Physics Chapter 4

Mass Resists Acceleration

The same force applied to: Twice the mass produces half the acceleration. 3 times the mass, produces 1/3 the acceleration. Acceleration~ 1/mass Acceleration is inversely proportional to mass.

Mass and Weight

1 kilogram weighs 10 newtons (9.8 newtons to be precise). Relationship between kilograms and pounds: 1 kg = 2.2 lb = 10 N at Earth's surface 1 lb = 4.45 N

Non-Free Fall—Example

A skydiver jumps from plane. Weight is the only force until air resistance acts. As falling speed increases, air resistance on diver builds up, net force is reduced, and acceleration becomes less. When air resistance equals the diver's weight, net force is zero and acceleration terminates. Diver reaches terminal velocity, then continues the fall at constant speed.

A. increases

A. increases As the skydiver falls faster and faster through the air, air resistance A. increases. B. decreases. C. remains the same. D. Not enough information.

B. decreases.

As the skydiver continues to fall faster and faster through the air, her acceleration A. increases. B. decreases. C. remains the same. D. Not enough information.

B. decreases

As the skydiver continues to fall faster and faster through the air, net force A. increases. B. decreases. C. remains the same. D. Not enough information.

C. Doubles Explanation: Acceleration = net force / mass Because, mass is in the denominator, acceleration increases as mass decreases. So, if mass is halved, acceleration doubles.

Consider a cart pushed along a track with a certain force. If the force remains the same while the mass of the cart decreases to half, the acceleration of the cart A. remains relatively the same. B. halves. C. doubles. D. changes unpredictably.

B. The heave person Explanation: They both have the same drag force (for the same speed). The man (heavier) has a greater downward force than the woman (lighter). The man has to drop farther to receive drag force equal to his downward force, so a higher terminal velocity.

Consider a heavy and light person jumping together with same-size parachutes from the same altitude. Who will reach the ground first? A. The light person B. The heavy person C. Both will reach at the same time. D. Not enough information.

B. 50 N Explanation: Then, ΣF = 0 and acceleration =

If a 50-N person is to fall at terminal speed, the air resistance needed is A. less than 50 N. B. 50 N. C. more than 50 N. D. None of the above.B. 50 N Explanation: Then, ΣF = 0 and acceleration = 0.

Halved

If the mass of an object is halved, the weight of the object is... A. halved. B. twice. C. depends on location. D. None of the above.

Newton's Second Law of Motion

In equation form: Acceleration= net force/mass Example: If net force acting on object is doubled ⇒ object's acceleration will be doubled. If mass of object is doubled ⇒ object's acceleration will be halved.

Newton's Second Law of Motion

Isaac Newton was the first to connect the concepts of force and mass to produce acceleration. Newton's second law (the law of acceleration) relates acceleration to force.

Mass and Weight

Mass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight.

Explanation: Acceleration = net force / mass If force doubles, acceleration will also double, But it does not, so mass must also be doubling to cancel out effects of force doubling.

Push a cart along a track so twice as much net force acts on it. If the acceleration remains the same, what is a reasonable explanation? A. The mass of the cart doubled when the force doubled. B. The cart experiences a force that it didn't before. C. The track is not level. D. Friction reversed direction.

Non-Free Fall

Terminal speed occurs when acceleration terminates (when air resistance equals weight and net force is zero). Terminal velocity same as terminal speed, with direction implied or specified.

Newton's Second Law of Motion

The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

Weight

The force upon an object due to gravity.

Free Fall

The greater the mass of the object... the greater its force of attraction toward the Earth. the smaller its tendency to move i.e., the greater its inertia. So, the acceleration is the same. It is equal to the acceleration due to gravity: 10 m/s2 (precisely 9.8 m/s2).

Mass

The quantity of matter in an object. It is also the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way.

A. Less than g Comment: Acceleration of a non-free fall is always less than g. Acceleration will actually be (20 N - 5 N)/2 kg = 7.5 m/s2.

When a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is A. less than g. B. more than g. C. g. D. terminated.

Free Fall

When acceleration is g—free fall Newton's second law provides an explanation for the equal accelerations of freely falling objects of various masses. Acceleration is equal when air resistance is negligible. Acceleration depends on force (weight) and inertia.

b. mass of the ball

When the string is pulled down quickly, the bottom string breaks, which best illustrates the A. weight of the ball. B. mass of the ball. C. volume of the ball. D. density of the ball.

A. Weight of the ball Explanation: Tension in the top string is the pulling tension plus the weight of the ball, both of which break the top string.

When the string is pulled down slowly, the top string breaks, which best illustrates the A. weight of the ball. B. mass of the ball. C. volume of the ball. D. density of the ball.

Mass versus weight

on the Moon and Earth: Weight of an object on the Moon is less than on Earth. Mass of an object is the same in both locations.