chapter 4 Flashcards
An object of mass m sits on a flat table. The Earth pulls on this object with force mg, which we will call the action force. What is the reaction force?
The object pulling upward on the Earth with force mg.
As a car drives with its tires rolling freely without slipping, the type of friction acting between the tires and the road is usually ________
static friction
A hockey puck slides across the ice with a constant velocity. What is the net force acting on the puck?
zero
1st law of motion (law of intertia)
an object moves at a constant velocity (or stays at rest) as long as no net force acts on it
inertia
tendency of a body to maintain motion at constant velocity or at a state of rest (zero velocity)
an object at rest…
no net force is acting on it
an object moves with constant velocity..
no net force is acting on it
You put your book on the car seat next to you. When you hit the brakes suddenly, the book slides forward off the seat. When does this happen?
no net force acts on it
- book continued to move forward, which was its intail state of motion
You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. What can you conclude?
A net force acted on it all along
2nd Law of Motion
The acceleration of an object is directly proportional to the net force (vector sum of the forces) acting on it and is inversely proportional to the objects mass
How much tension must a rope withstand if it is used to accelerate a 1210 kg car horizontally along a frictionless surface at 1.20 m/s^2?
F=ma
(1210 kg)(1.20 m/s^2) = 1452 N
A force F acts on mass m1 giving acceleration a1. The exact same force acts on a different mass m2 giving acceleration a2=2a1. If m1 and m2 are glued together and the same force F acts on this combination, what is the resulting acceleration?
-going to be smaller than a1
F=(m1+m2)a
F=(.5m1)(2a1)
2/3 a1
3rd Law of Motion
Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.
Forces are associated with interactions between objects.
T/F
True
- action/reaction pair
A large truck collides head-on with a small compact car. During the collision,
Which of Newton’s laws best explains why motorists should buckle-up?
the first law
When you sit on a chair, the resultant (net) force acting on you is ________
zero
In the absence of an external force, a moving object will ________
move with a constant velocity.
A rocket moves through empty space in a straight line with constant speed. It is far from the gravitational effect of any star or planet. Under these conditions, the force that must be applied to the rocket in order to sustain its motion is _________
zero
You are standing in a moving bus, facing forward, and you suddenly fall forward. You could imply from this that the bus’s __________
velocity decreased.
A constant net force acts on an object. Describe the motion of the object.
constant acceleration
The acceleration of an object is inversely proportional to ________
its mass
A net force F acts on a mass m and produces an acceleration a. What acceleration results if a net force 2F acts on a mass 4m?
a/2
An object of mass m sits on a flat table. The Earth pulls on this object with force mg, which we will call the action force. In this case, what is the reaction force?
The object pulling upward on the Earth with force mg.
The acceleration due to gravity is lower on the Moon than on Earth. Which of the following is true about the mass and weight of an astronaut on the Moon’s surface, compared to Earth?
Mass is same, weight is less.
An example of a force which acts at a distance is _______
weight
A 20-N weight and a 5.0-N weight are dropped simultaneously from the same height. Ignore air resistance. Which one accelerates faster?
They both accelerate at the same rate because they both have the same weight to mass ratio, g.
Why is it more difficult to start moving a heavy carton from rest than it is to keep pushing it with constant velocity?
Because μs > μk .
Two toy cars (16 kg and 2.0 kg) are released simultaneously on an inclined plane that makes an angle of 30° with the horizontal. Which statement best describes their acceleration after being released?
Both cars accelerate at the same rate.
A sports car of mass 1000 kg can accelerate from rest to 27 m/s in 7.0 s. What is the average forward force on the car?
3.9 x 103 N
If you push a 4.0-kg mass with the same force that you push a 10-kg mass from rest, ________
the 4.0-kg mass accelerates 2.5 times faster than the 10-kg mass.
An object has a mass of 60 kg on the Earth. What is the mass of the object on the surface of the Moon where the acceleration due to gravity is only 1/6th that on the Earth?
60 kg
A person of weight 480 N stands on a metric scale in an elevator. What will the scale be reading when the elevator is accelerating downward at 4.00 m/s2?
284 N
A person riding in an elevator stands on a metric scale. If the mass of the person is 60.0 kg and the elevator accelerates upward with an acceleration of 4.90 m/s2, what is the reading on the scale?
882 N
In the Atwood machine shown above, if M = 0.60 kg and m = 0.40 kg, what is the magnitude of the acceleration of the system? (Ignore friction and the mass of the pulley.)
2.0 m/s2
Two boxes of masses m and 2m are in contact with each other on a frictionless surface. (See the figure above.) What is the acceleration of the more massive box?
F/(3m)
A packing crate slides down an inclined ramp at constant velocity. Thus we can deduce that _________
a frictional force is acting on it.
An object of mass 6000 kg rests on the flatbed of a truck. It is held in place by metal brackets that can exert a maximum horizontal force of 9000 N. When the truck is traveling 15 m/s, what is the minimum stopping distance if the load is not to slide forward into the cab?
75 m
