Newton's Third Law of Motion - Action and Reaction
Two people have a tug-of-war on low-friction ice. One person has 2 times the mass of the other. Compared to the lighter person, how many times as fast does the heavier person accelerate?
1/2 times
T/F? A bicycle and a parked car have a head-on collision. The force of impact is greater on the bicycle.
False
T/F? A given force exerted on a large mass produces a greater acceleration than the same force exerted on a small mass.
False
T/F? In order to make a cart move forward, a horse must pull harder on the cart than the cart pulls on the horse.
False
According to Newton's third law, if you push gently on something, it will push
gently on you
A 60-kg person on in-line skates pushes against a wall with a force of 30 N and recoils. What acceleration does that person experience?
.5 m/s^2
If you push off the ground with a force of 350 N when you jump upward, what force pushes Earth downward?
350 N
What engine thrust (in newtons) is required for a rocket of mass 35 kg to leave the launching pad?
350 N
Two people pull on a rope in a tug-of-war. Each pulls with 400 N of force. What is the tension in the rope?
400 N
A person is attracted towards the center of Earth by a 440-N gravitational force. The force with which Earth is attracted toward the person is
440 N.
Suppose two people, one having three times the mass of the other, pull on opposite sides of a 20-meter rope while on frictionless ice. After a brief time, they meet. The more massive person slides a distance of
5 m
A fighter punches a sheet of paper in midair, and brings it from rest up to a speed of 40 m/s in 0.08 s. What is the force of impact on the paper if the mass of the paper is 0.01 kg?
5.0 N
A woman weighing 550 N sits on the floor. She exerts a force on the floor of
550 N
As a 600-N woman sits on the floor, the floor exerts a force on her of
600 N
Whenever an object exerts a force on another object, the second object exerts a force of the same magnitude, but in the opposite direction to that of the first object.
Always True
Bronco the skydiver falls toward Earth. The attraction of Earth on Bronco pulls him down. The reaction to this force is
Bronco pulling up on Earth.
Earth pulls on the moon, and similarly the moon pulls on Earth. This is evidence that the
Earth and moon are simply pulling on each other and Earth's and moon's pulls comprise an action-reaction pair.
If your car runs out of gas, why can't you push on the car's windshield from the inside to move it?
In order for the car to move, a force must be exerted by the car on the ground.
A force is a push or a pull. Newton's third law further defines the meaning of force. Explain.
Newton's third law explains that a force is not just a single push or pull, but that there is an action force and reaction force because in an interaction one object exerts a force on another and the second object exerts an equal and opposite force on the first object.
Apply Newton's third law to a falling boulder, identifying the action and reaction forces. If a force is exerted on Earth, why doesn't it move? Explain.
The action force is the boulder pushing on earth. Newton's third law states there must be an equal force in the opposite direction so the reaction force is the Earth pushing on the boulder. Because they are different masses they accelerate differently. The earth has such a bigger mass than the boulder, so the earth doesn't move.
Nellie Newton holds an apple in her hand. If action is Earth pulling on the apple, then reaction is
The apple is pulling on the earth
Explain how Newton's third law applies to a cannon launching a cannonball. Which has the larger acceleration, the cannonball or the cannon? What if the cannonball were just as massive as the cannon—how fast would the cannon move compared to the cannonball?
The cannon applies force to the cannonball and in return the cannonball exerts a force on the cannon, making them an action-reaction pair. The cannonball has the greater acceleration because it has the smaller mass. If they had equal mass, they would move at the same rate.
Suppose you accidentally drive into a tree. Which of the following correctly describes the interaction between your car and the tree?
The car and the tree mutually exert equal and opposite forces on each other.
If you exert a force of 12,000 N on a 3,000 kg car and a 6,000 kg truck that are both originally at rest, what will be the resulting accelerations of the objects?
The car will accelerate 4 m/s^2 and the truck will accelerate 2 m/s^2.
You drive past a farm, and you see a cow pulling a plow to till a field. You have just learned about Newton's third law, and you wonder how the cow is able to move forward if the plow is exerting an equal and opposite force on the cow. Which of the following explains the movement of the cow and plow?
The force exerted by the cow on the plow is smaller than the force that the cow exerts on the ground to move forward.
A player hits a ball with a bat. The action force is the impact of the bat against the ball. What is the reaction to this force?
The force of the ball against the bat
A player catches a ball. Consider the action force to be the impact of the ball against the player's glove. What is the reaction to this force?
The force the glove exerts on the ball
As a ball falls, the action force is the pull of Earth's mass on the ball. What is the reaction to this force?
The pull of the ball's mass on Earth
T/F? A rocketship is pushed forward by gases that are forced out the back of the ship.
True
T/F? If a train is pulling a caboose, the caboose will move if the train exerts a force on the ground that is greater than the force that the caboose exerts on the train.
True
T/F? Whenever one object exerts a force on another object, the second object always exerts a force back on the first object.
True
A pair of blocks, one 2 times as massive as the other, are connected by a compressed spring. When the spring is released, the blocks fly apart. Compared to the heavier block, how many times as fast does the lighter block accelerate?
Twice as fast
Consider the horizontal forces acting on a horse pulling a cart. Consider forces on the cart as well as on the horse. In terms of these forces, explain why the cart moves. Explain why the horse moves. Why don't action-reaction forces cancel each other?
When a horse is pulling a cart, the cart and horse are both exerting equal forces on each other. Because of this, the forces should cancel out and the cart shouldn't move. But because the cart and horse are both exerting forces on the ground and therefore have net forces that don't cancel out so the cart moves. Action and reaction forces do not cancel out when either of the forces is external to the system being considered.
If a horse pulls on a wagon at rest, the wagon pulls back equally on the horse. Can the wagon be set into motion?
Yes, because there is a net force acting on the wagon.
An archer shoots an arrow. Consider the action force to be the bowstring against the arrow. The reaction to this force is the
arrow's push against the bowstring.
Forces always occur
in pairs
A karate chop delivers a blow of 2300 N to a board that breaks. The force that acts on the hand during this feat
is 2300 N
An unfortunate bug splatters against the windshield of a moving car. Compared to the deceleration of the car, the deceleration of the bug is
larger
A rocket is able to accelerate in the vacuum of space when it fires its engines. The force that propels the rocket is the force
of the exhaust gases on the rocket.
A high school student hits a nail with a hammer. During the collision, there is a force
on the nail and also on the hammer.
A force is exerted on the tires of a car to accelerate the car along the road. The force is exerted by the
road
An unfortunate bug splatters against the windshield of a moving car. Compared to the force of the car on the bug, the force of the bug on the car is
the same
A large truck and a small car traveling at the same speed have a head-on collision. The vehicle to undergo the greater change in velocity will be
the small car.