Newton's Laws

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Newton's Laws Name:

Net Force and Acceleration Read from ​Lesson 3 ​of the ​Newton's Laws ​chapter at ​The Physics Classroom​: http://www.physicsclassroom.com/Class/newtlaws/u2l3a.html http://www.physicsclassroom.com/Class/newtlaws/u2l3b.html http://www.physicsclassroom.com/Class/newtlaws/u2l3c.html MOP Connection: ​Newton's Laws: sublevels 3 (front), 8 and 9 (back) 1. Luke Autbeloe drops a 5.0 kg fat cat (weight = ~50.0 N) off the high dive into the pool below (which on this occasion is filled with water). Upon encountering the water in the pool, the cat encounters a 50.0 N ​upward ​restraining force. Which ​one ​of the velocity-time graphs best describes the motion of the cat? ________ Accompany your answer with a description of the cat's motion.

A ​B ​C

Description of cat's motion while falling through air: The cat will have a downward (-) velocity, be speeding up, and have a downward (-) acceleration.

Description of cat's motion after hitting the water: The cat will have a downward (-) velocity with a constant value (due to the balanced forces).

2. Which ​one ​of the following dot diagrams best describes the motion of the falling cat from the time that they are dropped to the time that they hit the ground? ​Tape A ​The arrows on the diagram represent the point at which the cat hit the water. Support your answer with sound reasoning: Before hitting the water, the cat accelerates due to the 50 N downward force; this rules out Tape B. Upon hitting the water, there is a balance of forces and the cat's motion becomes a constant velocity motion; this rules out Tape C. 3 Several of Luke's friends were watching the motion of the falling cat. Being "physics types", they began discussing the motion and made the following comments. Indicate whether each of the comments are correct or incorrect? Support your answers.

Student Statement: Correct? a. Once the cat hit the pool, the forces are balanced and the cat will stop.

Reason: ​The forces are balanced and the cat is moving; an object in motion​ ​continues in motion when the forces are balanced.

No

b. Upon hitting the pool, the cat will accelerate upwards because the pool applies an upward force.

Reason: ​Upon hitting the water, there are two forces of equal magnitude and​ ​opposite direction; there is no net force. Acceleration requires a net force.

No

c. Upon hitting the pool, the cat will bounce upwards due to the upward force. Reason: ​W​hen the forces on a moving cat become balanced, the cat

No

keeps ​ ​moving at a constant velocity.

© The Physics Classroom, 2020 Page 1 Newton's Laws

4. For each force diagram, determine the net or resultant force (∑F), the mass and the acceleration of the object. Identify the direction (the second blank) of the two vector quantities. NOTE: F​ grav stands for the weight of the object.

NOTE: ​In each problem below, the net force (∑F) is determined by adding up all the individual force values ​as vectors;​ their direction must be considered when adding them. The mass is the down force (F​grav​) divided by 9.8 N/kg; the reported mass values are rounded at the second digit. The acceleration is determined by dividing the net force (∑F) by the mass (unrounded value); reported values are rounded at the second digit.

a.

b.

∑F = ​600 N​, ​down

∑F = ​200 N​, ​down

m = ​61 kg

m = ​61 kg

a = ​9.8 m/s​2​,​ ​down c.

a = ​3.3 m/s​2​,​ ​down d.

∑F = ​2000 N​,​ ​left

∑F = ​4000 N​,​ ​left

m = ​820 kg (816 kg)

m = ​820 kg (816 kg)

a = ​2.5 m/s​ ,​ ​left

a = ​4.9 m/s​2​,​ ​left

2​

e.

f.

∑F = ​8 N​,​ ​right

∑F = ​16 N​,​ ​right

m = ​2.0 kg (2.04 kg)

m = ​4.1 kg (4.08 kg)

a = ​3.9 m/s​ ,​ ​right

a = ​3.9 m/s​2​,​ ​right

2​

© The Physics Classroom, 2020 Page 2