing the acceleration?

b.  What is the magnitude and direction of the acceleration?

c.  What is the coefficient of friction between the block and the floor?

15.  (III) A boy exerts a 36 N horizontal force as he pulls a 52 N sled across a cement sidewalk at 0.25 m/s. What is the coefficient of kinetic friction between the sidewalk and the metal sled runners?

16.  (III) Suppose the sled from problem 12 runs on packed snow. The coefficient of friction is now only 0.12. If a person weighing 650 N sits on the sled, what force is needed to pull the sled across the snow at constant speed?

17.  (III) A 10.0 N force is applied at 20.0° above the horizontal to a 2.5 kg brick to drag it across a table. The coefficient of sliding (kinetic) friction is 0.35.

a.  What is the normal force acting on the brick?

b.  What is the force of friction acting on the brick?

c.  What is the acceleration of the brick?

12)  (a) 490 N (b) 147 N (c) 49 N / 13)  (a) 3430 N (b) 858 N (c) 14,017 N (c) 40.0 m/s2
14)  (a) 500 N (b) 2.2 m/s2 right (c) 0.27 / 15)  0.69 / 16)  84 N
17)  (a) 21.1 N (b) 7.39 N (c) 0.804 m/s2