Chapter 4- Newton's Laws Conceptual Examples
Terms in this set (10)
A school bus comes to a sudden stop, and all of the backpacks on the floor start to slide forward. What force causes them to do that?
It is not a "force". By Newton's First Law, the backpacks continue their state of motion, maintaining their velocity. The backpacks slow down if a force is applied, such as friction with the floor
Newton's Firt Law Of Motion
Every object continues in a state of rest, or of uniform velocity in a straight line, as long as no net force acts on it
-Law of Inertia
-An object at rest will stay at rest unless an outside force acts on it
Newton's Second Law
The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to the object's mass. The direction of the acceleration is in the direction of the net force acting on the object
Newton's Third Law
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
What makes a car go forward?
A common answer is that the engine makes the car move forward. But it is not so simple. The engine makes the wheels go around. But if the tires are on slick ice or wet mud, they just spin. Friction is needed. On firm ground, the tires push backward against the ground because of friction. By Newton's Third Law, the ground pushes on the tires in the opposite direction, accelerating the car forward.
Michaelangelo's assistant has been assigned the task of moving a marble block using a sled. He asks "When i exert a forward force on the sled, the sled exerts an equal but opposite force backwards. So how can I start moving?
The forward (action) force is exerted by the assistant on the sled, whereas the backwards (reaction) force is exerted by the sled on the assistant. If he pushes hard enough on the ground, the force on him exerted by the ground will be larger than the sled pulling back as the assistant accelerates forward. The sled accelerates forward when the force on it exerted by the assistant is greater than the frictional force exerted backward on it by the ground.
A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be friction-less. What all forces are acting on the hockey puck?
A mover is trying to lift a piano up to a second story apartment. He is using a rope looped over two pulleys. What force must he exert on the rope to slowly lift the piano's 1600-N weight?
The magnitude of the tension force within the rope is the same at any point along the rope if we assume we can ignore its mass. Weight of piano pulls down on the pulley. The tension in the rope, looped through the pulley, pulls up TWICE, once on each side of the pulley. The piano mover can exert a force equal to half of the piano's weight.
You can hold a box against a rough wall and prevent it from sliding down by pressing hard horizontally. How does the application of a horizontal force keep an object from moving vertically?
The horizontal force you apply produces a normal force on the box exerted by the wall. (net force horizontally is 0 because box does not move horizontally). The force of gravity, mg, acting downward on the box can now be balanced by an upward static friction force whose maximum magnitude is proportional to the normal force. The harder you push, the greater Fn is and the greater Ff can be.
Your little sister wants you to give her a ride on her sled. On level ground, what is the easiest way to accomplish this? Push or pull her?
If you push her, there is a vertically down component to your force. Hence the normal force upward exerted by the ground will be larger than mg
If you pull her your force has a vertically upward component so the normal force will be less than mg.
Because friction is proportional to the normal force, it will be easier to pull her.
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