Physics 11 - Chapter 4 FORCES 4.5 Using Newton s Law (Part 2)

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1 Name: Date: Physics 11 - Chapter 4 FORCES 4.5 Using Newton s Law (Part ) Q- A man wants to test a rope. He ties one end to a telephone pole and the other to a horse and makes the horse pull as hard as it can. It is not quite strong enough to break the rope. The man brings in a second horse of identical strength to take the place of the telephone poll. Will the rope break when the two horses pull in opposite directions as hard as they can? Explain your answer. Q- When a 0.0 kg child steps off a 3.0 kg stationary skateboard with an acceleration of 0.50 m/s, with what acceleration will the skateboard travel in the opposite direction? {The Net Force Causes Acceleration} In Newton s nd Law, F = ma, the force F that causes the mass to accelerate is the force acting on the mass. * The force is the vector sum of the applied and frictional forces. We must pay attention to the signs! Ex: Consider a 10.0 kg stone lying on the ground. The stone is at rest; the net force on it is zero. How can the stone (10 kg) be given an upward acceleration? Suppose a person exerts a 148 N upward force on the stone. {The Fall of Bodies in Air} Why do all objects free fall at the same rate of acceleration regardless of their mass? Is it because they all weigh the same?... Because they all have the same gravity?... Because the air resistance is the same for each? Why?

2 Consider the free-falling motion of a 1000-kg baby elephant and a 1-kg overgrown mouse. In the absence of air friction, all objects fall with the same. (Galileo) Earth Moon - Falling with Air Resistance As an object falls through air, it usually encounters some degree of air resistance. Air resistance is the result of collisions of the object's leading surface with air molecules. The actual amount of air resistance encountered by the object is dependent upon a variety of factors Q- In the diagrams below, free-body diagrams showing the forces acting upon an 85-kg skydiver (equipment included) are shown. For each case, use the diagrams to determine the net force and acceleration of the skydiver at each instant in time.. The diagrams above illustrate a key principle. As an object falls, it picks up speed. The increase in speed leads to an increase in the amount of air resistance. Eventually, the force of air resistance becomes large enough to balances the force of gravity At this instant in time, the net force is 0 Newton; the object will stop accelerating. The object is said to have reached a terminal velocity. We often encounter air resistance! (Drag force) It is like a friction force! If we sky dive from a plane we will start to accelerate! As our velocity the air friction After a certain amount of time our = Our velocity becomes constant this is called Homework: Read pgs , Do pg 10 #13-16

3 Name: Blk: Newton s nd Law assignment ( /36) Please solve the following problems giving the equation, the number substitution, and your circled final answer to the correct number of significant figures with the proper units! 1. A rocket which weighs 7840 N on Earth is fired. The force of propulsion is N. Determine: (1 mark each) (a) The mass of the rocket. (b) The upward acceleration of the rocket. (c) The velocity of the rocket at the end of 10.0 seconds.. A person weighing 490 N stands on a scale in an elevator. (1 mark each) (a) What does the scale read (in Newtons) when the elevator is at rest? (b) The elevator starts to go up and accelerates the person upward at.0 scale read now (in Newtons)? m / s. What does the (c) What is the reading (in Newtons) on the scale when the elevator is rising uniformly? (d) The elevator begins to slow down as it reaches the proper floor. Do the scale readings increase or decrease? (e) The elevator starts to accelerate down. Do the scale readings increase or decrease? (f) What does the scale read if the elevator descends at a constant speed? (g) If the cable snapped and the elevator fell freely, what would the scale read now? 3. What is the force needed to accelerate a 60.0 kg wagon from rest to 5.0 m/s in.0 s?

4 4. A frictionless wagon going.5 m/s is pushed with a force of 380 N and its speed increases to 6. m/s in 4.0 s. What is its mass? 5. Calculate the force that a pitcher exerts on a baseball whose mass is 0.14 kg if the acceleration is 30 m / s. 6. What acceleration would be given to a 7.5 kg bowling ball being swung with a propelling force of 10 N? 7. What force is required to accelerate a bicycle of 103 kg (including the rider) at 1.5 m / s? 8. A net force of 6.4 N accelerates an object at 10.8 m / s. What is the mass of the object? 9. What is the mass of a frictionless sled that will be accelerated at 3.0 m / s by a force of 130 N? 10. According to a simplified model of a mammalian heart, during each pulse approximately 0.00 kg of blood is accelerated from 0.5 m/s to 0.35 m/s in a time of 0.10 s. What is the magnitude of the force exerted by the heart muscle? 11. What force must the brakes and tires apply to a 800 kg truck going 30 m/s to bring it to rest in 8.0 s? 1. A 100 kg car traveling at 10 m/s experiences an air resistance of 5000 N and a road friction of 00 N. If the wheels push with a force of 9000 N, what is the car s acceleration? 13. A 1500 kg car accelerates at 3.0 m / s. If the motor pulls the car with a force of 6000 N, what is the coefficient of friction of the road? 14. A rock is dropped. How fast will the 5.0 kg rock be traveling in 4.0 s if a 5.0 N force of air resistance acts on it?

5 15. A 5.0 kg rock is sinking in water at a constant speed of 5.0 m/s. What is the upward force of the water on the stone? 16. At what rate will a 5.0 kg box accelerate if a 15 N force is applied to it and the friction opposing it is 3.7 N? 17. What thrust is needed to fire a 350 kg rocket straight up with an acceleration of 8.0 m / s? 18. A 45 kg piece of plywood falls off a 7 m high building under construction. The average force of air resistance is 15 N. (a) What is the weight of the piece of plywood? (b) What is the net downward force on the plywood? (c) What is the net acceleration of the piece of plywood? (d) How long will it take the plywood to fall to the ground? 19. A boy and girl on a sled have a combined mass of 97 kg. They have just come down a hill and are traveling horizontally across the snow at 6.5 m/s. The coefficient of friction between the sled and the ground is 0.5. (1 mark each) (a) What is the friction force on the sled? (b) What is the acceleration of the sled while traveling horizontally?

6 (c) How long will it take the sled to stop? (d) How far will the sled travel before coming to rest? 0. A girl weighing 600 N wishes to reach the ground from a tree top by sliding down a rope. The maximum upward force that the rope can exert without breaking is 300 N. (1 mark each) (a) Can the girl slide down at a constant speed? Explain. (b) What is the least acceleration with which the girl can slide down the rope without breaking it? 1. A.0 kg frictionless puck on a level table is attached by a nylon thread over a frictionless pulley to a hanging mass of.0 kg. What will the acceleration of the puck be? Assorted Answers: m / s forward, N backwards, 0.00 N, 43 kg,.44 kg, 19 N forward, 16 / s m / s down, 5 m / s down, 8.6 m,.7 s,.5 / s m/ s forward, 3 N forward, 410 kg, 150 N forward, 0.0 N, 0.1, 5.4 s, 5.0 m down, 30 N down, 440 N down, 600 N up,.3 m / s forward, 49 N up, 35 m/s down, 40 N backward, 4.9 m backward, 800. kg, 3.5 m / s up, 3.5 m/s up, 490 N, 590 N, 490 N, 490 N