PHY111 Lesson 04
Two boxes are suspended from a rope over a pulley. Each box has weight 50 N. What is the tension in the rope?
50 N
In general, the coefficient of static friction is _________ A. smaller than the coefficient of kinetic friction B. equal to the coefficient of kinetic friction C. greater than the coefficient of kinetic friction
C. greater than the coefficient of kinetic friction
You are riding in an elevator that is accelerating upward. Suppose you stand on a scale. The reading on the scale is A. less than your true weight B. equal to your true weight C. greater than your true weight
C. greater than your true weight
A large crate filled with physics laboratory equipment must be moved up an incline onto a truck The first physicist gets a second physicist to help. They both push on the crate, parallel to the surface of the incline, and it moves at constant speed up the incline. How does the force exerted by the two physicists on the crate compare with the force of friction on the crate?
F (twophysicists) is greater than F (friction).
Hooke's law is given by
F= k*ΔL, where ΔL is the amount of deformation (the change in length), F is the applied force, and k is a proportionality constant that depends on the shape and composition of the object and the direction of the force. The relationship between the deformation and the applied force can also be written as ΔL=(1/Y)(F/A)L0, where Y is Young's modulus, which depends on the substance, A is the cross-sectional area, and L0 is the original length.
You are going sledding with your friends, sliding down a snowy hill. Friction can't be ignored. Riding solo on your sled, you have a certain acceleration. Would the acceleration change if you let a friend ride with you, increasing the mass?
No, increasing the mass does increase the net force on the system, but it also increases the inertia. a=Fnet/m. Since both the net force and mass are increased they still cancel, leaving the acceleration the same.
The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force? The drag force increases. The drag force stays the same. The drag force decreases.
The drag force increases.
A large crate filled with physics laboratory equipment must be moved up an incline onto a truck A physicist attempts to push the crate up the incline. The physicist senses that if he applies slightly more force the crate will move up the incline but cannot muster enough strength to get the motion started. What can you say now about the force of friction acting on the crate?
The frictional force points down the incline
A large crate filled with physics laboratory equipment must be moved up an incline onto a truck. The crate is at rest on the incline. What can you say about the force of friction acting on the crate?
The frictional force points up the incline.
Two blocks are stacked on top of each other on the floor of an elevator. For each of the following situations, select the correct relationship between the magnitudes of the two forces given. The elevator is moving downward at a constant speed.
The magnitude of the force of the bottom block on the top block is equal to the magnitude of the force of the earth on the top block. The magnitude of the force of the bottom block on top block is equal to the magnitude of the force of the top block on bottom block.
Two blocks are stacked on top of each other on the floor of an elevator. For each of the following situations, select the correct relationship between the magnitudes of the two forces given. The elevator is moving downward at an increasing speed.
The magnitude of the force of the bottom block on the top block is less than the magnitude of the force of the earth on the top block. The magnitude of the force of the bottom block on the top block is equal to the magnitude of the force of the top block on the bottom block.
Two blocks are stacked on top of each other on the floor of an elevator. For each of the following situations, select the correct relationship between the magnitudes of the two forces given. The elevator is moving upward.
The magnitude of the force of the bottom block on the top block is unknown compared to the magnitude of the force of the earth on the top block. ***Even though the elevator is moving upwards, you do not know in which direction it is accelerating, or indeed whether the elevator is accelerating at all! The magnitude of the force of the bottom block on the top block is equal to the magnitude of the force of the top block on the bottom block.
An astronaut takes his bathroom scale to the moon and then stands on it. Is the reading of the scale his true weight? Select the correct explanation.
The reading on the moon will be the moon-weight, or the gravitational force of the moon on the astronaut. This would be about 1/6 of the astronaut's earth-weight or the gravitational force of the earth on the astronaut (while standing on the scales on the earth).
You are walking up an icy slope. Suddenly your feet slip, and you start to slide backward. Will you slide at a constant speed, or will you accelerate?
You will accelerate because the kinetic friction is less than the maximum static friction.
While standing in a low tunnel, you raise your arms and push against the ceiling with a force of 200 N . Your mass is 80 kg. What force does the ceiling exert on you? AND What force does the floor exert on you?
ceiling- 200N floor- 80*9.8+200=984 (round to asking sig figs)
deformation
change in shape due to the application of force
shear deformation
deformation perpendicular to the original length of an object
strain
ratio of change in length to original length
stress
ratio of force to area
tensile strength
the breaking stress that will cause permanent deformation or fraction of a material
An object will have constant acceleration if
the net force acting on it is constant in magnitude and direction.
An object cannot remain at rest unless
the net force acting on it is zero.
If a car is moving to the left with constant velocity, one can conclude that
the net force applied to the car is zero.
