Physics Ch. 4

What is the weight of a 1-kilogram brick resting on a table?

10 N

2.5×104 N

2.5 x 10^4

What is the net force acting on a falling 1-kg ball if it encounters 2 N of air resistance?

8.00 N

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

9.8 N

How would the acceleration change if the applied force on the elephant were increased?

Acceleration

How does acceleration depend on the net force?

Acceleration is proportional to the net force

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.

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.

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 acceleration of an object tells you nothing about its velocity--the direction and speed at which it is moving. In this case, the net force on (and therefore the acceleration of) the block is to the right, but the block could be moving left, right, or in any other direction.

How would the acceleration change if the mass of the elephant were increased?

It decreases

In the string-pull illustration in Figure 4.8, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball's weight or its mass?

Mass

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

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 dynamic friction.

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

Object A has three times the mass of object B. Identical forces are exerted on the two objects. Which statement is true? The acceleration of object B is three times that of object A. The accelerations of the two objects are equal. The speed of object A is three times that of object B after 3 seconds. The speeds of the two objects are equal after 2 seconds.

The acceleration of object B is three times that of object A. Since F = ma, the acceleration is given by a = F/m, so the object with the lower mass has a higher acceleration.

What two principal factors affect the force of air resistance on a falling object?

The force depends mostly on frontal area and speed.

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.

An object 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. If there is a net force acting on a body, regardless of whether the body is already moving, the body accelerates. If a body is moving with constant velocity, then it is not accelerating and the net force acting on it is zero. 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.

A bowling ball and a small marble will fall downward to the surface of the Moon at the same rate. Why?

The ratio of the force of gravity exerted on an object to the object's mass is the same. The acceleration is given by a = F/m, so all objects fall at the same rate since the force of gravity is proportional to the object's mass.

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.

Two parachutists have identical parachutes open and are falling toward the ground. The parachutist with the red jumpsuit is much more massive than the parachutist with the blue jumpsuit. How do the terminal velocities of the two parachutists compare?

The red parachutist has a higher terminal velocity. Since the red parachutist is more massive, he experiences a stronger force of gravity acting downward. In order for the wind resistance to balance this stronger force, he must have a faster terminal velocity

A parachutist is falling toward the ground. The downward force of gravity is exactly equal to the upward force of air resistance. Which statement is true?

The velocity of the parachutist is not changing with time. Since the net force is zero, the parachutist's acceleration is zero, so the velocity is constant in time.

Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

You push with a force equal to and opposite the dynamic friction force.

Select the correct equations that show that the acceleration of 3.0 m/s2 results from a net force of 12 N exerted on a 4.0-kg cart.

a = F net/m = 12N/4.0kg= 3.0m/s^2

Consider a 100-kg box of tools in the locations A(earth), B(moon), and C(Jupiter) From greatest to least, rank the masses of the 100-kg box of tools.

all are the same

Boxes of various masses are on a friction-free, level table. From greatest to least, rank the accelerations of the boxes. a) 8kg box, 4N left, 8N right b)4kg box, 8N left, 16N right c)3kg box, 5N left, 16N right d)20kg box, 2N left, 20N right

c, b, d, a solving: a= f/m divide force over mass of box

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

Boxes of various masses are on a friction-free, level table. From greatest to least, rank the net forces on the boxes a) 8kg box, 4N left, 8N right b)4kg box, 8N left, 16N right c)3kg box, 5N left, 16N right d)20kg box, 2N left, 20N right

d, c, b, a solving: subtract the forces

Consider a 100-kg box of tools in the locations A(earth), B(moon), and C(Jupiter) From greatest to least, rank the weights of the 100-kg box of tools.

jupiter, earth, moon

Two identical twins are falling toward the ground. The twin in the green jumpsuit is going down head first, and the twin in the red jumpsuit is going down belly first. The two twins have the same mass. Which twin reaches the fastest terminal velocity?

the twin in the green jumpsuit Since the twin in the green jumpsuit (going head first) has a smaller surface area plowing through the air, she must fall faster in order for the force of air resistance to have the same strength as the force of gravity

Calculate the acceleration of a 290000-kg jumbo jet just before takeoff when the thrust on the aircraft is 70000 N .

0.24m/s

Consider a mass of 1 kg accelerated 1 m/s2 by a force of 1 N. Find the acceleration for a force of 2 N acting on 2 kg.

1m/s^2

A person who weighs 800 N is squatting on a scale. Suddenly he starts jumping, extending his legs as quickly as possible, so that he can jump into the air. As the person is in the process of jumping (accelerating upward) the reading of the scale is ____________.

Greater than 800 N Since the person is accelerating upward, the scale must be pushing upward more strongly than the force of gravity.

A person who weighs 800 N (about 180 pounds) is standing on a scale inside an elevator. The elevator is moving upward yet slowing down. The reading on the scale is __________.

Less than 800 N Since the scale is pushing upward with a strength less than the force of gravity, the net force is directed downward, causing the person to decelerate.

A kilogram is a measure of an object's __________.

Mass An object far from any source of gravity still has mass, even if no forces of gravity are acting on it.

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.