Chapter 6 Test

For each of the following situations, identify what creates one of the forces that creates the motion described (there may be many). a. A flag flaps back and forth at the top of a flagpole. b. A soccer ball is passed from one player to another. c. A large piece of hail falls toward the ground. d. The ocean tide goes from high to low at the seashore (you might have to do a little research to get this one if you don't know already).

Answers are: a. wind; b. the foot kicking the ball; c. gravity; d. gravitational pull of the Moon and the Sun on Earth.

What are the natural states of motion? List all correct answers. a. Being stopped b. Moving with constant direction c. Moving with changing speed d. Moving with constant velocity

The answers are a and d.

A tow truck pulls a 1,500-kilogram car with a net force of 4,000 newtons. What is the acceleration of the car?

a=F m=(4,000N) (1,500kg)=2.67m/s2

Objects with more mass have more ____.

inertia

If the net force acting on an object is not zero, then the forces acting on the object are ____.

unbalanced forces

What conditions are necessary for acceleration to occur?

For acceleration to occur there must be unbalanced forces acting on an object to cause it to change its speed or direction.

Forces contribute to the net force on a car rolling down a ramp. a. Which force supports the car's weight? b. Which force accelerates the car down the ramp? c. Which force acts against the motion of the car?

(a) the normal force; (b) gravity; (c) friction

Explain how changing force or mass affects the acceleration of an object. Provide one example to support your answer.

Acceleration is directly proportional to force and inversely proportional to mass. For example, an empty pickup truck will accelerate faster than a full pickup truck. The increasing mass decreases the acceleration of the truck.

While an object is moving at a constant 20 m/s, a 5 N force pushes the object to the left. At the same time, a 5 N force is pushing the object to the right. What will the object's velocity be after 10 seconds?

20 m/s; since the net force is zero, the object continues moving at a constant velocity.

Identify whether the following scenarios involve balanced (B) or unbalanced (U) forces. a. ____ A car stopped at a red light. b. ____ A ball rolling down a hill. c. ____ An airplane flying at constant speed at the same altitude in one direction. d. ____ An airplane taking off. e. ____ A person sitting motionless on a chair. f. ____ A person running at constant speed around a circular track.

Answers: (a) B; (b) U; (c) B; (d) U; (e) B; (f) U

Explain the difference between "directly proportional" and "inversely proportional".

Directly proportional means that if you increase one variable, the other variable increases by the same factor. Inversely proportional means that if you increase one variable, the other variable decreases by the same factor.

How much force would you need to cause a 20-kilogram object to accelerate in a straight line to 20 m/s2?

F=a m;F=20m/s2 20kg=400kg-m/s2=400N

The momentum of an object depends on what two factors?

Momentum depends on the mass of an object and its velocity

____ says that objects continue the motion they already have unless they are acted on by an unbalanced force.

Newton's first law

An experiment measures the speed of a 250-kilogram motorcycle every 2 seconds (Figure 6.9). The motorcycle moves in a straight line. What is the net force acting on the motorcycle?

The acceleration of the motorcycle is 2.5 m/s . The mass is 250 kg. F=a m;F=2.5m/s2 250kg=kg-m/s2 =625N

Imagine whirling a ball on a string over your head. Suppose the knot holding the ball comes loose and the ball is instantly released from the string. What path does the ball take after leaving the string? Use Newton's first law to explain your answer.

The answer is (D). The ball would follow a straight line of motion once the string broke.

The engine of a jet airplane pushes exhaust gases from burning fuel backward. What pushes the jet forward?

The exhaust gases and the plane are an action-reaction pair. The exhaust gases push back on the plane and cause it to move forward.

You push backward against the ground to move a skateboard forward. The force you make acts against the ground. What force acts against you to move you forward?

The ground is pushing forward against your feet. At the same time, the skateboard is pushing on the ground and the ground is pushing on the skateboard. The skateboard moves forward because the ground is pushing your foot harder than the ground is pushing back on the skateboard. The unbalanced forces cause the skateboard to move forward.

A potato launcher uses a spring that can apply a force of 20 newtons to potatoes. A physics student launched a 100-gram potato, a 150-gram potato, and a 200-gram potato with the launcher. Which potato had the greatest acceleration?

The potato with the least mass will have the acceleration. The answer is the 100-gram potato.

What are the three main ideas associated with Newton's second law of motion? List these in your own words.

Three main ideas related to the second law are: that unbalanced forces result in acceleration, that force and acceleration are proportional, and that mass and acceleration are inversely proportional.

Give an example of the law of conservation of momentum from everyday life.

Whenabowlingballhitsastationarybowlingpin,theball slows down (loses momentum) because it has more inertia, and the pin moves quickly (gains the momentum lost by the bowling ball as long as friction is not considered).

The law of conservation of ____ can be used to predict motion of interacting objects after they collide.

momentum

In the following situation, which diagram (A, B, C, or D) best illustrates the net force experienced by the cart when the weight pulls downward?

A

Which has more inertia—a shopping cart full of groceries or an empty shopping cart?

A shopping cart full of groceries has more inertia.

Different forces are applied to cars of different masses. The acceleration is measured for each combination of force and mass. Graph the data and determine the acceleration. Force goes on the y-axis and mass goes on the x-axis. Be sure to label each axis and give your graph a title.

Acceleration is 5 m/s2 or 5 N/kg. Graph: Technology Mass transportation

Explain why action-reaction forces do not cancel each other out, resulting in zero net force.

Action-reaction forces do not cancel each other out because they work on different objects.

Emilio tries to jump to a nearby dock from a canoe that is floating in the water. Instead of landing on the dock, he falls into the water beside the canoe. Use Newton's third law to explain why this happened. Hint: First identify the action-reaction pair in this example.

The action force of Emilio pushing against the canoe creates a reaction force that pushes the canoe away from Emilio. The action and reaction forces are equal in strength and opposite in direction. Emilio clearly misjudged the effect of the action/reaction forces. If the canoe had held still, he might have landed on the dock!

Are these statements correct or incorrect? If incorrect, rewrite the sentence so that it is correct. a. In an action-reaction pair the forces work on the same object. b. Every action force creates a reaction force and the two forces are different in strength but act in the same direction.

a. The statement is incorrect. in an action-reaction pair the forces work on different objects. b. The statement is incorrect. Every action force creates a reaction force and the two forces are equal in strength and opposite in direction.

One kilogram-meter per second squared is also equal to what unit?

1 kg-m/s2 = 1 newton

The relationship between the force on an object, the mass of the object, and its acceleration is described by ____.

Newton's second law

____ states that every action force creates a reaction force that is equal in strength and opposite in direction.

Newton's third law

When a bug traveling west collides with the windshield of a car traveling east, what can be said about the collision? a. The bug feels a stronger force than the car. b. The bug and the car feel the same size force. c. The car accelerates more than the bug. d. The bug does not accelerate due to the force.

The answer is b.

A small rubber ball is thrown at a heavier, larger basketball that is still. The small ball bounces off the basketball. Assume there are no outside forces acting on the balls. a. How does the force on the small ball compare to the force on the basketball? b. Compare the total momentum of the two balls before and after the collision. c. The mass of the basketball is 600 grams and its velocity before the small ball hits is 0 m/s. The mass of the small ball is 100 grams and its velocity is +5 m/s before the collision and -4 m/s afterward. What is the velocity of the basketball after the collision?

a. Due to Newton's third law which states that forces come in action- reaction pairs and the forces are equal and opposite in direction, the balls will experience the same amount of force.b. Due to the law of conservation of momentum, the total momentum of the two balls before the collision equals the total momentum after the collision (as long as we do not consider any outside forces).c. Let the velocity of the basketball after the collision be v1. [600 kg (0 m/s)] + [(100 kg) +5 m/s] = [600 kg (1)] + [(100 kg) -4 m/s] 500 kg-m/s = 600kg (v1) + -400 kg-m/sc. Let the velocity of the basketball after the collision be v1. [600 kg (0 m/s)] + [(100 kg) +5 m/s] = [600 kg (1)] + [(100 kg) -4 m/s] 500 kg-m/s = 600kg (v1) + -400 kg-m/s

a. b. What is the car's acceleration? If the car had started at 29 m/s and ended at 5 m/s after 4 seconds, what would its acceleration be? How is this different from the answer above?

a. a=changeinv changeint=(29-5m/s) (4s)=6m/s2 b. -6 m/s2; equal in strength but opposite in direction

A 2-kilogram rabbit starts from rest and is moving at 6 m/s after 3 seconds. What net force must be exerted on the rabbit (by the ground) to cause this change in speed? (Figure 6.8)

a=changeinv changeint=(6m/s) (3s)=2m/s2 F=a m;F=2m/s2 2kg=kg-m/s2=4N The ground would need to exert 4 N of force on the rabbit.