Ch 4: Homework

You are conducting an experiment inside an elevator that can move in a vertical shaft. A load is hung vertically from the ceiling on a string. The tension in the string is measured to be exactly EQUAL to the force due to gravity on the load. No other forces are acting on the load. Which of the following statements about the elevator are correct?

- The elevator is an inertial frame of reference. - The elevator may be at rest. - The elevator may be moving at a constant velocity upward. - The elevator may be moving at a constant velocity downward.

You are conducting an experiment inside an elevator that can move in a vertical shaft. A load is hung vertically from the ceiling on a string, and is stationary with respect to you. The tension in the string is measured to be 10% less than the force due to gravity on the load. No other forces are acting on the load. Which of the following statements about the elevator are correct?

- The elevator is not an inertial frame of reference. - The elevator must be accelerating This is because if the elevator is the frame of reference suggest that the elevator would never move. if it is not a frame of reference then it can move in relative to the ground

The magnitudes of these forces are given. No other forces are acting on the object. In which cases may the object possibly remain at rest?

2 N; 2 N 200 N; 200 N 2 N; 2 N; 4 N 2 N; 2 N; 2 N 2 N; 2 N; 3 N 200 N; 200 N; 5 N *Triangle can be form if the two smallest side add up to be more than the remaining side

A 1000-kg car is moving along a straight road down a 30∘ slope at a constant speed of 20.0m/s. What is the net force acting on the car?

= 0N this is because it is going at constant speed therefore there are no acceleration and thus net force = 0 *the car is also an inertial frame of reference.

Which object provides an inertial frame of reference? -the tip of the moving second hand of a clock -a rock thrown vertically upward -a pendulum swinging with no air resistance -a skydiver falling at terminal (constant) velocity

Assuming that the earth provides an inertial frame of reference, an object moving at a constant velocity relative to the earth would also provide an inertial frame of reference

Recall the portion of the video in which the girl pushes her brother on the sled at constant velocity. The pushing force she exerts on the sled is _____ the frictional force the ground exerts on the sled.

Equal to. If the sled is moving with constant velocity, the pushing force is equal to the frictional force not more than because it will resulting net force of zero. If the pushing force is greater than the friction the sled will accelerate.

Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite directions. (Assume no other forces act on either body.)

False, although the net force may have the same magnitude according to F =ma that does not mean that acceleration will be the same since it depended on mass.

The two forces in each (3rd law) pair can either both act on the same body or they can act on different bodies.

False, they have to act on different body. Each pairwise interaction produces a pair of opposite forces, one acting on each body. Force 1: Act on body A by body B Force 2: Act on body B by body A

The two forces in each pair may have different physical origins (for instance, one of the forces could be due to gravity, and its pair force could be due to friction or electric charge).

False. Force in a reaction pair must have the same type of force.

Newton's second law relates an object's acceleration to its mass and the net force acting on it. Does Newton's second law apply to a situation in which there is no net force? Select the best explanation.

Newton second law apply to ALL situation in which force act. Therefore Newton's second law applies to all situations, whether there is a net force or not. In the case of zero net force, any finite mass object must have zero acceleration, which means that its velocity is constant.

The mass of a rocket decreases as it burns through its fuel. If the rocket engine produces constant force (thrust), how does the acceleration of the rocket change over time?

Newton second law state: Force = mass * acceleration If the force remain constant but the mass is decreasing this mean that the acceleration must be increasing to make up for the decreasing mass.

Consider the train car described in the previous part. Another experiment is conducted in it: A net force of 20N is applied to an object of mass 5kg. Can you determine the acceleration of the object with respect to the train, and, if so, what is its value?

Not enough info, this is because to know the acceleration in relative to the train we need to know the train's acceleration as well

Imagine holding a basketball in both hands, throwing it straight up as high as you can, and then catching it when it falls. At which points in time does a zero net force act on the ball? Ignore air resistance.

Only when you -hold the ball before throwing it -After you catch it at all other time it leave your hand the ball is being affected by gravity pulling it down so the acceleration is g and not zero.

A force pair is produced when a tennis racket strikes a tennis ball. Which of the following best explains why the tennis ball does not have zero net force acting on it?

The forces in the force pair are equal in size, act in opposite directions, and act on different objects. One half of the force pair acts on the tennis ball and the other half acts on the racket and both objects individually experience a non-zero net force

You are conducting an experiment inside a train car that may move along level rail tracks. A load is hung from the ceiling on a string. The load is not swinging, and the string is observed to make a constant angle of 45∘ with the horizontal. No forces other than tension and gravity are acting on the load. Which of the following statements are correct?

The train is not an inertial frame of reference The train may be instantaneously at rest The train may be moving at a constant peed in a circle The train must be accelerating. Since the tension and the force due to gravity are not directed opposite to each other, the net force cannot possibly be zero--and yet the load is at rest relative to the train car. Therefore, the car is not an inertial frame of reference. It must be accelerating relative to the earth, although it is not clear exactly how.

Consider two cars moving along the same straight road in opposite directions. Car A has a mass of 500kg and has a constant speed of 20m/s; car B has a mass of 800kg and a constant speed of 15m/s. What can you say about the net forces on the cars?

They are both experiencing net force of zero since they are going at a constant speed

Every force has one and only one 3rd law pair force

True. 3rd law, which states that a physical interaction always generates A PAIR of forces on the two interacting bodies. and the two force in a pair act in opposite direction

According to Newton's first law, when the net force acting on an object is zero, the object must _____.

When the net force is zero, the acceleration is zero, therefore velocity must be constant. ***zero velocity is simply a special case of constant velocity.

An elevator moves straight upward at a constant speed. Newton's first law predicts that the net force acting on the elevator will _____.

be zero. Constant speed mean, zero acceleration therefore net force is zero.

According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is

equal in magnitude but antiparallel to the force on the earth due to the moon. *antiparallel mean parallel but moving or oriented in opposite directions.

What explains the dramatically different magnitudes of accelerations that result when a mosquito collides head on with a moving truck?

unequal masses of the bus and the mosquito Think of Newton second law of : F = ma a = f/m, therefore smaller mass will result in greater acceleration.