State Newton's first law of motion.
An object at rest remains at rest as long as no net force acts on it. An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it.
Or
If the net force on an object is zero, its
State Newton's second law of motion.
If an object is acted on by a net force, it will experience an acceleration that is equal to the net force divided by the mass. Because the net force is a vector, the acceleration is also a vector. In fact, the direction of an object's acceleration is the
Why does it hurt more to fall from a certain height to the ground than it does to fall from the same height into a swimming pool?
You go much further into the water before coming to a stop, thus your acceleration is much smaller, and the force exerted by the water is much smaller than that exerted by the ground.
An astronaut in a space capsule is experiencing "weightlessness." Does the astronaut have to exert any effort to throw a bowling ball, since the bowling ball is also weightless?
Yes. The ball will undergo an acceleration, and the force required to give it this acceleration is equal to the mass of the ball times the acceleration.
State Newton's third law of motion.
For every force that acts on an object, there is a reaction force acting on a different object that is equal in magnitude and opposite in direction.
When you catch a ball, you feel a force against your hand. Why?
You have to exert a force against the ball to decelerate it and the ball exerts an equal and opposite force against your hand.
When a hunter fires a rifle, the hunter feels a force against her shoulder. Explain why.
To accelerate the bullet, the rifle exerts a forward force against the bullet. The bullet, in turn exerts a backward force on the rifle, making it recoil. The hunter's shoulder exerts a forward force on the rifle to slow it down, and the rifle, in turn, e
A student uses a plumb bob to verify that the doorjamb in a car is vertical. As the car pulls away from a stop, the student observes that the string now makes an angle ? with the doorjamb. Why does this happen?
The car and the plumb bob are both accelerating. The force exerted by the string on the plumb bob must have a forward component, and the string pulls along its own length, so it cannot be vertical.
A 47.0-kg person steps on a scale in an elevator. The scale reads 461 N. What is the elevator doing?
It is either at rest or traveling with a constant velocity.
Force is a vector quantity.
TRUE
Mass is a measure of how difficult it is to change the velocity of an object.
TRUE
force is required for an object to move.
FALSE
The acceleration of an object does not have to be in the same direction as the net force applied to it.
FALSE
The mass of the Moon is about 1/80th of the mass of Earth. The force exerted by Earth on the Moon is about 80 times that exerted by the Moon on Earth.
FALSE
The force you exert on Earth is insignificant compared to the force Earth exerts on you.
FALSE
The mass of an object is fixed, but its weight varies from location to location.
TRUE
In the absence of an external force, a moving object will
E) move with constant velocity.
An object is moving with constant velocity. Which of the following statements is true?
D) The net force on the object is zero.
When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal velocity. This means that
E) the force of air resistance is equal to the weight of the parachutist.
A constant net force acts on an object. Describe the motion of the object.
B) constant non-zero acceleration.
Imagine that the metal head of a hammer is loose. In order to get the hammerhead tight again you should
B) drop the hammer with the handle end down.
You apply the same force to two objects. Object 1 has mass M and object 2 has mass 5M. The acceleration of object 2 is
D) one-fifth as that of object 1.
A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car. Since a lot of damage is done on the car
B) the force on the truck is equal to the force on the car.
A truck is towing a car whose mass is one quarter that of the truck. The force exerted by the truck on the car is 6000 N. The force exerted by the car on the truck is
D) 6000 N.
A golf club hits a golf ball with a force of 2400 N. The golf ball hits the club with a force
B) exactly 2400 N.
An object of weight W is in free-fall close to the surface of Earth. What is the force that the object exerts on Earth?
C) a force equal to W
An object of mass m sits on a flat table. The Earth pulls on this object with force mg, which we will call the action force. What is the reaction force?
D) The object pulling upward on the Earth with force mg.
A locomotive is pulling a number of identical wagons along a level track and accelerating. Friction is negligible. Starting from the last wagon, the ratio of the forces between adjacent wagons is
A) 1:2:3:4 . . .
A student uses a plumb bob to verify that the doorjamb in a car is vertical. As the car pulls away from a stop, the student observes that the string now makes an angle ? with the doorjamb. What is the acceleration of the car?
C) g tan ?
A ball is thrown up into the air. Ignore air resistance. When it is rising and reaches half of its maximum height, the net force acting on it is
A) equal to its weight.
A fireman is sliding down a fire pole. As he speeds up, he tightens his grip on the pole, thus increasing the vertical frictional force that the pole exerts on the fireman. When this force equals the weight of the fireman, what happens?
D) The fireman continues to descend, but with constant speed.
A person is lowering a bucket into a well with a constant speed. The force exerted by the rope on the bucket is
A) equal to the bucket's weight.
You ride on an elevator that is moving downward with constant speed while standing on a bathroom scale. The reading on the scale is
A) equal to your true weight, mg.
You ride on an elevator that is moving with constant downward acceleration while standing on a bathroom scale. The reading on the scale is
B) less than your true weight, mg.
What does the word "normal" mean in the phrase "normal force"?
D) the component of the force exerted by a surface perpendicular to the surface
A block of mass M slides down a frictionless plane inclined at an angle ? with the horizontal. The normal reaction force exerted by the plane on the block is
C) Mg cos ?.
A block of mass M slides down a frictionless plane inclined at an angle ? with the horizontal. The normal reaction force exerted by the plane on the block is directed
C) perpendicular to the plane.
A block of mass M slides down a frictionless plane inclined at an angle ? with the horizontal. The gravitational force is directed
D) toward the center of the Earth.
An object rests on an inclined surface. If the inclination of the surface is made steeper, what does the normal force on the object do?
B) decrease
Two objects have masses m and 5m, respectively. They both are placed side by side on a frictionless inclined plane and allowed to slide down from rest.
E) The two objects reach the bottom of the incline at the same time.