Reviewing Newton’s Laws, elevator problems, and more Atwood’s
1.
An object near the surface of the earth with a weight of 100 newtons is accelerated horizontally at 4 m/s2. What is the net force on the object?
(A) 25 N (B) 40 N (C) 250 N (D) 400 N (E) 2500 N
F = ma, where m = W/g = 10 kg
2.
A 50-kg student stands on a scale in an elevator. At the instant the elevator has a downward acceleration of 1.0 m/s2 and an upward velocity of 3.0 m/s, the scale reads approximately
(A) 350 N (B) 450 N (C) 500 N (D) 550 N (E) 650 N
Elevator physics: F
N represents the scale reading. ΣF = ma; F
N
– mg = ma, or F
N
= m(g + a). B
The velocity of the elevator is irrelevant.
3.
The “reaction” force does not cancel the “action” force because:
(A) The action force is greater than the reaction force. (B) The action force is less than the reaction force.
(C) They act on different bodies. (D) They are in the same direction.
(E) The reaction exists only after the action force is removed.
4.
When an object exerts a force on a second object, the second object exerts an equal and opposite force back on the first object.
5.
What happens to the inertia of an object when its velocity is doubled?
(A) the object’s inertia becomes 2 times greater
(B) the object’s inertia becomes 2 times greater
(C) the object’s inertia becomes 4 times greater
(D) the object’s inertia becomes 8 times greater
(E) the object’s inertia is unchanged
Inertia is mass
A block of mass 3 kg, initially at rest, is pulled along a frictionless, horizontal surface with a force shown as a function of time t by the graph above. The acceleration of the block at t = 2 s is
(A) 3/4 m/s2 (B) 4/3 m/s2 (C) 2 m/s2 (D) 8 m/s2 (E) 12 m/s2
At t = 2 s the force is 4 N. F = ma

6.
If a net force F applied to an object of mass m will produce an acceleration of a, what is the mass of a second object which accelerates at 5a when acted upon by a net force of 2F?
(A) (2/5)m (B) 2m (C) (5/2)m (D) 5m (E) 10m
If F = ma, then m = F/a. For the second object m′ = 2F/5a = 2/5(F/a) = (2/5)m
7.
A book leans against a crate on a table. Neither is moving. Which of the following statements concerning this situation is CORRECT?
(A) The force of the book on the crate is less than that of crate on the book.
(B) Although there is no friction acting on the crate, there must be friction acting on the book or else it will fall.
(C) The net force acting on the book is zero.
(D) The direction of the frictional force acting on the book is in the same direction as the frictional force acting on the crate.
(E) The Newton’s Third Law reaction force to the weight of the book is the normal force from the table
If they are not moving, the net force must be zero. While the book and crate are pushing each other apart, there is friction from the table pointing inward against each object on the table to keep them at rest.
The figure above shows an 80-kilogram person standing on a 20-kilogram platform suspended by a rope passing over a stationary pulley that is free to rotate. The other end of the rope is held by the person. The masses of the rope and pulley are negligible. You may use g = 10 m/ s
2
. Assume that friction is negligible, and the parts of the rope shown remain vertical. a.
If the platform and the person are at rest, what is the tension in the rope?
The person now pulls on the rope so that the acceleration of the person and the platform is 2 m/s
2 upward.
b. What is the tension in the rope under these new conditions?

c. Under these conditions, what is the force exerted by the platform on the person?
A 10-kilogram block rests initially on a table as shown in cases I and II above. The coefficient of sliding friction between the block and the table is 0.2. The block is connected to a cord of negligible mass, which hangs over a massless frictionless pulley. In case I a force of 50 newtons is applied to the cord. In case II an object of mass 5 kilograms is hung on the bottom of the cord. Use g = 10 meters per second squared. a. Calculate the acceleration of the 10-kilogram block in case I. b. On the diagrams below, draw and label all the forces acting on each block in case II
Calculate the acceleration of the 10-kilogram block in case II.