Chapter 4

Three identical blocks are pulled as shown, on a horizontal frictionless surface. If tension in the rope held by the hand is 30N, what is the tension in the other ropes?

10N 20N

Which contains more apples, a 1 pound bag of apples on Earth or a 1 pound bag of apples on the moon? Which contains more apples a 1 kilogram bag of apples on Earth or a 1 kilogram bag of apples on the moon?

1lb bag of apples on the moon contains more apples because gravity on earth is much more than the gravitational pull on the moon. They both have the same number of apples because kg-mass so they have the same mass and number of apples

A grocery bag can withstand 300N of force before it rips apart. How many kilograms of apples can it safely hold?

30.6 kg

A 400 kg bear grasping a vertical tree slides down at a constant velocity. What is the friction force that acts on the bear?

4000N

What is the net force acting on a 1 kg ball in free fall?

9.8N

On a long alley, a bowling ball slows down as it rolls. Is any horizontal force acting on the ball? How do you know?

A force opposite the ball is acting - likely air resistance and friction.

When a car is moving in reverse, backing from a driveway, the driver applies the brakes. In what direction is the car's acceleration?

Acceleration is opposite to velocity (direction car is moving).

Aristotle claimed that the speed of a falling object depends on its weight. We now know that objects in free fall, whatever their weights, undergo the same gain in speed. Why does weight not affect acceleration.

Acceleration is the ratio force/mass which in free fall is just weight/mass. Since weight is proportional to mass, the ratio weight/mass is the same whatever the weight of a body.

Upon which air resistance be greater - a sheet of falling paper or the same paper wadded into a ball that falls at a faster terminal speed?

Air resistance will be the same on each

A parachutist after opening her parachute, finds herself gently floating downward no longer gaining speed. She feels the upward pull of the harness while gravity pulls her down. Which of these two forces is greater? Or are they equal in magnitude.

Both forces have the same magnitude.

Consider a heavy crate resting on the bed of a flatbed truck. When the truck accelerates, the crate also accelerates and remains in place. Identify and discuss the force that accelerates the crate.

Friction is the force that keeps the crate picking up the same amount of speed as the truck. With no friction, the accelerating truck would leave the crate behind.

A shooting star is usually a grain of sand from outer space that burns up and gives off light as it enters the atmosphere. What exactly causes this burning?

High speed grains of sand grazing the Earths atmosphere burn up because of friction against the air.

Your empty hand is not hurt when it bangs lightly against a wall. Why does your hand hurt if it is carrying a heavy load? Which of Newton's laws is most applicable here?

If you are carrying a load then the mass that needs to stop is larger than when the hand was empty. Newton's Second Law

When blocking in football, why does a defending lineman often attempt to get his body under that of his opponent and push upward? What effect does this have on the friction force between the opposing lineman's feet and the ground?

It lessens the impact of his weight when he is parallel to the ground.

Why is a massive cleaver more effective for chopping vegetables than an equally sharp knife?

Its greater mass contributes to greater tendency to keep moving as the cleaver severs the food.

Does a dieting person more accurately lose mass or lose weight?

Losing mass

Gravity on the surface of the moon is only 1/6 as strong as gravity on earth. What is the weight of a 10kg object on the moon and on earth? What is ist mass on each?

Mass: 10kg Weight (earth): 98N Weight (moon): 16.3N

When a junked car is crushed into a compact cube, does its mass change? Its volume? Its weight?

Neither the mass or the weight change.

Free fall is motion in which gravity is the only force acting. Is a skydiver who has reached terminal speed in free fall? Is a satellite above the atmosphere that circles Earth in free fall?

No Yes

Is it possible to move in a curved path in the absence of a force? Defend your answer.

No an object can move in a curved path only when a force acts on it.

Since an object weighs less on the surface of the Moon than on Earth's surface, does it have less inertia on the Moon's surface?

No, inertia involves mass not weight.

In an orbiting space shuttle you are handed two identical boxes, one filled with sand and the other filled with feathers. How can you determine which is which without opening the boxes.

Shake the boxes. Whichever provides more resistance to movement is the more massive box and the one with the sand.

A crate remains at rest on a factory floor while you push on it with a horizontal force F. How big is the friction force exerted on the crate by the floor?

The force of friction on he crate must be - F because then the net force on it is zero.

As you leap upward in a standing jump, how does the force that you exert on the ground compare with your weight?

The force that you exert on the ground is greater than your weight as you momentarily accelerate upward.

A race car travels along a raceway at a constant velocity of 200km/h. What horizontal net force acts on the car?

The forces acting horizontally are the driving force provided by friction between the tires and the road, and resistive forces are mainly drag. These forces cancel and the car is in dynamic equilibrium with a net force of zero.

Hold a ping-pong ball and a golf ball at arm's length and drop them simultaneously. You'll see them hit the floor at about the same time. But if you drop them off the top of a high ladder, you'll see the golf ball hit first. What is your explanation?

The golf ball has a higher mass to cross-sectional area ratio and thus a higher terminal velocity. The ping pong ball is lighter than the golf ball even though they're about the same size, so the drag force from the air affects it more.

Does the mass of the astronaut change when he or she is visiting the International Space Station? Defend your answer.

The mass will remain the same.

To pull a wagon across a lawn with constant velocity you have to exert a steady force. Reconcile this fact with Newton's first law, which says that motion with constant velocity requires no force.

The net force on the wagon your pull plus friction is zero.

Why will a sheet of paper fall more slowly than one that is wadded into a ball?

The surface area is much lower than the sheet of paper.

How does the terminal speed of a parachutist before compare with the air resistance it encounters before it reaches terminal velocity? After reaching terminal velocity?

The terminal velocity before opening the parachute is much greater than after.

When a coin is tossed upward, what happens to its velocity while ascending? It's acceleration? (ignore air resistance)

The velocity of the coin decreases. The acceleration remains constant.

When your car moves along the highway at constant velocity, the net force on it is zero. Why then do you continue running your engine?

The zero net force on the car results from the driving force that your engine supplies against the friction drag force.

How does the force of gravity on a raindrop compare with the air drag it encounters when it falls at constant velocity?

When anything falls at constant velocity air drag and gravitational force are equal in magnitude.

A common saying goes, "It's not the fall that hurts you it's the sudden stop." Translate this into Newton's laws of motion.

When you stop suddenly your velocity changes rapidly, which means a large acceleration of stopping. By Newton's second law it means that the force acting on you is also large.

Does a falling object increase in speed if its acceleration of fall decreases?

Yes

Can the velocity of an object reverse direction while maintaining constant acceleration? Give an example.

Yes, a ball thrown vertically into the air. Its velocity is initially up and finally down all the while acceleration is at a constant.

What weight change occurs when your mass increases by 2 kg?

about 20N

An astronaut tosses a rock on the Moon. What force(s) act(s) on the rock during its curved path?

gravity

A friend says that as long as a car is at rest no forces act on it. What do you say to correct that statement.

two forces - weight of the car and normal force

When a parachutist opens her parachute after reaching terminal speed, in what direction does she accelerate?

upwards - she slows down