Chapter 6, 7 Newton’s Second and Third Law

Question For a constant force, how does an increase in mass affect an object’s acceleration?

Answer Decreases acceleration

Question What causes accelerations? Be as specific as possible!

Answer A nonzero net force Unbalanced force

Question What quantity is directly proportional to an object’s acceleration?

Answer Force

Question Calculate the acceleration of a 100-kg cart when the net force on it is 50 N.

Answer A = 0.5 m/s²

Question Calculate the horizontal force needed to make a 1-kg hockey puck accelerate at 1000 m/s²

Answer 1000 N

Question Heather can apply a force of 72 N to a wagon in which sits her little brother, Bryce. The combined mass of Bryce and the wagon is 48 kg. If starting from rest, how much speed will the wagon pick up after Heather has pushed for 3 s?

Answer 4.5 m/s

Question A golf ball leaves a tee at a speed of 75 m/s. If the ball has a mass of 0.05 kg and gains this speed in 0.02 s, what is the average force of impact of the club on the ball?

Answer N

Question What is the pressure on a table when a 15-N dictionary with a 0.05 m² cover lies flat on it?

Answer 300 N/m²

Question A 500-kg subcompact car and a 1500-kg standard car are given equal accelerations. How much greater is the force that acts on the more massive car?

Answer The force on the big car is 3 times that on the small car.

Question Scarlet Skydiver, who has a mass of 60 kg, jumps form a stationary helicopter. What is the net force on her 10 s into the dive, when she has reached terminal speed?

Answer Net force = 0

Question What factors affect the force of friction between surfaces? (2; 1 point each)

Answer Kinds of material in contact How much the surfaces are pressed together (the support force)

Question After being pushed and released, a 50-kg crate slides across a factory floor. Friction on the sliding crate is 200 N. What is the crate’s acceleration?

Answer - 4 m/s² in the direction opposite motion

Question What is the acceleration of a 20-kg container of paint being pulled upward (not sideways) with a force of 300 N?

Answer 5 m/s²

Question A 10.0-kg block slides down a ramp inclined 30° to the horizontal. How great is the normal force by the ramp on the block?

Answer 86.6 N

Question Find the acceleration of a plastic block of mass 3 kg that slides down a friction free plane inclined at 42°to the horizontal.

Answer 6.7 m/s²

Question For a constant force, how does an increase in area affect the resulting pressure?

Answer It decreases pressure

Question True or False: Gravity exerts a greater force on more massive objects.

Answer True

Question What 2 factors affect the amount of air resistance an object experiences? (1 point each)

Answer Speed and area

Question The ratio of _____ to _____ is the same for all objects. This proves that all objects undergoing free fall (in the absence of air resistance) will accelerate at the same rate.

Answer weight

Question Scarlet Skydiver, who has a mass of 60 kg, jumps from a stationary helicopter. What is her acceleration when she reaches terminal speed?

Answer a = 0 m/s²

Question Mass and weight are proportional. This means that heavier objects also are more massive. If gravity pulls on heavier objects with a greater force, how is it that all objects undergoing free fall accelerate at the same rate in the absence of air resistance?

Answer More massive objects have more inertia

Question When accounting for air resistance, which will reach terminal speed first, a feather or a coin?

Answer A feather

Question For a constant area, how will an increase in force affect the resulting pressure?

Answer Increase in pressure

Question A 5 kg box slides down an incline at an angle of 15° to the horizontal. Find the coefficient of friction between the box and the incline.

Answer 0.27

Question A 65-kg astronaut in the space station kicks a soccer ball with a force of 13 N. At what rate does the astronaut accelerate backward?

Answer a = 0.20 m/s²

Question Daniel (mass 40 kg) and Justin (mass 60 kg) face each other at rest while wearing roller blades. Daniel pushes Justin and accelerates him at 1.6 m/s². What is Daniel’s acceleration in the other direction?

Answer 2.4 m/s²

Question Forces always occur in __________

Answer pairs

Question Consider hitting a baseball with a bat. If we call the force the bat exerts against the ball that action force, identify the reaction force.

Answer A baseball hits a bat

Question When a cannon is fired, why do the cannonball and cannon have very different accelerations?

Answer Because of their different masses

Question A small car bumps into a van at rest in a parking lot. Upon which vehicle is the force of impact greater?

Answer Neither; force is the same on each vehicle

Question A speeding bus makes contact with a bug that splatters on the windshield. Because of the sudden force, the unfortunate bug undergoes a sudden deceleration. Is the resulting deceleration of the bus greater than, less than, or that same as that of the bug?

Answer Less than

Question Suppose you exert 200 N on your refrigerator and push it across the kitchen floor at constant velocity. Friction opposes your push with 200 N of force as well. Does the friction force make up the reaction force to your push?

Answer No; the reaction to your push on the refrigerator is the refrigerator’s push back on you

Question 2 people attempt tug of war on low friction ice. One person has 4 times the mass of the other. Relative to the acceleration of the heavier person, what will be the acceleration of the lighter person?

Answer The lighter person will have 4 times the acceleration of the heavier person.

Question Why don’t action-reaction forces cancel each other out?

Answer Because one of the forces is external to the system being considered.

Question Use Newton’s Third Law to explain how rockets move forward.

Answer Rocket pushes gas backward; gas pushes rocket forward