Physics Conceptual Questions Chapter 6

13. Boxes A and B both remain at rest. Is the friction force on A larger than, smaller than, or equal to the friction force on B? Explain. Box A: m=20 kg, F=30N, coefficient of static friction=0.4 Box B: m=10kg, F=30N, coefficient of static friction=0.5

Calculate Fmax FmaxA=usmBg=0.4*20*10=80 N FmaxB=usmAg=0.5*10*10=50 N The force applied on both of the boxes is 30N so neither box will move. Therefore, the friction force on A is equal to the friction force on B

7c. True or False: Mass and weight describe the same thing in different units.

False. Mass is a property of matter, and weight is a FORCE

7a. True or False : The mass of an object depends on its location

False. The mass of an object is a measure of its inertia, a property which is the same regardless of location. You have the same mass on Earth and on the Moon.

15. A block pushed along the floor with velocity vox slides a distance d after the pushing force is removed. a. If the mass of the block is doubled but its initial velocity is not changed, what distance does the block slide before stopping? b. If the initial velocity is doubled to 2vox but the mass is not changed, what distance does the block slide before stopping?

Ff=uN=umg a=ug v^2=vo^2+2ad => d=(vo)^2/(2a) a) d1=d=(vo)^2/(2a) = d b) d2=d=(2vo)^2/(2a) = 4d

12. A hand presses down on the book in fig. 6.12. Is the normal force of the table on the book larger than, smaller than, or equal to mg?

If normal force is N and some force by hand pressing is F then from equilibrium of book will be greater than mg. N-mg-F=0 N=mg+F

3. Kat, Matt, and Nat are arguing about why a physics book on a table doesn't fall. According to Kat, "Gravity pulls down on it, but the table is in the way so it can't fall." "Nonsense," says Matt. "An upward force simply overcomes the downward force to prevent it from falling." "But what about Newton's first law?" counters Nat. "It's not moving so there can't be any forces acting on it." None of the statements is exactly correct. Who comes closest, and how would you change his or her statement to make it correct?

Kat is closest. Kat: Gravity is in the downward direction on the book and the table pushes up on the book with same magnitude. NET FORCE is zero. Matt: Upward force overcoming a downward force would mean the book is flying. Should have said the upward forces EQUAL the downward forces so the NET force is zero Nat: Not moving does not mean that there are no forces acting - the sum of all the forces (net force) is zero

2. A ball tossed straight up has v=0 at its highest point. Is it in equilibrium? Explain.

No, speed is constantly changing. The velocity is momentarily zero but it will not stay.

4. If you know all of the forces acting on a moving object, can you tell the direction the object is moving? If yes, explain how. If no, give an example.

No. Example: baseball player sliding to first base. No force is pushing him forward (in the direction he is moving) when he begins to slide. Friction between him and the ground is a force acting in the OPPOSITE direction of his motion (causing him to slow down and eventually stop moving). Example of net force acting in one direction and the motion being opposite of this direction. Therefore, cannot tell the direction of motion just by knowing the net force.

16. A crate of fragile dishes is in the back of a pickup truck. The truck accelerates north from the stop sign, and the crate moves without slipping. Does the friction force on the crate point north or south? Or is the friction force zero? Explain.

North. The only force acting on the crate on the horizontal plane is the friction force, which, since it provides the acceleration due North, must point due North.

10. Suppose you attempt to pour out 100g of salt, using a pan balance for measurements, while in a rocket accelerating upward. Will the quantity of salt be too much, too little, or the correct amount? Explain.

Pan balance measures MASS, not weight. Therefore, the quantity of salt you pour out will be the correct amount because gravity is not taken into consideration as you are not calculating weight.

11. An astronaut orbiting the earth is handed two balls that have identical outward appearances. However, one is hollow while the other is filled with lead. How can the astronaut determine which is which? Cutting or altering the balls is not allowed.

The astronaut can push the two balls and determine which ball was harder to push (which one required the largest force to obtain the same acceleration) This ball will be the filled one, since it has a greater mass.

9. The four balls in Fig. 6.9 have been thrown straight up. They have the same size, but different masses. Air resistance is negligible. Rank in order, from largest to smallest, the magnitude of the net force acting on each ball. Some may be equal. Give your answer in form a>b>c=d and explain your ranking. Ball A: v=5 m/s, m=200g. Ball B: v=4m/s, m=300g. Ball C: v=3m/s, m=300g. Ball D: v=3m/s, m=400g.

The same force acting on the ball: gravity=9.8m/s^2. So F=mg therefore, d>c=b>a.

5. An elevator, hanging from a single cable, moves upward at constant speed. Friction and air resistance are negligible. Is the tension in the cable greater than, less than, or equal to the gravitational force on the elevator? Explain. Include a free-body diagram as part of your explanation.

The tension in the cable is equal to the force of gravity, since the net force must be zero in order for the elevator to move with constant speed. (FBD pointing up (T) and pointing down (Fg=mg) with the same magnitude)

6. An elevator, hanging from a single cable, moves downward and is slowing. Friction and air resistance are negligible. Is the tension in the cable greater than, less than, or equal to the gravitational force on the elevator? Explain. Include a free-body diagram as part of your explanation.

The tension must be greater than the weight, because the difference in these forces will provide the acceleration. It is clear, that since the elevator is going down and slowing down, the acceleration points upwards. Therefore, the greater force is the tension force. (FBD pointing up (T) and pointing down (Fg=mg) but the Tension arrow has a greater magnitude as the answer describes)

7b. True or False: The weight of an object depends on its location.

True. The weight of an object is a measurement of how much force an object presses down on a surface with. It varies depending on location and whether the object is accelerating. Your weight is not the same on Earth as it would be on the Moon.

8. An astronaut takes his bathroom scale to the moon and then stands on it. Is the reading of the scale his weight? Explain.

Yes, the scale shows his weight on the Moon. Because the force of gravity is different from the Moon and the Earth, the astronaut will not have the same weight but he will still have weight.

1. Are the objects described here in equilibrium while at rest, in equilibrium while in motion, or not in equilibrium at all? Explain. a. A 200 pound barbell is held over your head. b. A girder is lifted at constant speed by a crane. c. A girder is being lowered into place. It is slowing down. d. A jet plane has reached its cruising speed and altitude. e. A box in the back of a truck doesn't slide as the trunk stops.

a. Static equilibrium. No acceleration, velocity zero. b. Dynamic equilibrium. No acceleration, but the girder has a nonzero constant velocity. c. Not in equilibrium. Slowing down means there is acceleration in the direction opposing motion (deceleration, negative acceleration) d. Dynamic equilibrium. No acceleration but the plane has a nonzero constant velocity. e. Not in equilibrium. The box slows down as the truck does, so has a nonzero (negative) acceleration

14. Suppose you push a hockey puck of mass m across frictionless ice for 1.0s, starting from rest, giving the puck speed v after traveling distance d. If you repeat the experiment with a puck of mass 2m, pushing with the same force, a. How long will you have to push for the puck to reach the same speed v? b. How long will you have to push for the puck to travel the same distance d?

a. use equations of motion and newton's second law: t=2s b. use equation of motion: t=sqrt(2)s

17. Five balls move through the air as shown in Fig. 6.17. All five have the same size and shape. Air resistance is not negligible. Rank in order, from largest to smallest, the magnitudes of the accelerations aa to ae. Some may be equal. Give your answer in the form a>b=c>d>e and explain your ranking. Ball A: m=50g, vy=0, just released Ball B: m=100g, vy=0, just released Ball C: m=50g, vy=-20m/s Ball D: m=100g, vy=-20m/s Ball E: m=50g, vy=20 m/s

ad>ae=ac>aa=ab