Newton's Laws Examples

Physics Newton’s Laws Unit Examples

Day 1:

Objective 4.1.1

1. Three boys are fighting over a ball at recess. They apply the following forces, as seen from above. Find the net force on the ball.

2. If a fourth boy were to join in the fight, how hard and in what direction would he have to pull in order for the net force on the ball to be zero?

Homework Chapter 4:

E2–4

Name_______________________________________________ Period___________

Day 2:

Objective 4.1.1

2 3. A given force produces an acceleration of 5 m/s on an object with mass of 1 kg. When an equal force is applied to a carton of ice cream, it produces an acceleration 2 of 11 m/s . What is the mass of the carton of ice cream? 4. You’re stranded in space away from your spaceship. Fortunately, you have a propulsion unit that provides a constant force F for 3 s. After 3 s you have moved 2.25 m. If your mass is 68 kg, find F. Hint: you need to use a kinematics equation to find acceleration 5. A person in an elevator is holding a 10-kg block by a cord rated to withstand a tension of 150 N. When the elevator starts up, the cord breaks. What was the minimum acceleration in of the elevator? 6. An object of mass m is suspended from the ceiling of an elevator that is descending with an acceleration of a. What is the tension in the string?

Homework:

Chapter 2:

Physics Newton’s Laws Unit Examples Q 1, 3, 5; E 5, 7, 11

Days 3 and 4:

Objective 4.6.1

rd 7. A horse refuses to pull a cart. The horse reasons, “according to Newton’s 3 law, whatever force I exert on the cart, the cart will exert an equal and opposite force on me, so the net force will be zero and I will have no chance of accelerating the cart.” What is wrong with this reasoning? 8. An advertisement claims that a particular automobile can “stop on a dime”. What net force would actually be necessary to stop a 850-kg automobile traveling initially at 45.0 km/h in a distance equal to the diameter of a dime, which is 1.8 cm?

Homework: Chapter 4:

Q 6, 10; E 8 – 10, 14, 15, 17; P 26

Day 5: 9. Example: A 52-N sled is pulled across a cement sidewalk at constant speed. A horizontal force of 36 N is exerted. What is the coefficient of kinetic friction between the sidewalk and the metal runners of the sled? 10. On your own: The sled from the example is now on packed snow. The coefficient of friction is 0.12. If a person weighing 650 N sits on the sled, what force is needed to slide the sled across the snow at a constant speed? 11. Example: A runaway baby buggy is sliding without friction across a frozen pond toward a hole in the ice. You race after the buggy on skates. As you grab it, you and the buggy are moving toward the hole at speed v . The coefficient of friction 0 between your skates and the ice as you turn out the blades to brake is  . D is k the distance to the hole when you reach the buggy, M is the mass of the buggy, and m is your mass. What is the lowest value of D such that you stop the buggy before it reaches the hole in the ice?

Homework: Chapter 5:

Q 1, 7; E 2 – 5

Physics Newton’s Laws Unit Examples

Days 6, 7, and 8: Objective 5.3.1 12. Example: A 100-N body is shown suspended from a system of cords. What is the tension in the horizontal cord?

13. On your own: A traffic light is supported by two wires as shown below. Which wire has the greatest tension? Wire 1 Wire 2

14. Example: A skier of mass 65.0 kg is pulled up a snow-covered slope at constant speed by a tow rope that is parallel to the ground. The ground slopes upward at a constant angle of 26° above the horizontal and you can ignore friction. Calculate the tension in the tow rope. 15. On your own: A man is dragging a trunk up the loading ramp of a mover’s truck. The ramp has a slope angle of 20.0° and the man pulls upward with force F whose direction makes an angle of 30.0° with the ramp. How large a force F is required in order for the component F parallel to the ramp to be 28.0 N? How large will x the component F then be? y 16. Example: A 20-kg box rests on a frictionless ramp with a 15° slope. A mover pulls up on a rope attached to the box to pull it up the incline. If the rope makes an

Physics Newton’s Laws Unit Examples angle of 40° with the horizontal, what is the force the mover must exert on the box 2 to give it an acceleration of 1 m/s ? 17. On your own: The system shown below is in equilibrium, the incline is frictionless, and the pulley is massless and frictionless. What is the mass m?

m

3.5 kg

18. Example: Two children sitting on a sled at rest in the snow ask you to pull them. You oblige by pulling on the sled’s rope, which makes an angle of 40° with the horizontal. The children and sled have a combined mass of 50 kg. The coefficients of friction are  = 0.2 and  = 0.15. Find the acceleration of the s k sled and children, if the tension in the rope is a. 100 N b. 140 N

Physics Newton’s Laws Unit Examples 19. On your own: A 5-kg block is held at rest against a vertical wall by a horizontal force. What is the minimum horizontal force needed to prevent the block from falling if the coefficient of friction between the wall and the block is  =0.40? s

Homework: Chapter 5:

Q 1, 9, 11, 13; E 10, 20, 23, 36; P 41, 43, 57

Review: 1. What is equilibrium? 2. What are the units on the coefficients of friction? 3. a) What is the equation for Newton’s second law? b) What are the four kinematics equations for motion with constant acceleration? 4. In emergency situations with major blood loss, the doctor will order the patient placed in the Trendelberg position, which is to raise the foot of the bed to get maximum blood flow to the brain. If the coefficient of static friction is 0.80, what is the maximum angle at which the bed can be tilted with respect to the floor before the patient begins to slide?

5. Block A sits on a table and is connect by a wire over a pulley at the table’s edge to block B which hangs freely over the edge of the table.

Block A has weight wA and the coefficient of kinetic friction between the table and the block is k. Calculate the weight wB of the hanging block required if this block is to descend at constant speed once it has been released. 6. A book whose mass is 2.00 kg is projected up a long 30.0° incline with an initial speed of 16.0 m/s. The coefficient of kinetic friction between the book and the plane is 0.30. a. find the magnitude of the friction force acting on the book as it moves up the plane. b. For how much time does the book move up the plane before momentarily stopping? c. How far does the book move up the plane before momentarily stopping?