Chapter 4

• A space shuttle is carried out into space by powerful rockets which accelerate, increasing in speed rapidly.
• The rockets exert a force on the gases they push out from the rear, according to Newton’s second law.
• The ejected gases exert an equal and opposite force on the rockets in the forward direction, according to Newton’s third law.
• The reaction force exerted on the rockets by the gases, labeled, accelerates the rockets forward.
• The force exerted on the smaller player during a collision by the heavier player is most accurate if it is equal and opposite to the force exerted by the smaller player, according to Newton’s third law.
• The force that the earth presses up against our feet is a friction force, according to the poet T.S. Eliot.
• Objects move as they do because a force is required.
• Intuitively, we experience force as any kind of a push or a pull on an object.
• The tiny friction force is in the direction opposite to the motion, and the puck’s velocity decreases, even if very slowly.
• No net force acts on the puck, and the puck slides at constant velocity across the ice.
• The correct free-body diagram for a hockey puck sliding across frictionless ice is (b).
• The force exerted on the puck in the free-body diagram (b) is the force needed to maintain the motion.
• In the real world, where even smooth ice exerts at least a tiny friction force, the correct answer is (c).
• A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless.
• Contact forces are exerted when one object comes in contact with another object.
• An object falls because of the force of gravity, which is not a contact force.
• To start an object moving from rest requires force, which is a force to accelerate an object from zero velocity to a nonzero velocity.
• For an object already moving, if you want to change its velocity — either in direction or in magnitude — a force is required.
• To accelerate an object, a force is always required.
• Action and reaction forces act on different objects.
• The force exerted on the nail is exerted by the hammer.
• Forces come from observations such as a horse pulling a wagon, a person pushing a grocery cart, a hammer pushing on a nail, and a magnet attracting a paper clip.
• Newton’s laws are valid from an inertial reference frame outside the car, on the street.
• The nail exerts an equal force back on the hammer, causing the hammer’s speed to be rapidly reduced to zero upon contact.
• The acceleration of the cup is calculated as -1.1 * 10^4 N.
• The two objects must be treated on an equal basis according to Newton’s third law of motion: Whenever one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first.
• The equation for the velocity of the cup is: v = 0, v0 = 100 km/h = 27.8 m/s.
• If your hand pushes against the edge of a desk, the desk pushes back against your hand.
• Newton’s second law of motion describes quantitatively how forces affect motion.
• The equation for the acceleration of the cup is: v^2 = v0^2 + 2a (x - x0).
• The force that pushes the cup off the dashboard is calculated as (1500 kg)A - 7.0 m s^2.
• The magnitude (or strength) of a force can be measured using a spring scale.
• Newton’s third law states that whenever one object exerts a force on a second object, the second object always exerts an equal force on the first object which is opposite in direction: (4 ; 2)
• Inertia is the tendency of an object to resist a change in its motion.
• For solving problems involving the forces on one or more objects, it is essential to draw a free-body diagram for each object, showing all the forces acting on only that object.
• If an object is moving, is it possible for the net force acting on it to be zero? Explain.
• Newton’s second law states that the acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass: (4 ; 1)
• Mass is a measure of the inertia of an object.
• Only one force acts on an object.
• If the objects are at rest relative to each other, then the force of friction is just large enough to hold them at rest and satisfies the inequality: (4 ; 5)