Newton's 1st Law

Physics 31N: Newton’s First Law

Item 1
Two forces have the same magnitude F.
Part A
What is the angle between the two vectors if their sum has a magnitude of 2F? θ = 0∘

Part B
What is the angle between the two vectors if their sum has a magnitude of 2√F? θ = 90∘
Part C
What is the angle between the two vectors if their sum has a magnitude of zero? θ = 180∘

Item 2
An object is moving in the absence of a net force. Which of the following best describes the object's motion?
*The object will continue to move with a constant velocity.

Item 3
An object is moving with constant velocity. Which of the following best describes the force(s) acting on the object?
*The net force acting on the object is zero.

Item 4
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.

Item 5
The same net force is applied to two different objects. The second object has twice the mass of the first object. Compare the acceleration of the two objects.
*The acceleration of object 1 is twice the acceleration of object 2.

Item 6
An object is at rest on a tabletop. Earth pulls downward on this object with a force equal in magnitude to mg. If this force serves as the action force, what is the reaction force in the action–reaction pair?
*The object pulling upward on Earth

Item 7
A large truck collides head-on with a small car. The car is severely damaged as a result of the collision. According to Newton's third law, how do the forces acting between the truck and car compare during the collision?
*The force on the truck is equal to the force on the car.

Item 8
You swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is
*Exactly 1500 N whether or not the box moves.

Item 9
A stalled car is being pushed up a hill at constant velocity by three people. The net force on the car is
*Zero

Item 10
In order to get an object moving, you must push harder on it than it pushes back on you.
*False

Item 11
In order to lift a bucket of concrete, you must pull up harder on the bucket than it pulls down on you.
*False

Item 12
A box of mass m is pulled with a constant acceleration a along a horizontal frictionless floor by a wire that makes an angle of 15° above the horizontal. If T is the tension in this wire, then
*T>ma

Item 13
You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is
*No forces were acting on you to cause you to fall.

Item 14
Consider what happens when you jump up in the air. Which of the following is the most accurate statement?
*When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.

Item 15
A fish weighing 16 N is weighed using two spring scales, each of negligible weight, as shown in the figure. What will be the readings of the scales?
[pic]
*Each scale will read 16 N

Item 16
An object is moving forward with a constant velocity. Which statement about this object MUST be true?
*The net force on the object is zero.

Item 17
A ball is tossed vertically upward. When it reaches its highest point (before falling back downward)
*The velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.

Item 18
A car is being towed at constant velocity on a horizontal road using a horizontal chain. The tension in the chain must be equal to the weight of the car in order to maintain constant velocity.
*False

Item 19
Two dogs pull horizontally on ropes attached to a post; the angle between the ropes is 61.0∘. Dog Aexerts a force of 282N and dog B exerts a force of 312N.

Part A
Find the magnitude of the resultant force.
F = 512 N

Part B
Find the angle the resultant force makes with dog A's rope. θ = 32.2 ∘

Item 20
A 1190-kg car is held in place by a light cable on a very smooth (frictionless) ramp, as shown in the figure. The cable makes an angle of 31.0 ∘ above the surface of the ramp, and the ramp itself rises at 25.0 ∘above the horizontal.

Part A
Draw a free-body diagram for the car.
Draw the force vectors with their tails at the front bumper of the car. The location and orientation of your vectors will be graded. The exact length of your vectors will not be graded.

[pic]

Part B
Find the tension in the cable. T = 5760 N
Part C
How hard does the surface of the ramp push on the car? N = 7600 N

Item 21
A large wrecking ball is held in place by two light steel cables (Figure 1).
[pic]
Part A
If the mass m of the wrecking ball is 4090kg , what is the tension TB in the cable that makes an angle of 40∘ with the vertical?
Express your answer using two significant figures.
*52000 N

Part B
If the mass m of the wrecking ball is 4090kg , what is the tension TA in the horizontal cable?
Express your answer using two significant figures.
*34000N