Chapter 4 physics Newton's second law of motion
What is the net force that acts on a 1-kg freely falling object?
10 N
What is the weight in newtons of a 1-kilogram brick resting on a table?
10 N
What net force does a sliding crate experience when you exert a force of 110 N and the friction between the crate and the floor is 100 N?
10 N in the direction of your push 110N-100N. The crate accelerates.
A 10-kg brick and a 1-kg book are dropped in a vacuum. The force of gravity on the 10-kg brick is
10 times as much
A 300-kg bear grasping a vertical tree slides down at constant velocity. The friction force between the tree and the bear is
3000 N
B) What is the combined force of air resistance that acts all over the plane's outside surface
80,000N, else Fnet would not be zero => acceleration => non-constant velocity
Is the acceleration of a pushed object directly proportional to mass, or is it inversely proportional to mass? What law of physics illustrates this?
Acceleration is inversely proportional to mass according to Newton's second law.
State Newton's second law of motion in words.
Acceleration is proportional to net force, is in the direction of the net force, and is inversely proportional to mass.
How does acceleration depend on the net force?
Acceleration is proportional to the net force
Which of the following statements is true about a falling object that has reached terminal velocity?
Air resistance and weight are equal and opposite forces.As established by the Equilibrium rule
Two balls are dropped simultaneously from rest, starting 20 meters above the ground. Ball 1 is twice as massive as ball 2. Neglecting air resistance, which ball hits the ground first?
Both balls reach the ground at the same time. The acceleration and thus the amount of time it takes to fall is independent of the objects mass.
A 100 kg ball and a 5 kg ball are dropped at the same and begin falling downward due to gravity. Which object has the higher acceleration?
Both objects have the same acceleration. The acceleration is given by the ratio of the weight and it's mass, and both objects have the same ratio.
If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force of friction, and why?
Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.
You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N to do so. What is the force of friction? Why?
Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.
Consider a heavy crate resting on the bed of a flatbed of a truck. When the truck accelerates, the crate also accelerates and remains in place. Identify 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
After he gets the block to move, what does Dr. Hewitt do to drag the block across the table at a steady rate?
He continues to apply a steady force.
Three identical blocks are pulled, as shown, on a horizontal frictionless surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?
If the acceleration a is constant that means that the ratio of force divided by mass is also constant. Note that 30 N pulls 3 blocks. To pull 2 blocks then requires a 20-N pull, which is the tension in the rope between the second and third block. Tension in the rope that pulls only the third block is therefore 10 N. (Note that the net force on the first block, 30 N - 20 N = 10 N, is the force needed to accelerate that block
What relationship does mass have with inertia?
Inertia is a property of matter that resists changes in an object's motion. An object's inertia is directly proportional to its mass; the heavier an object is, the more inertia it has. Hence, a body's mass measures its inertia
A block of mass 2kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's motion?
It could be moving to the left, moving to the right, or be instantaneously at rest.
The mass of a rocket decreases as it burns through its fuel. If the rocket engine produces constant force (thrust), how does the acceleration of the rocket change over time?
It increases.The constant-force rocket's acceleration is inversely related to its mass. Because its mass decreases over time, its acceleration increases.
In a vacuum, a coin and a feather fall at the same rate, side by side. Would it be correct to say that equal forces of gravity act on both the coin and the feather when in a vacuum?
No, no, no, a thousand times no! These objects accelerate equally not because the forces of gravity on them are equal but because the ratios of force to mass for each are equal. Although air resistance is not present in a vacuum, gravity is. (Spacecraft orbiting Earth in the vacuum of space are accelerated by Earth's gravity, which is why they don't move off in straight lines.)
How many forces act on an object in free fall?
One force, the force due to gravity
Two parachutists are moving downward at a constant, terminal velocity. Parachutist 1 is twice as massive as parachutist 2, but both are using identical parachutes. Which parachutist is moving faster?
Parachutist 1. Since they weigh more, he needs to move faster in order for air resistance to balance the force of gravity.
If you were a passenger in the International Space Station and were confronted with two cans, one filled with refried beans and the other empty, how could you determine which is full and which is empty?
Shake the cans back and forth! Or move both in any way, and you'll immediately judge which one has a greater resistance to changes in motion. The can of refried beans is harder to shake, so it has more inertia, which is to say it has more mass.
Shake something to and fro and you're measuring its what? Lift it against gravity and you're measuring its what?
Shaking measures mass, whereas lifting measures weight.
Which is normally greater, static friction or sliding friction on the same object?
Static friction is normally greater than sliding friction
Why does a heavy parachutist fall faster than a lighter parachutist who wears a parachute of the same size?
Terminal speed must be higher to make air drag equal the gravity force for a heavier person
If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?
The acceleration is reduced to 1/3 of its original value
Does a 2-kg iron block possess twice as much inertia as a 1-kg iron block? Twice as much mass as a 1-kg iron block? Twice as much volume as a 1-kg iron block? Twice as much weight as a 1-kg iron block weighed in the same location?
The answer is yes to all of these questions. A 2-kg block of iron has twice as many iron atoms, and therefore twice the amount of inertia, mass, and weight. The blocks consist of the same material, so the 2-kg block also has twice the volume.
You push on a crate that sits on a smooth floor, and it accelerates. If you apply four times the net force, how much greater will be the acceleration?
The crate will have four times as much acceleration.
If you push with the same increased force on the same crate on a very rough floor, how will the acceleration compare with pushing the crate on a smooth floor?
The crate will have less acceleration because friction will reduce the net force.
What two principal factors affect the force of air resistance on a falling object?
The force depends mostly on frontal area and speed.
Marie exerts a 100-N horizontal force on a table on the floor, and it doesn't slide. This indicates that 100 N isn't great enough to make the table slide. How does the friction force between the table and the floor compare with Marie's push?
The force of friction is 100 N in the opposite direction, which opposes motion that would occur otherwise. The fact that the table is at rest is evidence that static equilibrium.
How does the force of friction for a sliding object vary with speed?
The force of friction is approximately independent of speed.
What general rule can you conclude about the force needed to keep an object in motion at a steady rate?
The force that you need to apply to keep an object moving at a steady rate is equal to the force resisting the motion of the object.
In the video, forces acting on the car that are parallel to the direction of motion are analyzed. How are these forces related?
The forces are equal in size, act in opposite directions, and produce a zero net force
When a crate is pushed horizontally and doesn't slide along a level floor, how great is the force of friction on the crate?
The friction force is equal and opposite to your push.
A person is pushing on an elephant which is wearing roller skates. For a specific applied force, the mor massive the elephant,
The lower the acceleration. Since F=ma, a is inversely proportional to mass
An object at rest cannot remain at rest unless which of the following holds?
The net force acting on it is zero. If there is a net force acting on a body, regardless of whether it is a constant force, the body accelerates. If the body is at rest and the net force acting on it is zero, then it will remain at rest. The net force could be zero either because there are no forces acting on the body at all or because several forces are acting on the body but they all cancel out.
If a block is moving to the left at a constant velocity, what can one conclude?
The net force applied to the block is zero..
What net force acts on the sliding table when pushed with 125 N while the friction with the floor is 115 N?
The net force is 10 N, because In this case, Ef= the crate accelerates.125N- 115N
If a constant net force is applied to an object, which of the following statements is/are true? Select all that apply.
The object has a constant acceleration, The velocity of the object changes linearly.
If a non-zero net force is exerted on an object, which statement must be true?
The object must be accelerating. Since F=m x a, the acceleration is always due to a net force.
Why doesn't a heavy object accelerate more than a light object when both are freely falling?
The ratio of the weight to mass is the same for all objects in the same locality.
mass resists acceleration
The same force applied to - twice the mass produces half the acceleration. - three times the mass, produces one-third the acceleration. Acceleration is inversely proportional to mass
What three controls in an automobile allow the driver to produce acceleration? How does the automobile's mass affect the accelerations?
The three controls are the accelerator (to increase speed), the brake pedal (to decrease speed), and the steering wheel (to change direction). The greater the mass of the vehicle, the greater the resistance to these changes. (Drivers of massive vehicles adjust to the sluggishness of velocity changes).
A person is pushing on a box with a force of 5 newtons. The box is sliding across the horizontal floor. If the force of friction is 3 newtons,
The velocity of the box is increasing in time. The force exerted by the person is greater than the force of friction, so the box accelerates
A person is pushing on a box with a force of 5 newtons. The box is sliding across the horizontal floor. If the force of friction is 5 newtons,
The velocity of the box is not changing in time. Since the force of friction is equal and opposite the force exerted by the person, the net force on the box is zero
Which varies with location, mass or weight?
Weight varies with location, but mass does not.
Newton's second law relates an object's acceleration to its mass and the net force acting on it. Does Newton's second law apply to a situation in which there is no net force? Select the best explanation.
Yes. The law applies and it tells us that the object has constant velocity.
A parachutist is moving downward at a constant, terminal velocity. The net force acting on her is
Zero. If the velocity is constant, the acceleration is zero and therefore the net force is zero
newtons 2nd law equation
a=F/m. or F=ma
nonfree fall occurs when
air resistance is nonnegligible. - depends on two things: ▪ speed and ▪ frontal surface area
Two forces, of magnitude 4N and 10N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object
cannot have a magnitude equal to 5N
When Sanjay pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is
equal and opposite to Sanjay's push.
what will be the acceleration of a rock thrown straight upward at the moment it reaches the tippity-top of its trajectory.
g. although its speed and velocity at the top will both instantaneously be zero, its acceleration will be g, or 9.8 m/s2.
As a skydiver gains speed in falling through the air, air resistance
increases. The greater the speed, the greater the number of air molecules the object encounters per second and the greater the force of molecular impact
When Sanjay pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is
less than Sanjay's push.The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium.
Fill in the blanks: Shake something to and fro and you're dealing with its
mass
a 1-kg rock is thrown at 10 m/s straight upward. neglecting air resistance, what is the net force that acts on it when it is halfway to the top of its path? what would be the net force halfway up, if you were to throw it at 100 m/s?
mg. in the absence of air resistance, the only force exerted on the 1-kg rock is simply the force of gravity-mg. that's 9.8 newtons, at any speed and anywhere along its trajectory.
A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be
moving with a constant nonzero acceleration. Since there is a net force acting, the body does not move at a constant velocity, but it accelerates instead. However, the force acting on the body is constant. Hence, according to Newton's 2nd law of motion, the acceleration of the body is also constant.
terminal speed
occurs when acceleration terminates (when air resistance balances weight, so that net force is zero)
The direction of the force of friction on a sliding crate is
opposite to the direction of sliding. Friction always is opposite to the direction of the relative motion.
terminal velocity
refers to terminal speed along with the direction of motion, which is downward.
non free fall depends on
speed and frontal surface area
Friction results from
surface irregularities and mutual attractions (stickiness) between atoms in the surfaces of sliding objects.
The unit of mass is the kilogram, and the unit of weight is
the Newton
Newton's Second Law
the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. F=ma
A 1-kg stone and a 10-kg stone have the same acceleration in free fall because
the ratio of weight to mass is the same for each
A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be
the same
Phil Physiker playfully tosses his pet puppy vertically upward and then catches it. at the tippity top of its vertical path, the puppys acceleration is
the same as halfway down
if an object is to move at constant velocity, a force equal to the opposing force of friction must be applied so that
the two forces exactly cancel each other. The zero net force then results in zero acceleration and constant velocity.
Whenever a (non-zero) net force acts on an object, there is a change in the object's
velocity. A non-zero net force will produce an acceleration and, therefore, a change in velocity.
Lift it against gravity and you're dealing with its
weight
A crate sits at rest on a factory floor. Friction between the crate and floor occurs
when the crate is pushed horizontally, whether sliding or not. As per the equilibrium rule, if the crate is at rest, friction equals the external force.
A sack of potatoes weighing 200 N falls from an airplane. As the velocity of fall increases, air resistance also increases. When air resistance equals 200 N, the sack's acceleration in meters per second per second is
zero
Suppose in a high-flying airplane the captain announces over the cabin public address system that the plane is flying at a constant 900 km/h and the thrust of the engines is a constant 80,000 Newtons. A) What is the acceleration of the airplane?
zero... constant velocity is the magic word(s)