Chapter 4: Forces and Newton's laws of motion, Mastering Physics 4, physics exam 2
Free body diagrams
1. Identify all the forces acting on the object 2. Draw a coordinate system 3. Represent the object as a dot at the origin of the coordinate axes 4. Draw vectors representing each of the identified forces 5. Draw and label the net force vector
Force vectors
1. Represent the object as a particle. 2. Place the tail of the force vector on particle. 3. Draw it pointing in the direction that the forces acts with a length proportional to the size of the force 4. Give the vector an appropriate label
What are the eight forces?
1. Weight 2. Spring force 3. Tension force 4. Normal Force 5. Friction 6. Drag 7. Thrust 8. Electric and magnetic forces
It could be moving to the left, moving to the right, or be instantaneously at rest.
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 must be moving to the left. It must be moving to the right. It must be at rest. It could be moving to the left, moving to the right, or be instantaneously at rest.
Gravity
A downward force of magnitude 5 N is exerted on the book by the force of the table gravity inertia
Newton's second law
A force causes an object to accelerate. The acceleration (a) is directly proportional to the force F and inversely proportional to the mass M a = F/M Or F = Ma Note that if we double the size of the force F, the acceleration a will double, as we found experimentally. And if we triple the mass m, the acceleration will be only 1/3 as great.
What is a force?
A force is a push or a pull. A force acts on an object and requires an agent. A force is also a vector. It can also be either a contact force or a long range force.
moving with a constant nonzero acceleration
A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________. continuously changing direction moving at constant velocity moving with a constant nonzero acceleration moving with continuously increasing acceleration
a contact force
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. The force acting on the block and directed to the left is __________. a contact force a long-range force
a contact force
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. The force acting on the block and directed to the right is __________. a contact force a long-range force
friction
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. What is the force acting on the block and directed to the left called? tension normal force weight friction
tension
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. What is the force acting on the block and directed to the right called? tension normal force weight friction
the surface of the table
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. Which object exerts a force on the block that is directed toward the left? the block itself the earth the surface of the table the string
the string
A string is attached to a heavy block. The string is used to pull the block to the right along a rough horizontal table. Which object exerts a force on the block that is directed toward the right? the block itself the earth the surface of the table the string
You've just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these are forces acting on the rock? List all that apply. A. Gravity, acting downward B. The normal force, acting upward C. The force of the kick, acting in the direction of motion D. Friction, acting opposite the direction of motion E. Air resistance, acting opposite the direction of motion
A, B, and D. Friction and the normal force are the only contact forces. Nothing is touching the rock to provide a "force of the kick." You can ignore air resistance unless the problem specifically calls for it.
|| Two children fight over a 200 g stuffed bear. The 25 kg boy pulls to the right with a 15 N force and the 20 kg girl pulls to the left with a 17 N force. Ignore all other forces on the bear (such as its weight). At this instant, can you say what the velocity of the bear is? If so, what are the magnitude and direction of the velocity? At this instant, can you say what the acceleration of the bear is? If so, what are the magnitude and direction of the acceleration?
A.No b. 10 m/s2 to the left
equal in magnitude but antiparallel to the force on the earth due to the moon.
According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is greater in magnitude and antiparallel to the force on the earth due to the moon. greater in magnitude and parallel to the force on the earth due to the moon. equal in magnitude but antiparallel to the force on the earth due to the moon. equal in magnitude and parallel to the force on the earth due to the moon. smaller in magnitude and antiparallel to the force on the earth due to the moon. smaller in magnitude and parallel to the force on the earth due to the moon.
The net force acting on it is zero.
An object cannot remain at rest unless which of the following holds? The net force acting on it is zero. The net force acting on it is constant and nonzero. There are no forces at all acting on it. There is only one force acting on it.
Newton's first law
An object has no forces acting on it. If it is at rest, it will remain at rest. If it is moving, it will continue to move in a straight line at a constant speed.
An object is moving with constant velocity. Which of the following best describes the force(s) acting on the object? An object is moving with constant velocity. No forces are acting on the object. A constant net force is acting on the object in a direction opposite of the motion. A constant net force is acting on the object in a direction perpendicular to the direction of motion. A constant net force is acting on the object in the direction of the motion.
An object is moving with constant velocity. No forces are acting on the object. Because f=m*a a depends on change in velocity. but since velocity is constant acceleration is 0 and anything times 0=0
5N/ book
An upward force of magnitude _____ is exerted on the _____ by the table.
A constant force is applied to an object, causing the object to accelerate at 8.0 m/s2. What will the acceleration be if The force is doubled? The object's mass is doubled? The force and the object's mass are both doubled? The force is doubled and the object's mass is halved?
Answer : a. 16 m/s2 b. 4.0 m/s2 c. 8.0 m/s2 d. 32 m/s2
An object cannot remain at rest unless which of the following holds? The net force acting on it is zero. The net force acting on it is constant and nonzero. There are no forces at all acting on it. There is only one force acting on it.
Answer-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.
Two rubber bands stretched to the standard length cause an object to accelerate at 2 m/s2. Suppose another object with twice the mass is pulled by four rubber bands stretched to the standard length. What is the acceleration of this second object? 1 m/s2 2 m/s2 4 m/s2 8 m/s2 16 m/s2
Answer: 2 m/s^2 because Acceleration is proportional to force, so doubling the number of rubber bands doubles the acceleration of the original object from 2 m/s2 to 4 m/s2. But acceleration is also inversely proportional to mass. Doubling the mass cuts the acceleration in half, back to 2 m/s2.
The motion of a very massive object can be minimally affected by what would seem to be a substantial force. Consider an oil supertanker with mass 3.0*10^8 kg. Suppose you strapped two jet engines (thrust) onto the sides of the tanker. Ignoring the drag of the water (which, in reality, is not a very good approximation), how long will it take the tanker, starting from rest, to reach a typical cruising speed of 6.0 m/s?
Answer: 5 hours
A car has a maximum acceleration of 10.0 m/s2. What will the maximum acceleration be if the car is towing another car of the same mass?
Answer: 5 m/s2
A boy is using a rope to pull a sled to the right. What are the directions of the tension force and the friction force on the sled, respectively? A. Right, right B. Right, left C. Left, right D. Left, left
Answer: B.
A block has acceleration a when pulled by a string. If two identical blocks are glued together and pulled with twice the original force, their acceleration will be A. (1/4)a B. (1/2)a C. a D. 2a E. 4a
Answer: C
Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on the ocean floor. If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water so that the thrust force on the scallop is . A. upward, upward B. upward, downward C. downward, upward D. downward, downward
Answer: C.
Tennis balls experience a large drag force. A tennis ball is hit so that it goes straight up and then comes back down. The direction of the drag force is Always up. Up and then down. Always down. Down and then up.
Answer: Down and then up
A beach ball is thrown straight up, and some time later it lands on the sand. Is the magnitude of the net force on the ball greatest when it is going up or when it is on the way down? Or is it the same in both cases? Explain. Air resistance should not be neglected for a large, light object.
Answer: Greater on the way up
A car has a maximum acceleration of 8.4 m/s2 . What will the maximum acceleration be if the car is towing another car of the same mass? Neglect the additional friction force.
Answer: a= 4.2 m/s^2
A falcon is hovering above the ground, then suddenly pulls in its wings and begins to fall toward the ground. Air resistance is not negligible. Indentify the forces on the bird.
Answer: drag and weight.
A car is moving along a straight road at a constant speed. A hockey puck slides along a horizontal icy (frictionless) surface.
Consider the following situations: A car is moving along a straight road at a constant speed. A car is moving along a straight road while slowing down. A car is moving along a straight road while speeding up. A hockey puck slides along a horizontal icy (frictionless) surface. A hockey puck slides along a rough concrete surface. A cockroach is speeding up from rest. A rock is thrown horizontally; air resistance is negligible. A rock is thrown horizontally; air resistance is substantial. A rock is dropped vertically; air resistance is negligible. A rock is dropped vertically; air resistance is substantial. Which of these situations describe the motion shown in the motion diagram at point A?
A car is moving along a straight road while slowing down. A hockey puck slides along a rough concrete surface.
Consider the following situations: A car is moving along a straight road at a constant speed. A car is moving along a straight road while slowing down. A car is moving along a straight road while speeding up. A hockey puck slides along a horizontal icy (frictionless) surface. A hockey puck slides along a rough concrete surface. A cockroach is speeding up from rest. A rock is thrown horizontally; air resistance is negligible. A rock is thrown horizontally; air resistance is substantial. A rock is dropped vertically; air resistance is negligible. A rock is dropped vertically; air resistance is substantial. Which of these situations describe the motion shown in the motion diagram at point B?
A rock is thrown horizontally; air resistance is negligible.
Consider the following situations: A car is moving along a straight road at a constant speed. A car is moving along a straight road while slowing down. A car is moving along a straight road while speeding up. A hockey puck slides along a horizontal icy (frictionless) surface. A hockey puck slides along a rough concrete surface. A cockroach is speeding up from rest. A rock is thrown horizontally; air resistance is negligible. A rock is thrown horizontally; air resistance is substantial. A rock is dropped vertically; air resistance is negligible. A rock is dropped vertically; air resistance is substantial. Which of these situations describe the motion shown in the motion diagram at point C?
no
Do the downward force in Part A (gravity) and the upward force in Part B (upward force of magnitude 5N exerted on the book on the table) constitute a 3rd law pair?
Electric and Magnetic forces.
Electricity and magnetism, like gravity, exert long-range forces. The forces of electricity and magnetism act on charged particles. (Don't really need to know this).
true
Every force has one and only one 3rd law pair force. true false
Newton's third law
Every force occurs as one member of an action/reaction pair of forces. The two members of an action/reaction pair act on two different objects. The two members of an action/reaction pair point in opposite directions and are equal in magnitude.
a long-range force
First, consider a book resting on a horizontal table. The downward force acting on the book is __________. a contact force a long-range force
the earth
First, consider a book resting on a horizontal table. Which object exerts a downward force on the book? the book itself the earth the surface of the table
the surface of the table
First, consider a book resting on a horizontal table. Which object exerts an upward force on the book? the book itself the earth the surface of the table
a contact force
First, consider a book resting on a horizontal table. The upward force acting on the book is __________. a contact force a long-range force
weight
First, consider a book resting on a horizontal table. What is the downward force acting on the book called? tension normal force weight friction
normal force
First, consider a book resting on a horizontal table. What is the upward force acting on the book called? tension normal force weight friction
Drag
Friction at a surface is one example of a resistive force, a force that opposes or resists motion. This kind of resistive force—the force of a fluid on a moving object—is called drag and is symbolized as D. Like kinetic friction, drag points opposite the direction of motion.
Friction
Friction, like the normal force, is exerted by a surface. Unlike the normal force, however, the frictional force is always parallel to the surface, not perpendicular to it. (In many cases, a surface will exert both a normal and a frictional force.) The symbol for friction is f. 1. Kinetic friction, denoted by fk, acts as an object slides across a surface, it is always "opposed the motion". 2. Static friction, denoted by fs, is the force that keeps an object "stuck" on a surface and prevents its motion relative to the surface. It points in the direction necessary to prevent motion.
False
Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite directions. (Assume no other forces act on either body.) true false
The net force applied to the block is zero.
If a block is moving to the left at a constant velocity, what can one conclude? There is exactly one force applied to the block. The net force applied to the block is directed to the left. The net force applied to the block is zero. There must be no forces at all applied to the block.
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 must be moving to the left. It must be moving to the right. It must be at rest. It could be moving to the left, moving to the right, or be instantaneously at rest.
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.
Net force
It is the vector sum of all the forces. F1 +F2+F3 ... = Fnet
Spring force
Springs exert one of the most basic contact forces. A spring can either push (when compressed) or pull (when stretched). The symbol for Spring Force is Fsp.
gravitational force acting on the piano (piano's weight) force of the floor on the piano (normal force) force of Chadwick on the piano
Suppose that you are asked to solve the following problem: Chadwick is pushing a piano across a level floor (see the figure). (Figure 1) The piano can slide across the floor without friction. If Chadwick applies a horizontal force to the piano, what is the piano's acceleration? To solve this problem you should start by drawing a free-body diagram Identify the forces acting on the object of interest. From the list below, select the forces that act on the piano. Check all that apply. acceleration of the piano gravitational force acting on the piano (piano's weight) speed of the piano gravitational force acting on Chadwick (Chadwick's weight) force of the floor on the piano (normal force) force of the piano on the floor force of Chadwick on the piano force of the piano pushing on Chadwick
An astronaut is taking a space walk near the shuttle when her safety tether breaks. What should the astronaut do to get back to the shuttle? Attempt to "swim" toward the shuttle. Take slow steps toward the shuttle. Take a tool from her tool belt and throw it away from the shuttle. Take the portion of the safety tether still attached to her belt and throw it toward the shuttle.
Take a tool from her tool belt and throw it away from the shuttle. As the astronaut throws the tool away from the shuttle, she exerts a force in the direction away from the shuttle. Then, by Newton's 3rd law, the tool will exert an opposite force on her. Thus, as she throws the tool, a force directed toward the shuttle will act on the astronaut. Newton's 2nd law stipulates that the astronaut would acquire an acceleration toward the shuttle.
Two objects attract each other gravitationally. If the distance between their centers decreases by a factor of 2, how does the gravitational force between them change? The gravitational force increases by a factor of 2. The gravitational force increases by a factor of 4. The gravitational force decreases by a factor of 4. The gravitational force decreases by a factor of 2. The gravitational force remains unchanged.
The gravitational force increases by a factor of 4.
Weight
The gravitational pull of the earth on an object on or near the surface of the earth. The symbol for weight is w. An object's weight vector always points vertically downward, no matter how the object is moving.
If a block is moving to the left at a constant velocity, what can one conclude? There is exactly one force applied to the block. The net force applied to the block is directed to the left. The net force applied to the block is zero. There must be no forces at all applied to the block.
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 constant net force acts on an object. Which of the following best describes the object's motion? The object is moving with a constant acceleration. The object is moving with a decreasing acceleration. The object is moving with a constant velocity. The object is moving with an increasing acceleration. The object is at rest; its position is constant.
The object is moving with a constant acceleration. newtons 2nd law. f=ma
5 N / table / book / downward
The reaction to the force in Part B (upward force of magnitude 5N exerted on the book on the table) is a force of magnitude _____, exerted on the _____ by the _____. Its direction is _____.
friction
The same block is placed on the same rough table. However, this time, the string is disconnected and the block is given a quick push to the right. The block slides to the right and eventually stops. The following questions refer to the motion of the block after it is pushed but before it stops. What is the force acting on the block that is directed to the left called? tension normal force weight friction
1
The same block is placed on the same rough table. However, this time, the string is disconnected and the block is given a quick push to the right. The block slides to the right and eventually stops. The following questions refer to the motion of the block after it is pushed but before it stops. How many forces are acting on the block in the horizontal direction? 0 1 2 3
Assuming that the astronaut can throw any tool with the same force, what tool should be thrown to get back to the shuttle as quickly as possible? You should consider how much mass is left behind as the object is thrown as well as the mass of the object itself. The tool with the smallest mass. The tool with the largest mass. Any tool, since the mass of the tool would make no difference
The tool with the largest mass. The force that acts on the astronaut must equal in magnitude the force that she exerts on the tool. Therefore, if she exerts the same force on any tool, the force acting on her will be independent of the mass of the tool. However, the acceleration that the astronaut would acquire is inversely proportional to her mass since she is acted upon by a constant force. If she throws the tool with the largest mass, the remaining mass (the astronaut plus her remaining tools) would be smallest—and the acceleration the greatest!
false
The two forces in each pair may have different physical origins (for instance, one of the forces could be due to gravity, and its pair force could be due to friction or electric charge). true false
Thrust
Thrust occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a contact force, with the exhaust gas being the agent that pushes on the engine. Thrust is a force opposite the direction in which the exhaust gas is expelled. There's no special symbol for thrust, so we will call it Fthrust.
cannot have a magnitude equal to 5N
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 __________. Check all that apply. cannot have a magnitude equal to 5N cannot have a magnitude equal to 10N cannot have the same direction as the force with magnitude 10N must have a magnitude greater than 10N
A baseball player is sliding into second base. Identify the forces on the baseball player.
Weight, normal force by ground, kinetic friction force by ground
A skier is sliding down a 15° slope. Friction is not negligible. Identify the forces on the skier.
Weight, normal force by slope, kinetic friction force by slope
A mountain climber is hanging from a vertical rope, far above the ground and far from the rock face. Identify the forces on the mountain climber.
Weight, tension force by rope
The net force is zero.
What is the direction of the net force acting on the object at position A? upward downward to the left to the right The net force is zero.
to the left
What is the direction of the net force acting on the object at position B? upward downward to the left to the right The net force is zero.
downward
What is the direction of the net force acting on the object at position C? upward downward to the left to the right The net force is zero.
Tension force
When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force, represented by T. The direction of the tension force is always in the direction of the string or rope.
Newton's 3rd law
Which of Newton's laws could we have used to predict that the forces in Parts B (upward force of magnitude 5N exerted on the book on the table) and E (force of magnitude 5N exerted on the table by the book downward) are equal and opposite? Check all that apply. Newton's 1st law Newton's 2nd law Newton's 3rd law
1,2,3,4,5
Which of these diagrams correspond to a situation where the moving object (not necessarily the one shown in the motion diagram) is changing its velocity?
6
Which of these diagrams may possibly correspond to the situation at point A on the motion diagram?
3,5
Which of these diagrams may possibly correspond to the situation at point B on the motion diagram?
2,4
Which of these diagrams may possibly correspond to the situation at point C on the motion diagram?
If the astronaut throws the tool with a force of 16.0 N, what is the magnitude of the acceleration a of the astronaut during the throw? Assume that the total mass of the astronaut after she throws the tool is 80.0 kg. Express your answer in meters per second squared.
a= 0.200 m/s^2 F=ma F/m=a 16N/80 kg = a
Two forces, of magnitude 4Nand 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 cannot have a magnitude equal to 10N cannot have the same direction as the force with magnitude 10N must have a magnitude greater than 10N
cannot have a magnitude equal to 5N
A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________. continuously changing direction moving at constant velocity moving with a constant nonzero acceleration moving with continuously increasing acceleration
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.
Normal force
the force exerted by a surface (the agent) against an object that is pressing against the surface. The symbol for the normal force is n. The normal force is perpendicular to the surface.
