Forces
Reading: Arons, Section 3.101. A small rubber ball rolls across the floor. For a period of time it passes over a piece of felt. After leaving the felt, it collides with a wall and rebounds, passive the felt a second time. Assume that the motion of the ball is along a straight-line path. The strobe diagram for the motion of the ball is shown below. Assume that the time interval between strobes is 0.5 s.
A. Identify the forces exerted on the ball prior to reaching the felt for the first time, and indicate the agent causing each force. Is your answer consistent with Newton’s First Law? Explain.
B. Use Newton’s second law to determine when and where there is a net force exerted on the ball. Identify the unbalanced force for each instance.
C. Where is the net force on the ball the greatest? Explain, using Newton’s laws.
D. For each instance that the ball is accelerating, compare the direction of the net force with the direction of the acceleration.
below: The man has a mass of 85. kg.
A. Construct a force diagram for the man.
B. What force does the floor exert on the man?
3. The elevator now accelerates upward at 2.0 m/s2.
A. Construct a force diagram for the man.
4. Upon reaching the top of the building, the elevator accelerates downward at 3.0 m/s2.
A. Construct a force diagram for the man.
B. What force does the floor now exert on the man?
5. While descending in the elevator, the cable suddenly breaks. What is the force of the floor on the man?
6. A person takes a step forward on a horizontal surface. Draw a force diagram for this situation.
8. A 4600 kg helicopter accelerates upward at 2.0 m/s2. What lift force is exerted by the air on the propellers?
9. The maximum force that a grocery bag can withstand without ripping is 250 N. Suppose that the bag is filled with 20. kg of groceries and lifted with an acceleration of 5.0 m/s2. Do the groceries stay in the bag?
For these problems, you will have to use kinematic formulas as well as Newton's 2nd Law.
10. A racecar has a mass of 710 kg. It starts from rest and travels 40.0m in 3.0s. The car is uniformly accelerated during the entire time. What net force is acting on the car? Draw a force diagram for this situation.
12. A 65 kg person dives into the water from the 10. m platform.
A. What is her speed as she enters the water?
B. She comes to a stop 2.0 m below the surface of the water. What net force did the water exert on the swimmer?
13. During a head-on collision, a passenger in the front seat of a car accelerates from 13.3 m/s (≈ 30 miles/hour) to rest in 0.10 s.
A. What is the acceleration of the passenger?
B. The driver of the car holds out his arm to keep his 25 kg child (who is not wearing a seat belt) from smashing into the dashboard. What force must he exert on the child?
C. What is the weight of the child? Convert these forces from N to pounds. (x 1 lb
horizontally to a pulley at the edge of the table. A 10 kg mass (B) hangs at the end of the cord.
a) Clearly draw and label the force vectors acting on each object.
b) Calculate the acceleration of the block and mass.
15.An elevator is rising at a speed of 5.00 m/s. It comes to a halt in 4.0 s. The guide rails on the side of the elevator each exert a 110 N frictional force on the elevator. The pulley has negligible friction and mass for the purposes of this analysis.
a. Find the tension in the cable during the slowing of the elevator.
b. Find the mass M that the counterweight must have in order for the elevator to stop as stated above.
500 kg elev ator car
Counterweight
