physics ch.5 plr

Click on the reset button, and stack one 50kg. crate on top of the other, so that the total mass is 100kg. The Friction should be set to halfway between None and Lots. Adjust the Applied Force (force exerted) slowly, and determine when the crates begin accelerating. What is the minimum force that must be exerted on the crates so that they accelerate?

A little more than 250 N. The maximum force of static friction is given by fmax=μmg, where μ is the coefficient of static friction, m is the mass, and g is the acceleration due to gravity. The force of static friction will be exactly equal to the applied force up to this amount. If the applied force is greater than this amount, the crates accelerate.

Click the reset button, and set the Applied Force (force exerted) to 350 N. After about 3 s , set the Applied Force back to zero. After the applied force (force exerted) is turned off, what happens to the crate?

It slows down with a negative acceleration. Because the net force on the crate points to the left, which is defined as negative in this simulation, the acceleration is a=Fnet/m, and is also negative.

Adjust the mass of the refrigerator by stacking different objects on top of it.If the mass of the refrigerator is increased (with the Applied Force held constant), what happens to the acceleration?

The acceleration decreases. Since Fnet=ma the acceleration is inversely proportional to the mass.

When a board with a box on it is slowly tilted to a larger and larger angle, common experience shows that the box will at some point "break loose" and start to accelerate down the board. The box begins to slide once the component of its weight parallel to the board, w||w||w_||, equals the maximum force of static friction. Which of the following is the most general explanation for why the box accelerates down the board after it begins to slide (rather than sliding with constant speed)?

The coefficient of kinetic friction is less than the coefficient of static friction. At the point when the box finally does "break loose," the component of the box's weight that is parallel to the board, w||w||w_||, is equal to μsn (the maximum force of static friction). For the box to then accelerate, there must be a nonzero net force acting on the box parallel to the board. In other words, w||w||w_|| must be greater than the force of kinetic friction, fk=μkn. Therefore the force of kinetic friction, μkn , must be less than the force of static friction, μsn, which implies μk<μs, as expected.

In general, how does the coefficient of static friction compare to the coefficient of kinetic friction for the same two materials?

The coefficient of static friction is greater than the coefficient of kinetic friction.

Click the reset button, and adjust the Friction back to None. With the 50kg crate selected, use the slider to apply a force of about 100N. After 3 s have elapsed, release the slider so that the Applied Force is now zero, and the figure is standing still. What happens to the crate after the Applied Force (force exerted) goes to zero?

The crate's acceleration becomes zero, and its velocity remains constant. Because the friction is turned off, there are no horizontal forces acting on the crate if the applied force is set to zero. This means that the acceleration must be zero, causing the velocity to remain constant. The cratea never slows down or stops!

A packing crate is sitting at rest on an inclined loading ramp. How does the magnitude of the force of static friction compare to the other forces acting on the crate?

The magnitude of the force of static friction is equal to the magnitude of the component of the weight of the crate parallel to the inclined ramp.

When two objects slide against one another, which of the following statements about the force of friction between them is true?

The magnitude of the frictional force is always equal to μkn.

When two objects are in contact with no relative motion, which of the following statements about the frictional force between them is true?

The magnitude of the frictional force may be either equal to or less than μsn. For static friction, the actual magnitude of the friction force is such that it, together with any other forces present, will cause the object to have zero acceleration. The magnitude of the force due to static friction cannot, however, exceed μsn. If the magnitude of static friction needed to keep the acceleration equal to zero exceeds μsn, then the object will slide and will be subject to the force of kinetic friction. Do not automatically assume that fs=μsn unless you are considering a situation in which an object is just on the verge of slipping.

Choose the 200 kgkg refrigerator. Set the applied force to 400 NN (to the right). Be sure friction is turned off.What is the net force acting on the refrigerator?

The magnitude of the net force is 400NN, directed to the right. Since there is no friction, the only horizontal force is due to the applied force. The normal force (directed upward) and the force of gravity (directed downward) exactly cancel out.

An object is hanging by a string from the ceiling of an elevator. The elevator is moving upward with a constant speed. What is the magnitude of the tension in the string?

The magnitude of the tension in the string is equal to the magnitude of the weight of the object.

An object is hanging by a string from the ceiling of an elevator. The elevator is slowing down while moving upward. What is the magnitude of the tension in the string?

The magnitude of the tension in the string is less than the magnitude of the weight of the object.

What is the acceleration a of the refrigerator 4 s after the person begins pushing on it with a force of 400 N?

a=2m/s^2 Applying Newton's 2nd law, a=Fnet/m=(400N)/(200kg)=2m/s^2. The acceleration is constant.

If the Applied Force (force exerted) on both crates with the current Friction setting is 350N, the acceleration is

a constant 1 m/s^2 The net force on the crates is Fnet=(350−250)N=100N, so the acceleration is a=100N/(100kg)=1m/s^2. This acceleration is constant in time since the force of friction does not depend on the velocity.

Click on the crate to bring it to a stop, then replace it with the refrigerator. Use the slider to apply a force of about 400 N. After 2 s have elapsed in the simulation, decrease the Applied Force (force exerted) slowly back to zero. Try to do this adjustment in roughly 2 s . While the Applied Force (force exerted) is decreasing, the velocity is___

increasing. Even though the acceleration is decreasing as the applied force is decreasing, the acceleration is still positive, so the velocity must still be increasing. A smaller acceleration causes the rate of change of the velocity to decrease. You should see this on the Speed indicator; as the acceleration goes down, the needle rotates more slowly.