Tournament Review Questions Forces 1. Mass and acceleration are proportional to each other when a force is applied. 2. A pilot announces that the 500-kg plane is flying at a constant 900 km/hr. What is the acceleration of the plane? (I bet I get you on this one.) 3. If a 500 kg plane is using 1000 N of thrust to fly through the air, what is the acceleration of the plane if you ignore air resistance? 4. A 500 kg plane is using a thrust of 80,000 N to propel it through the air at a constant speed and altitude. What is the lift force on this plane? (Hint: lift is opposite of what force, what are you trying to overcome?) 5. What two forces are involved when calculating the coefficient of friction between two surfaces? 6. Felicia, the ballet dancer, has a mass of 45 kg. What is her weight on earth? 7. A 20-g sparrow flying towards a bird feeder mistakenly flies into a windowpane. The force of impact was 2 N. What was the bird s acceleration? (Don t forget appropriate units.) 8. Felicia, the ballet dancer, has a mass of 45 kg. If she were on the planet Jupiter and experienced its gravitational pull of 25 m/s 2, what would her mass be? 9. While trying to practice the poise part of a beauty pageant, Ellen walked around the room with a book on her head. Use Newton s 3 rd Law to explain the forces involved concerning the book. 10. Sammy Sosa swings at a 0.15 kg baseball and accelerates it at a rate of 300 m/s 2. How much force was exerted on the ball? 11. What would be the weight of an 8-kg bowling ball? 12. What is the mass of a 100-N crate of delicious candy? 13. While pushing a 10 kg box across the floor at a constant velocity, 100 N of force was used. What is the coefficient of kinetic friction between the floor and the box? 14. A 500-kg subcompact car and a 1500-kg standard car are set in motion by forces so that each car has equal acceleration. How much greater is the force that acts on the more massive car? 15. Heather pulls a wagon with her little brother Bryce in it. The mass of the wagon and Bryce is 48 kg. She pulls the wagon until it reaches a speed of 8m/s then she lets go. Bryce applies the brakes on the wagon and it comes to rest in 6 seconds. What was the braking force of the wagon? 16. Scarlet skydiver, who has a mass of 60 kg, jumps from a helicopter. What is her net force as she emerges from the helicopter? 17. If Scarlet skydiver, who has a mass of 60 kg, jumps from a helicopter, what would be her net force 10 s into the dive when she has reached her terminal velocity? 18. Once Bronco the skydiver reaches his terminal velocity is his acceleration 10 m/s 2 or 0 m/s 2? Explain. 19. Buster gathers two bathroom scales (which only measure in standard units!!) that are in the house, sets them next to each other and then stands on the top of both of them. If Buster s mass is 45 kg, what will be the weight reading on each of the scales? 20. Benji, whose mass is 60 kg, pushes off a wall with a force of 30 N while wearing his fancy new roller blades, which are extra frictionless. What will be his recoil (backward) acceleration?
21. A boxer punches a sheet of paper in midair, and brings it from rest to a speed of 40 m/s in 0.05 seconds. If the mass of the paper is 0.01 kg, what force did the boxer exert on the paper? 22. The family decides to take the boat to the lake for a nice day of water skiing. The boat trailer is hooked up to the car and the car begins to pull the trailer. According to Newton s 3 rd Law of Motion, for every action force there is an equal and opposite reaction force. Thus the car pulls on the trailer with the same force that the trailer pulls on the car. So then why does the trailer move? 23. Jenny pushes a 100 N chair across the floor at a constant speed of 2 m/s. What is the force of friction on the chair? 24. Sally places a cute 8 N kitty cat in Sandy s hands. What is the force pairs involving Sandy s hands? 25. Astronaut Darnell is standing on the nose of the space shuttle in outer space and does a beautiful swan dive off the front. Describe what happens to all things involved. 26. Astronaut Ito s mass on earth is 84 kg. What is his mass and weight in outer space? 27. The coefficient of static friction between ice and a 70-kg figure skater is 0.2. How much force will he need to apply to get moving? 28. A heavy person and a light person jump from a plane at the same time and open their chutes at the same time. Assuming all other factors the same (chute size, same altitudes, etc.), who reaches the ground first? Explain. 29. Two forces act on a book resting on a perfectly flat table: its weight and the normal (support) force from the table. Does a force of friction act as well? Explain. 30. Scrappy wants to amaze his friends by pulling the tablecloth from under the dishes on the table without the dishes moving. You are his physics-inspired expert assistant, what would you suggest and why? 31. A mass is pushes with a force that causes it to accelerate. If the mass is doubled but the force remains constant, what will happen to the acceleration? 32. If air resistance is basically the same for a falling tennis ball and a falling baseball, which will have the greater terminal velocity? Explain why. 33. Does a stick of dynamite contain force? Explain. 34. A 60-kg boulder falls off a cliff and reaches its terminal velocity before impact with the ground. What was the force of air resistance on the boulder when it reached its terminal velocity? 35. Describe one of the force pairs that exist when a rocket is blasting off. 36. If you jump up towards the ceiling, does the Earth move up, down or both? 37. We know that the Earth pulls on the moon. Does the moon pull on the Earth? If so, which is stronger? 38. Identify one set of the action-reaction forces involved when firing a rifle. 39. A 200-kg cannon fires a 20-kg cannonball and recoils with an acceleration of 2 m/s 2. What is the acceleration of the cannonball? 40. A student pushes a book on a lab table with a force of 20 N with no acceleration. If the coefficient of friction between the book and the table is 0.5, what is the weight of the book?
Forces Review Answers 1. inversely (meaning as the mass increases, the acceleration will decrease as long as the force is constant) 2. zero because the speed is constant 3. if you ignore air resistance, then the net force is the thrust force of the plane; a = F/m = 1000 N / 500 kg = 2 m/s 2 4. the weight (downward force) of the plane is 500 kg X 10 m/s 2 = 5000 N, therefore, the lift force is 5000 N so that forces are in balance (constant altitude); you will not use the horizontal force of 80,000N 5. the friction force and the weight of the object 6. 450 N 7. First you have to convert the mass into kg before you can use the formula; 20 g = 0.02 kg; next a = F/m = 2 N / 0.02 kg = 100 m/s 2 8. her mass would be 45 kg, regardless of where she is 9. one of the force pairs would be the weight of the book pushing down (the force due to gravity)(action), the other force is the normal (a normal force is a force that supports an object as it is being pulled by gravity, i.e. your hand holds an apple above the ground) force of her head supporting (or pushing) the book upward above her head(reaction), another set of forces could be the earth pulls on the book (action) and the book pulls on the earth (reaction) 10. F = ma = 0.15kg (300 m/s 2 ) = 45 N 11. 80 Newtons 12. 10 kg 13. First, the horizontal motion is constant so there is no net force; this means the horizontal forces of friction and the applied force are equal. Thus, the applied force of 100N is also the kinetic force of friction (100N). Likewise, there is no vertical motion so the downward force of gravity (the objects weight) is equal to the support force supplied by the floor (normal force). So, w = mg = (10kg)(10) = 100 N. Finally, µ = f / N = 100 N / 100 N = 1 14. since the more massive car has three times the mass, it will require three times the force to achieve equal accelerations 15. F = ma = m ( v/ t) = 48 kg (8 m/s /6 s) = 64 N 16. her net force is her weight (since no air resistance has been encountered yet) which is W = mg = 60 (10) = 600 N 17. if she has reached terminal velocity, her weight (the downward force) will equal air resistance (the upward force) and a net force of zero will result 18. his acceleration will be zero because at terminal velocity, velocity is constant; therefore, no change in velocity is occurring even though he is still plowing through the air 19. Buster s weight = mg = 45 kg (10m/s 2 ) = 450 N, therefore, both scales should read 225 N (as his weight force is divided equally between the two scales) 20. a = F/m = 30 N/ 60 kg = 0.5 m/s 2 21. first find the acceleration of the paper, a = 40 m/s / 0.05 s = 800 m/s 2 ; next, F= ma = (0.01 kg)(800 m/s 2 ) = 8 N 22. although the net force between the car and the trailer may be zero, the more massive car exert more force against the road than the less massive boat trailer, therefore, the overall (net) forces between the car and the road are greater than the
overall (net) forces between the trailer and the road; thus the car wins the tug of war 23. since the chair is moving at a constant speed, acceleration must be zero, meaning the net force applied to the chair is zero; therefore, the opposing friction force must be equal to the pushing force of 100 N 24. the weight of the cat is the downward force and Sandy s hands pushing upward on the cat represents the normal force 25. Darnell is propelled forward into space with a significant acceleration while the space shuttle is propelled in the opposite direction but with a miniscule acceleration because its mass is much larger than Darnell s; however it is important to point out that both objects will move away from each other 26. his mass is the same 84 kg, however his effective weight is zero because there is essentially no earthly gravity in space 27. since the skater is stationary, the force applied to get the skater moving will be equal to the static friction force between the skater and the ice. Also, since the skater is not going up in the air, only forward, we can determine the normal force on the skater by knowing the weight of the skater. The skater s weight is mg = (70kg)(10)= 700 N = normal force, Thus f = µn = (0.2)(700N) = 140 N. This is the force needed to make the skater move on the ice. 28. the heavy person will reach the ground first because the lighter person will reach their terminal velocity sooner, thus the heavier person can continue to increase in speed and slowly pull away from the lighter person 29. no friction force is at play if the book is at rest; however, friction is the reaction force to an action force that tries to create motion, so as soon as the table is tilted or someone pushes on the book, friction would resist the book s motion 30. he should pull the cloth as quickly as possible; based on Newton s 1 st Law of Motion (Law of Inertia), the dishes and such have mass and will want to remain at rest until a force acts upon them; also the heavier the dishes (more inertia), the more likely they will stay in place, so he should get heavy plates and dinner ware 31. the acceleration will be half as much as before 32. although air resistance is equal, the mass of the two object is not; the greater the mass the greater the net force; the greater the net force, the longer the time of acceleration, so the baseball should have the greater terminal velocity 33. unlike mass or energy, force is not something an object can possess; forces are either generated as a field or applied; in either case they are the result of objects interacting 34. when the boulder reaches terminal velocity, the net force on the boulder is zero which means the force of air resistance (upward) is equal to the weight of the boulder(downward); therefore, if the weight of the boulder is (60kg)(10) = 600 N, then the force of air resistance is 600 N 35. action: exhaust gases push on atmosphere, reaction: atmosphere pushes on exhaust (spaceship), action: gravity pulls down on the spaceship, reaction: the spaceship pulls up on the Earth, action: spaceship gases push against the earth, reaction: earth pushes against the spaceship gases/exhaust 36. both, when you accelerate away from the Earth, the Earth accelerates away from you; when the Earth pulls you back down towards the ground, you pull the Earth up towards you (however, because the Earth s mass is so large compared to yours, we never notice any Earthly movement) 37. both object are pulling on each other with an equal force, neither pull is stronger
38. there are several to choose from: the bullet pushes the rifle/rifle pushes the bullet, rifle pushes (recoils) your shoulder, shoulder pushes rifle, explosive gun powder pushes bullet out of the gun, bullet pushes on the exhaust gases 39. according to Newton s 3 rd law the force on the cannon and the cannonball will be equal; therefore, (F = ma), ma (cannon) = ma (cannonball) 200 kg (2 m/s 2 ) = 20 kg (a), acceleration of cannonball is 20 m/s 2 40. in the following formula, f = µn, the f (force of friction) is equal to the applied force since the object is not accelerating; so f = 20 N. Also, the normal force and weight of the book are the same since there is no vertical motion. Therefore, by finding the normal force (N), we ll also be finding the weight (W) of the book). So, to find N we ll divide: N= f / µ = 20 N/ 0.5 = 40 N. So the book weighs 40 N