Chapter 4 Conceptual Questions

(a) Give an example of different net external forces acting on the same system to produce different accelerations. (b) Give an example of the same net external force acting on systems of different masses, producing different accelerations. (c) What law accurately describes both effects?

(a) A 30 N force is applied to a crate on the floor causing it to accelerate. If a 40 N force is applied to the same crate, it will have a greater acceleration. (b) A 30 N force is applied to an 80 kg sled causing it to accelerate. If a 30 N force were applied to a 60 k g sled, the 60 N sled will have a greater acceleration than the 80 kg sled. (c) Newton's 2nd law describes both effects. The acceleration of a system is proportional to the net external forces on the system, and inversely proportional to the mass of the system.

Which statement is correct? (a) Net force causes motion. (b) Net force causes change in motion.

A net force causes an object to accelerate. An object can already be moving without the application of net force. Ex. If you kick a ball in the air, it will accelerate, and then slow down as it reaches its max height and then accelerate again as it lands on the ground. The ball's change in motion is net force (when it accelerates and slows down).

A system can have a nonzero velocity while the net external force on it is zero. Describe such a situation.

An object being pushed across the floor at constant velocity.

What properties do forces have that allow us to classify them as vectors?

Force has magnitude and direction, so it's considered a vector quantity.

A rock is thrown straight up. What is the net external force acting on the rock when it is at top of its trajectory?

Gravitational force

Newton's third law of motion tells us that forces always occur in pairs of equal and opposite magnitude. Explain how the choice of the "system of interest" affects whether one such pair of forces cancels.

If our system of interest doesn't include directly the origin of all forces, then Newton's third law doesn't apply.

If the acceleration of a system is zero, are no external forces acting on it? What about internal forces?

If the acceleration of a system is zero, then the sum of the external forces is zero. There can be external forces, the sum of those forces just has to be zero. The internal forces on a system will have no effect on whether a system is accelerating or not. If any part of a system is exerting a force on another part of a system, it would be considered an internal force.

How are inertia and mass related?

Inertia is measured by the mass of the object. The more mass the object has, the more inertia.

What is the relationship between weight and mass? What is an intrinsic, unchanging property of a body?

Mass is the measure of amount of matter in an object. It is an intrinsic, unchanging property of a body. Mass cannot be destroyed nor be created. Weight is the production of mass and acceleration due to gravity. It changes with value of accelerate due to gravity. That's why it's not an intrinsic property, it's a changing property for a body.

Describe a situation in which the net external force on a system is not zero.

Net force causes acceleration, which means a change in velocity. An object in circular motion is an example of that. The net force constantly changes objects speed direction, but magnitude stays constant (like a vector)

Why can we neglect forces such as those holding a body together when we apply Newton's second law of motion?

Newton's 2nd law applies to external forces acting on a system. The forces holding a body together are internal forces.

Describe a situation in which one system exerts a force on another and, as a consequence, experiences a force that is equal in magnitude and opposite in direction. Which of Newton's laws of motion apply?

Newton's 3rd law of motion

Explain how the choice of the "system of interest" affects which forces must be considered when applying Newton's second law of motion.

Only forces that act upon the chosen object are taken into consideration.

If a constant, nonzero force is applied to an object, what can you say about the velocity and acceleration of the object?

The object must accelerate, which means the velocity of that object must be changing.

An American football lineman reasons that it is senseless to try to out-push the opposing player, since no matter how hard he pushes he will experience an equal and opposite force from the other player. Use Newton's laws and draw a free-body diagram of an appropriate system to explain how he can still out-push the opposition if he is strong enough.

The person with less friction will get pushed because they are the one who has less friction on the surface they are standing.

Why does an ordinary rifle recoil (kick backward) when fired? The barrel of a recoil-less rifle is open at both ends. Describe how Newton's third law applies when one is fired. Can you safely stand close behind one when it is fired?

The rifle shoots with a huge amount of force and the bullet applies a backward force on the rifle. If a rifle is open at both ends, the air is pushed backwards rather than the rifle itself. Yes, you can stand close behind one if you put it close to your shoulder, so that the whole body absorbs the sudden force.

When you take off in a jet aircraft, there is a sensation of being pushed back into the seat. Explain why you move backward in the seat; is there really a force backward on you?

They move backward because the plane moves forward, there is no backward force on their body. Due to Newton's 3rd law, the person exerts an equal force back onto the seat causing them to move backward into their seat.