Solutions to Additional Problems for Newton's Laws

1. 60,000 N
2. 150 N, yes the bag lifts it without breaking.
3. The elevator accelerates him upward at 2.5 m/s2, then travels with a constant velocity, and then slows while traveling upward at a rate of 3 m/s2.
4. 5 m/s2
5. 133m/s2, 3,325 N, 250 N, unlikely - the driver would have to exert a force that is 13 x greater than the child's weight and react in a tenth of a second
6. At all times the propeller pushes backwards on the water and the water pushes forward on the propeller. There is also a big resistant force on the front of the boat: the boat pushes the water forward and the water pushes the boat backwards. When the boat travels at constant speed (and with no acceleration) the forces between the propeller and the water must be equal to the resistive force on the front of the boat. When the boat is accelerating the propellor must push harder causing the water to push harder on the back of the boat and producing a net force forward on the book. When the propeller is turned off, the boat slows down, because there is now a net force backwards afainst the velocity. This occurs bbecause the re is still warer resistance pushing backward on the boat, but no longer a forward force due to the propeller.

7. While it is true that the forces between the propeller pushing backward on the water and the water behind the boat (propeller) pushing forward on the boat (propeller), these forces do not cancel each other resulting in zero net force. These forces don't cancel because they are on different things - the water and the boat. Similarly, the water pushes the boat backward when the boat pushes the water forward as it pushes through the water. These forces also don't cancel each other becasue they are on different objects - the water and the boat.. The boat travels at constant speed, because the two forces on the boat (from the water resisting in front, and from the water pushing on the propeller in the back) are qual and opposite to each other.

8. If the earth pulls on a satellite with 1000 N then the satellite pulls on the earth with 1000 N of force. The force does not affect the path of the earth through space because the acceleration resulting from 1000N divided by the enormous mass of the earth is next to nothing.



9. 200 N (forces are balanced becuase the velociy is constant) The friction force is equal and opposite to the push resulting in 0 net force, but it is not the reaction force. The reaction force to your pushing the refrigerator is the refrigerator pushing on you. The reaction force to the floor pushing on the refrigerator with friction is the refrigerator pushing on the floor.



10 When you weigh yourself on a scale, the scale pushes up on you as hard as you push down on it. Normally you push down on the scale with as much force as the earth pulls down on you with gravity, thus normally the scale reading is the same as your weight, the force due to gravity. If though, you push up on the bottom of the sink then you will push into the scale with more force, the scale pushes up with you for more force and the reading goes up. Similarly if you push down on the top of the sink then the sink will push up on you, supporting some of your weight and the scale will support less of your weight. You will push down less on the scale and the scale will push up on you less.

11. At a point halfway up the ball is moving upward because its inertia is causing it to continue the motion it has. It is slowing down because there is a single, unbalanced force on it - gravity - which is producing a downward acceleration against the velocity. On the way down, there is still a single force, gravity, that is now causing it to accelerate downward.

12. a. ball bumps head, bug hits windshield, ball hits bat, nose touches hand, flower pulls hand, bar pushes athlete down, balloon pushes air inward.

11.

Physics Online Tutorial

This looks like a great site for conceptual expanations. It also has problems with answers.
The Physics Tutorial.

For review over Thanksgiving holidays, you should look at the section called Newton's Laws.

Answers to physics homework due 11/7

Worksheet:
1. 2.5 m/s2
2. a. 50 N, b. 2.5 N c.
3. 0.3 m/s2
4. 600N
5. 15 m/s2, 30 m/s, 30 m

Book
25. 6300 N
27. 250 kg
29 10.5 m/s2 down
32 300 N, no
36 a. 1.04 m/s2, -0.633 m/s2,, c stopping