Chapter 5 Review Questions pp.71-72 #1-18

1. Distinguish between the relationship that defines the acceleration and the relationship that states how it is produced. Definition: acceleration = change in velocity divided by the change in time How is acceleration produced? Acceleration = the net force divided by the mass

2. What is meant by the net force that acts on an object? The net force is equal to the sum of all the forces acting on the object. The net force is the resultant of all the applied forces.

The acceleration will double. 3. Suppose a cart is being moved by a certain net force. If the net force is doubled, by how much does the car’s acceleration change? The acceleration will double. Acceleration is directly proportional to the net force. As forces increases acceleration increases.

The acceleration will be cut in half. 4. Suppose a cart is being moved by a certain net force. If a load is dumped into the cart, so its mass is doubled, by how much does the acceleration change? The acceleration will be cut in half. Mass is inversely proportional to acceleration. So, if more mass is added, the acceleration slows down.

5. Distinguish between the concepts directly proportional and inversely proportional. Support your statement with examples. Directly: as one variable increases the other increases by the same factor. Inversely: as one variable increases the other decreases by the same factor. Examples: the force applied to a cart doubles, therefore the acceleration doubles Example: the mass of a cart doubles, therefore the acceleration decreases by half.

6. State Newton’s second law in words and then in the form of an equation. The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force and is inversely proportional to the mass of the object.

7. How much force does a 20 000 kg rocket develop to accelerate 1 m/s2? 20, 000 N

8. What is the cause of friction, and in what direction does it act with respect to the motion of a sliding object? Friction is caused when two surfaces come into contact with one another. The irregularities on the surfaces rub against one another and friction results. Friction opposes motion. That is to say, friction acts in the direction opposite to motion.

100 N of force must be applied to maintain constant velocity. 9. If the force of friction acting on a sliding crate is 100 N, how much force must be applied to maintain a constant velocity? What will be the net force acting on the crate? What will be the acceleration? 100 N of force must be applied to maintain constant velocity. The system is in equilibrium so the net force acting on the crate is zero. There is no acceleration because there is constant velocity and a net force of zero.

10. Distinguish between force and pressure. Force, simply stated, is the amount of push or pull on a system. Pressure is the amount of force encountered over a certain area.

11. Which produces more pressure on the ground, a person standing up or the same person lying down? The person standing up produces more pressure on the ground because when you stand on two feet the surface area in contact with the ground is less. So, if the same amount of force is applied to less area the pressure is greater. When you are lying down there is more surface area in contact with the ground. So the same amount of force is applied to a greater area, and the pressure is less.

12. The force of gravity is twice as great on a 2-kg rock as on a 1-kg rock. Why does the 2-kg rock not fall with twice the acceleration? Neglecting air resistance, a 2 kg rock will not fall with twice the acceleration because twice the force acts on twice the mass.

Neglecting air resistance, they have the same weight to mass ratio. 13. Why do a coin and a feather in a vacuum tube fall with the same acceleration? Neglecting air resistance, they have the same weight to mass ratio. a = F/M a = f/m

14. Why do a coin and a feather fall with different accelerations in the presence of air? Because air resistance has more of an effect on the feather, thus causing it to reach terminal velocity sooner. The air resistance builds up quickly and counteracts its tiny weight.

Remember that air resistance is a fluid frictional force. 100 N 15. How much air resistance acts on a 100-N bag of nails that falls at its terminal speed? Remember that air resistance is a fluid frictional force. 100 N When an object reaches terminal speed the net forces are equal to zero.

16. How do the air resistance and the weight of a falling object compare when terminal speed is reached? Air resistance and weight are equal to one another when terminal speed is reached

A greater weight is more effective at “plowing through” the air. 17. All other things being equal, why does a heavy sky diver have a terminal speed greater than a light sky diver? What can be done so that the terminal speeds are equal? Terminal speed is when the air resistance acting on an object equals the weight of the object. Lighter objects obtain terminal speed sooner than heavier objects. A greater weight is more effective at “plowing through” the air. Terminal speeds can be equalized if the heavier skydiver orientates his/her body to encounter more air resistance. Spreading out your body’s surface area (wing suit flying).

Zero, terminal velocity is reached 18. What is the net force acting on a 25-N freely falling object? What is the net force when the object encounters 15 N of air resistance? When it falls fast enough to encounter 25 N of air resistance? 25 N downward 10 N downward Zero, terminal velocity is reached