Ch 4-- Newton's 2nd law of motion
A friend says that as long as a car is at rest, no forces act on it. What do you say if you are in the mood to correct your friend?
The car has weight so some forces must be acting on it. But since the car is at rest, it is not accelerating so the net force must be zero.
If a mass of 1kg is accelerated 1 m/s/s by a force of 1N, what would be the acceleration of 2kg acted on by a force of 2N?
1 m/s/s a=∑forces/mass
What is the acceleration of a rock at the top of its trajectory when thrown straight upward? Is your answer consistent with Newton's 2nd Law of Motion?
10 m/s/s down. Yes.
What is the weight of a 1kg weight near the earth's surface?
10N
Calculate your metric mass and metric weight on earth
150 lb= 68 kg= 680N
Convert kilograms to Newtons
1kg= 10N
If you stand next to a wall on a frictionless skateboard and push the wall with a force of 30N, how hard does the wall push on you? If your mass is 60kg, what is your acceleration?
30N in the opposite direction 1/2 m/s/s
What is the net force that acts on a 10N falling object when it encounters 4N of air resistance? 10N of air resistance?
6N, 0N
A cart is pulled to the left with a force of 100N and to the right with a force of 30N. What is the net force on the cart?
70N to the left
What is the acceleration of an object that moves at a constant velocity? What is the net force on the object in this case.
Acceleration is 0 m/s/s, Net force is zero. An object moving at constant velocity does not accelerate and does not have an unbalanced force acting on it.
If the net force acting on a sliding block is somehow tripled, by how much does the acceleration increase?
Acceleration is directly proportional to net force. If net force triples, acceleration triples.
Is acceleration directly or inversely proportional to mass?Give an example.
Acceleration is inversely proportional to mass. If mass doubles, acceleration is cut in half. SUVs are generally more massive than cars and have poor acceleration (unless a lot of extra horsepower is added)
if the mass of a sliding block is tripled while a constant net force is applied, by how much does acceleration decrease?
Acceleration will be one third its original value acceleration is inversely proportional to mass.
What is meant by free fall?
An object is in free fall when the only forces acting on it are gravity and possibly air friction. Air friction is often ignored when its value is small when compared to the force of gravity on the object.
State Newton's 2nd law of motion
An object's acceleration is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object
If we find an object that is not moving even though we know it to be acted on by a force, what inference can we draw?
Because it is not accelerating, we know that the sum of all forces that are acting on the object is zero. Since we are told that a force acts on the object, we know that there must be one or more forces acting on the object whose sum is equal in magnitude and opposite in direction to the first force.
If we say one quantity is proportional to another, does this mean they are equal to each other? Explain using mass and weight as an example.
Being proportional does not mean being equal. Your weight is proportional to your mass but not equal to your mass. My mass is about 70kg and my weight is about 700N. Being proportional means that if I doubled my mass to 140kg, my weight would also double to 1400N. Mass and weight are not equal but they are proportional to each other.
What is the net force that acts on a 10 N freely falling object?
If we ignore air friction, the net force on an object in free fall is its weight, in this case 10N
When a junked car is crushed into a compact rectangle, does a) its mass change? b) its weight change?
Mass and weight do not change, volume has changed?
Distinguish between mass, weight and volume
Mass is a measure of an object's resistance to change its velocity, weight is the force of gravity on an object, volume is the amount of space that an object occupies
Which is more fundamental, mass or weight? which varies with location?
Mass is more fundamental, weight varies with location because it is affected by gravity
If the earth exerts a force of 1000N on an orbiting communications satellite, how much force does the satellite exert on the earth?
Newton's 3rd law of motion tells us that the satellite must be exerting a 1000N force on the earth.
Can an object round a curve without any force acting on it?
No, going around a curve involves changing the direction of travel. An object cannot change its speed or its direction without the application of an unbalanced force.
In the orbiting space shuttle, you are handed two identical boxes, one filled with sand and the other filled with feathers. How can you tell which is which without opening the boxes?
Push each of them, the one that is more difficult to accelerate is the one that is more massive-- the one with sand in it.
What kind of path would the planets follow if suddenly no net forces acted on them?
Since there are no unbalanced forces acting on the planets, newton's 1st law of motion tells us that they would travel in a straight line at a constant speed. All the planets, including earth, would stop revolving around the sun and we would not continue to live happily ever after.
A 400kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear?
Since velocity is constant, the force of friction must equal the weight of the bear. 400kg= 4000N The friction force is 4000N in the opposite direction the bear is sliding.
When your car moves along the highway at constant velocity, the net force on it is zero. Why then do you continue running your engine?
The engine is supplying the energy that allows the wheels to push the car forward. Wind friction (and other sources of friction) are pushing the car backwards. When the car is moving at constant velocity, the sum of all these forces must be zero. If you turn off the engine, frictional forces will slow the car down.
Your weight is the result of a gravitational force of the earth on your body. What is the corresponding reaction force?
The mass of your body is pulling back on the earth.
What two principal factors affect the force of air resistance on a falling object?
The speed of the falling object and its shape. (the larger the surface area perpendicular to the direction of motion, the larger the air friction.)
If it takes 1N of force to push horizontally on your book to make it slide at constant velocity, how much force of friction acts on the book?
There must be a 1N force of friction back in the opposite direction. The forces will cancel each other out and the net force will be zero-- the book must then move at a constant velocity.
If the mass of a sliding block is tripled at the same time the net force on it is tripled, how does the acceleration change?
There will be no change A= f/m a= 3f/3m
How does the direction of acceleration compare to the direction of the net force applied to the object?
They are always in the same direction (part of Newton's 2nd law)
Gravitational force on the moon is only 1/6 that of the gravitational force on earth. What would be the weight of a 10kg object on Earth and on the moon? What would its mass be on the moon and the earth?
Weight on Earth= (10kg)(10N)= 100N Weight on Moon= 100N/6= 17 N Mass would not change with location.
Once a crate you're pushing is sliding, how hard do you push to keep it moving at a constant velocity?
With a force that is equal to the kinetic or sliding friction
Can the velocity of an object reverse direction while maintaining constant acceleration? Give an example
Yes, throwing a ball straight up and then it comes down. the acceleration is always 10 m/s/s but the velocity changes when the ball comes back down.
What is the net force on a mercedes convertible traveling along a straight road at a steady speed of 100 km/hr?
Zero, the convertible is not accelerating so no unbalanced force is acting on it.
What is the acceleration of a falling object that has reached its terminal velocity?
Zero, when air resistance equals the objects weight, terminal velocity has been achieved and acceleration terminates
Consider a woman weighing 500N who stands with her weight equally divided on a pair of bathroom scales. a) What is the reading on each scale? b) if she shifts her weight so one of the scales reads 300N, what will the other scale read?
a) 250N per scale b) 200N
If you push horizontally with a force of 50N on a crate and make it slide at constant velocity a) how much friction acts on the crate? b) if you increase your force, will the crate accelerate?
a) 50N in the opposite direction b) if you increase your force, then yes, the crate will accelerate since the force that you apply is larger than the frictional force, you will have an unbalanced force. Whenever an unbalanced force is applied, an object accelerates.
a) What is the net force on an apple that weighs 1N when you hold it at rest above the floor? b) what is the net force on it when you release it?
a) zero b) 1N
another equation for acceleration (not change speed over time)
a=∑forces/mass
What is the acceleration of a 40kg block of cement when pulled sideways with a net force of 200n?
a=∑forces/mass a= 200N/40kg= 5m/s/s
A rocket becomes progressively easier to accelerate as it travels through space. Why is this so? (hint: about 90% of the mass of a newly launched rocket is fuel.)
acceleration is inversely proportional to mass. If the mass of the ship has been cut in half because half of the fuel is used, the acceleration doubles.
A shooting star is usually a grain of sand from outer space that gives off light as it burns up. What exactly causes this burning?
air friction
What relationship does mass have with weight?
an object's weight is directly proportional to its mass. weight=mg if you triple the object's mass, the weight is tripled
inertia
an objects resistance to change its velocity
If the net force on a sliding block is tripled, by how much does the acceleration increase?
it triples, net force is directly proportional to acceleration
What relationship does mass have with inertia?
mass is a measure of an object's inertia
What is the SI unit for mass? What is the SI unit for force?
mass= kilogram force= Newton
Mass
measure of quantity of matter and its inertia
If an object has no acceleration, can you conclude that no forces are exerted on it?
no, you can only conclude that the net forces are zero.
terminal velocity
the speed and direction at which the acceleration of a falling object terminates (is zero) because air resistance equals the object's weight.
How great is the force of friction compared with your push on a crate that doesn't move on a level floor? If you increase your push, will the friction on the crate increase also?
they are equal in magnitude and opposite in direction if you increase your push, the friction on the crate will also increase
