Physics chapter 4
A woman is standing on the Earth. In terms of magnitude, is her gravitational force on the Earth
equal to,
A large crate of mass is placed on the back of a truck but not tied down. As the truck accelerates forward with an acceleration , the crate remains at rest relative to the truck. What force causes the crate to accelerate forward?
the force of friction between the crate and the floor of the truck
If an object is in equilibrium, which of the following statements is not true?
the object must be at rest
A weight lifter stands on a bathroom scale. As she pumps a barbell up and down, what happens to the reading on the scale? Suppose she is strong enough to actually throw the barbell upward. How does the reading on the scale vary now?
(a) The barbell always exerts a downward force on the lifter equal in magnitude to the upward force that she exerts on the barbell. Since the lifter is in equilibrium, the magnitude of the upward force exerted on her by the scale(that is, the scale is reading) equals the sum of her weight and the downward force exerted by the barbell. (pushing the barbell up will accelerate her motion thus increasing the weight on the scale - lowering it down decelerates thus decreasing the reading on the scale) As the barbell goes through the bottom of the cycle and is being lifted upward, the scale reading exceeds the combined weights of the lifter and the barbell. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weight. (b) If the barbell is moving upward, the lifter can declare she has thrown it just by letting go of it for a moment. Thus, the case is included in the previous answer.
A volume of rock has a mass roughly three times a similar volume of ice. Suppose one world is made of ice whereas another world with the same radius is made of rock. If is the acceleration of gravity on the surface of the ice world, what is the approximate acceleration of gravity on the rock world?
3g
An exoplanet has twice the mass and half the radius of the Earth. Find the acceleration due to gravity on its surface, in terms of , the acceleration of gravity at Earth's surface.
8g
what is a force?
A force is commonly imagined as a push or a pull on some object, perhaps rapidly, as when we hit a tennis ball with a racket. (See Fig. 4.1.) We can hit the ball at different speeds and direct it into different parts of the opponent's court. That means we can control the magnitude of the applied force and also its direction, so force is a vector quantity, just like velocity and acceleration.
If the car is parked on a more gentle slope, how will the time required for it to slide to the bottom of the hill be affected? Explain.
A gentler slope means a smaller angle and hence a smaller acceleration down the slope. Consequently, the car would take longer to reach the bottom of the hill.
what is mass?
Although inertia is the tendency of an object to continue its motion in the absence of a force, mass is a measure of the object's resistance to changes in its motion due to a force.
Which of the following statements are true?
An astronaut's mass is the same on the International Space Station as it is on Earth.
How could this simple machine be used to raise objects too heavy for a person to lift?
Attach one end of the cable to the object to be lifted and the other end to a platform. Place lighter weights on the platform until the total mass of the weights and platform exceeds the mass of the heavy object.
If mass M2 is increased, does the acceleration of the system increase, decrease, or remain the same? Does the tension increase, decrease, or remain the same?
Both the acceleration and the tension increase when is increased.
True or False: In general, the magnitude of the acceleration of an object is determined by the magnitudes of the forces acting on it.
False. The object's mass and the angles at which the forces are applied are also important in determining the magnitude of the acceleration vector.
A space explorer is moving through space far from any planet or star. He notices a large rock, taken as a specimen from an alien planet, floating around the cabin of the ship. Should he push it gently, or should he kick it toward the storage compartment? Explain.
If it has a large mass, it will take a large force to alter its motion even when floating in space. Thus, to avoid injuring himself, he should push it gently toward the storage compartment.
What are contact forces?
If you pull on a spring (Fig. 4.2a), the spring stretches. If you pull hard enough on a wagon (Fig. 4.2b), the wagon moves. When you kick a football (Fig. 4.2c), it deforms briefly and is set in motion. These are all examples of contact forces, so named because they result from physical contact between two objects.
If only one force acts on an object, can it be in equilibrium? Explain.
In order for an object to be in equilibrium, the resultant force acting on it must be zero. Thus, it is not possible for an object to be in equilibrium when a single force of non-zero magnitude acts on it.
A truck loaded with sand accelerates along a highway. The driving force on the truck remains constant. What happens to the acceleration of the truck as its trailer leaks sand at a constant rate through a hole in its bottom?
It increases at a steady rate.
A crate remains stationary after it has been placed on a ramp inclined at an angle with the horizontal. Which of the following statements must be true about the magnitude of the frictional force that acts on the crate?
It is greater than the component of the gravitational force acting down the ramp.
As a block slides down a frictionless incline, which of the following statements is true?
Its speed increases and its acceleration remains constant.
What other forces act on the airboat? Describe them.
Other than the forces mentioned in the problem, the force of gravity pulls downwards on the boat. Because the boat doesn't sink, a force exerted by the water on the boat must oppose the gravity force. (In Topic 9 this force will be identified as the buoyancy force.)
SI unit of force
The SI unit of force is the newton.
ow would the answer be affected if the puck were struck by an astronaut on a patch of ice on Mars, where the acceleration of gravity is 0.35 g, with all other given quantities remaining the same?
The coefficient of kinetic friction would be larger than in the example by a factor of 1/0.35 = 2.6
If you push on a heavy box that is at rest, you must exert some force to start its motion. Once the box is sliding, why does a smaller force maintain its motion?
The coefficient of static friction is large than that of kinetic/dynamic friction. To start the box moving, you must counterbalance the maximum static friction force. This force exceeds the kinetic friction force that you must counterbalance to maintain constant velocity of the box once it starts moving.
n Figure 4.4, a locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall?
The force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive.
A passenger sitting in the rear of a bus claims that she was injured as the driver slammed on the brakes, causing a suitcase to come flying toward her from the front of the bus. If you were the judge in this case, what disposition would you make? Explain.
The inertia of the suitcase would keep it moving forward as the bus stops. There would be no tendency for the suitcase to be thrown backward toward the passenger. The case should be dismissed.
Analyze the motion of a rock dropped in water in terms of its speed and acceleration as it falls. Assume a resistive force is acting on the rock that increases as the velocity of the rock increases.
The net force acting on the object decreases as the resistive force increases. Eventually, the resistive force become equal to the weight of the object, and the net force goes to zero. In this condition, the object stops accelerating, and the velocity stays constant. The rock has reached its terminal velocity.
what is net force?
The net force on an object is defined as the vector sum of all external forces exerted on the object. External forces come from the object's environment. If an object's velocity isn't changing in either magnitude or direction, then its acceleration and the net force acting on it must both be zero.
What is inertia?
The tendency of an object to continue in its original state of motion is called inertia.
In a tug-of-war between two athletes, each pulls on the rope with a force of 200 N. What is the tension in the rope? If the rope doesn't move, what horizontal force does each athlete exert against the ground?
The tension in the rope is the maximum force that occurs in both directions. In this case, then, since both are pulling with a force of magnitude 200 N, the tension is 200 N. If the rope does not move, then the force on each athlete must equal zero. Therefore, each athlete exerts 200 N against the ground.
Choose the best answer. A car traveling at constant speed has a net force of zero acting on it.
True
Suppose you are driving a car at a high speed. Why should you avoid slamming on your brakes when you want to stop in the shortest possible distance? (Newer cars have antilock brakes that avoid this problem.)
When a tire is rolling, the point on the tire in contact with the ground is momentarily at rest relative to the ground. Thus, static friction exists between the tire and the ground under these conditions. When the brake lock, the tires begin to skid over the ground and kinetic friction now exists between the tires and the ground. Since the kinetic friction force is less than the maximum static friction force, the friction force tending to slow the car is less with the brakes locked than while the tires continue to roll.the kinetic coefficient is smaller so you have a weaker force to stop the car once you are sliding.
