Why does the bowling ball (pendulum) at the front of the room constantly move? Does that happen because of the earth's rotation? If not, why?
The pendulum moves for two reasons. First, it retains energy from the last time it was touched. This energy shifts back and forth between two forms, kinetic and gravitational potential, as the ball swings. While air resistance and friction in the support wire gradually slow the ball's motion, it takes an extremely long time to stop completely. Second, the ball can be made to swing by sympathetic vibration-any motion of the room that occurs at the ball's natural resonance frequency will tend to cause the ball to swing at that frequency. This type of energy transfer occurs whenever an object in your room begins to buzz when the stereo plays a particular note. In this case, the frequency of motion is way too slow for you to hear.
If you dropped a bullet and at the same time, fired a bullet directly at the ground, wouldn't the bullet fired at the ground hit the ground first?
Sure it would. The fired bullet will only hit the ground at the same time as the dropped bullet if the fired bullet is shot exactly horizontally. If you fire the bullet at the ground, then it starts out with an enormous downward component to its velocity. The falling bullet doesn't have this initial downward component to its velocity and never catches up.
Is one's weight downward the same force as gravity?
Yes, you weight is the force that gravity exerts on you due to your proximity to the earth.
We learned about opposite forces and how one acts to equal out the other. What is involved in a football player who is running being tackled by another player?
If the two players collide hard, they will both exert enormous forces on one another. The player running toward the right will experience a force to the left and will accelerate backward (slowing down). The player running toward the left will experience a force to the right and will accelerate backward (slowing down). The forces involved would cause bruises if they weren't wearing pads. The pads reduce the magnitudes of the forces on their skin by prolonging the accelerations (smaller forces exerted for longer times). If one player simply trips up the other player, then the player who falls will still come to a stop. However, that player will experiencing most of the stopping force from the ground by way of sliding friction.
What's going on with the wheels when a car accelerates?
As a car heads forward, its freely-turning wheels begin to rotate. The torque that starts them rotating comes from static friction with the ground. The ground pushes backward on the bottoms of the wheels to keep them from sliding and this backward frictional force exerts a torque on the wheels. They begin to rotate so that their bottom surfaces head backward and their top surfaces head forward. The car's powered wheels turn for a different reason: they are driven by a torque from the car's engine. As you step on the accelerator, the engine exerts a torque on the wheels and they begin to turn. They would skid backward across the ground where it not for static friction between the wheels and the ground. This static friction opposes the skidding by exerting a forward force on the bottom surface of the wheels. This static frictional force produces a torque on the wheels and that torque partly balances the torque from the engine. The wheels don't skid. However, the forward frictional force on the wheel's bottom surface isn't balanced elsewhere in the car and the car accelerates forward.
Please explain how a hockey stick is a 3rd class lever.
When you swing a hockey stick, the effective pivot is somewhere near the stick's handle and the effort (the force causing the stick undergo angular acceleration) is only a short distance down the stick away from the handle. The business end of the stick, where it hits the puck, is a long way away from the pivot.
In the "Check Your Understanding" #5, in Section 1.3, I'm having trouble visualizing the explanation.
When you squeeze the two handles of a pair of scissors together, you produce torques on the blades. The blades undergo angular accelerates in opposite directions and their cutting edges rotate toward one another. Anything that you put between those cutting edges will also exert torques on the blades but these torques will tend to make them undergo angular acceleration the wrong way; the cutting edges will rotate away from one another. To minimize this last effect, you put paper or cardboard deep into the cutting blades, near the pivot, so that they exert as little torque on the blades as possible. That way, the blades can rotate their cutting edges together with as little opposition as possible.
In the "Check Your Figures" #2, in Section 1.3, I'm unclear at how adding length to a screw would help.
This problem suggests adding length the wrench that you're using to loosen a screw. Increasing the length of wrench allows you to exert an extremely large torque on a screw. By moving the point at which you push on the wrench away from the pivot (the screw itself), you lengthen the lever arm and increase the torque you're producing. The only disadvantage is that you will have to move that long handle a large distance to make the screw turn even a little bit.
