Chapter 4 Newton S Laws and Forces

Physics

Chapter 4—Newton’s Laws and Forces

1) The picture below shows the forces acting on a block that is accelerating to the right along a level table. Treat the handle as part of the block. Friction is also present.

a) Write the Fnet = ma equation of the block for the horizontal direction, assuming the positive direction is to the right.

b) Write the Fnet = ma equation of the block for the vertical direction, assuming the positive direction is upward, and assuming that the block does not leave the table.

2) Your 10 kg dog is dying for a bone. He grabs hold of a string on the 20 kg box of bones to tow them away just as you pull on the other end with 50 N of force. The two of you get into a game of tug of war.

Determine the acceleration of your dog and the box and the tension in the string:

a.) When the two of you are on frictionless ice (assume you don't slide because you're wearing ice-pick shoes).

b.) When you're on a surface where the friction force is one tenth of the normal force.

3) A person has a reasonable chance of surviving an automobile crash if the deceleration is no more than 30 "g's" (thirty times the acceleration due to gravity)

a.) Calculate the force on a 70 kg person accelerating at this rate.

b.)What distance is traveled if the person comes to rest from 80 km/hr with this acceleration?

4) On takeoff, the propeller of a helicopter exerts an 8000-N force on the helicopter, directed upward at an angle of 65 degrees above the horizontal. The helicopter rises with constant velocity in the vertical direction while continuing to accelerate in the horizontal direction.

a) What is the weight of the helicopter?

b) What is its horizontal acceleration?

THINKING PROBLEMS:

1) A body's center of mass moves only if an external force is applied, but you can get to the other side of the room in a chair without letting your feet touch the floor. If all of your twisting and contorting are internal forces, what provides the external force?

2) Two spaceships oat in space and are at rest relative to each other. They are connected by a string, which is strong, but cannot withstand an arbitrary amount of stretching. At a given instant, the spaceships simultaneously (relative to their initial inertial frame) start accelerating in the same direction (along the line between them) by putting their identical engines on identical settings. Will the string ever break?

There is a rather hard and involved answer to this problem which uses no more relativity than we teach you in this class. It turns out there's another lovely and very elegant answer, but it requires a teeny bit of general relativity. Google the `correspondence principle' if you're interested.