Download Physics 1P21/1P91 Tutorial for the week of 29

Physics 1P21/1P91
Tutorial for the week of 29 Sept 3 to Oct 2014
S. D’Agostino
Exploring Newton’s Laws of Motion
Brief Answers
Your task is to carefully examine each of the following statements and determine whether each
statement is correct or incorrect. If a statement is correct, explain why. If a statement is incorrect,
explain why and then correct the statement.
Your explanation may include diagrams, if you wish, and should include a brief paragraph of a
few sentences at most. (My sample solutions, which will be posted at our course web page at the
end of the week, might be significantly longer, for your potential benefit.)
Strive for a brief, clear, and complete explanation, demonstrating your complete understanding
of the situation and the relevant physical principles, which will include Newton’s laws of motion.
1. (a) When you pull a desk across the floor, you exert a force on the desk and by Newton’s
third law of motion, the desk exerts an equal and opposite force on you. Thus, the net
force is zero. (b) This is an example of Newton’s first law of motion, because motion
occurs with no force.
Brief answer: (a) False. The phrase, “the net force is zero” is false. Remember that you should
only add forces that are acting on the same object. The two forces mentioned act on different
objects. (b) False. It’s not true that “motion is occurring with no force.” There are a number of
forces acting on the desk, and without more information we don’t know whether the net force on
the desk is zero or not.
2. You drag a desk across a floor at a constant speed by exerting a constant force on the
desk. This means that Newton’s first law of motion does not apply to this situation,
because an object is moving in a straight line at a constant speed, and yet there is a force
acting on it.
Brief Answer: False. Certainly you are exerting a force on the desk, but the total of all forces
acting on the desk must be zero if the desk is moving at a constant speed. This means at least one
other force must be acting on the desk.
3. Newton’s first law of motion states that if no force acts on an object, then the object stays
at rest. But at the grocery store, if I push the grocery cart and then let go of it, the grocery
cart soon comes to rest. The moving grocery cart that comes to rest when I stop pushing it
is therefore a good example of Newton’s first law of motion.
Brief Answer: The moving cart would continue moving at a constant speed after I let go of it if
no forces were acting on it. The fact that it slows down and then stops means that some force
(friction, in this case) must be acting on it and opposing its motion.
The implication of the statement, which is that the cart comes to rest because no force is acting
on it any more once I let go of it, is false. If no force were acting on the cart when I let go of it, it
would continue moving in a straight line at a constant speed. The moving cart that slows down
when I let go is therefore not a good illustration of Newton’s first law.
The first sentence in the statement is incomplete, because Newton’s first law of motion says
more than this.
4. If the net force acting on a moving object is zero, then the object will slow down and
come to rest, because it takes a force to keep an object moving. (Think about the grocery
cart that comes to rest when I stop pushing it in the previous item.)
Brief answer: False. A moving object slows down and stops because a force acts on it to oppose
its motion. If the net force acting on a moving object is zero, then the object will continue to
move in a straight line at a constant speed.
5. If a constant force acts on an object, then the object moves at a constant velocity. A good
example of this is pushing a grocery cart in a grocery store; I push on the cart with a
constant force and it moves at a constant velocity.
Brief answer: False. If the net force acting on an object is constant, then the object’s acceleration
is constant, not the object’s velocity. The net force on the grocery cart is zero; I may exert a
constant force on it, but friction opposes the cart’s motion, so that the net force on the cart is
zero.
6. When I push on a transport truck, it doesn’t move. This shows that Newton’s first law of
motion doesn’t apply to every object, because a net force acts on the transport truck and
yet it doesn’t move.
Brief Answer: False. Newton’s law does indeed apply to the transport truck. The frictional force
acting on the truck is opposite to my applied force, and the two forces have equal magnitude, so
the net force acting on the truck is zero. (Be careful: These two forces are not equal and opposite
because of Newton’s third law; they are not connected by Newton’s third law because they act
on the same object.)
7. When you toss a ball straight up, it stops momentarily at the peak of its motion. The total
force acting on the ball at this moment is zero, because the ball is momentarily at rest.
Brief Answer: False. Even though the ball stops momentarily, it still has downward acceleration,
and so by Newton’s second law the net force acting on the ball must also be downward. (If its
velocity and acceleration were both zero, then it would hover indefinitely.)
8. When you throw a ball straight up, the ball slows down because the upward force of the
throw gradually decreases because of the downward force of gravity.
Brief Answer: False. Once the ball leaves your hand, your hand no longer exerts a force on the
ball. The force of your hand is not carried along with the ball. The ball slows down because the
downward force of gravity opposes its motion, not because the downward force of gravity
opposes some upward force; there is no upward force once the ball leaves your hand.
9. Consider a passenger in a car driving forward. When the driver slams on the brakes, the
passenger is thrown forward by the force of the car’s motion.
Brief Answer: The car’s motion does not carry a force along with it. When the car begins to slow
down, the passenger’s upper body tends to continue moving forward at the original speed, by
Newton’s first law of motion. The waist, however, slows down because of the seatbelt exerting a
force on it. Thus, the body leans forward. As the body leans forward, the lower body exerts a
greater and greater force on the upper body, slowing it down so that it stays inside the car, which
is also slowing down.
Note that the forces exerted on the body are towards the back of the car, slowing the body down.
There is no force throwing the body forward; the passenger feels that his or her body is thrown
forward because he or she is self-centred, observing the situation from a non-inertial reference
frame. An observer from outside the car sees correctly that the passenger and the car both slow
down because forces are exerted on them towards the back of the car.
10. Consider a passenger in a car driving around a curve that bends to the left. The passenger
is thrown to the outside of the curve (i.e., towards the passenger’s right) by an outward
force that is caused by the car’s motion.
Brief Answer: False. There is no outward force. The passenger tends to keep going in a straight
line, by Newton’s first law of motion, and the curving car pulls the passenger towards the centre
of the curve. Thus, the force on the passenger is towards the centre of the curve.
11. Newton’s first law of motion states that, “A body at rest tends to stay at rest, and a body
in motion tends to stay in motion.” This means that for an object going at a constant
speed in a circle, the net force acting on the object is zero; the object has a tendency to
stay in motion, so just keeps going in a circle without any force being needed.
Brief Answer: False. The statement of Newton’s first law of motion given here is false because it
is too vague. Remember, Newton’s first law of motion says that a body in motion tends to
remain in motion in a straight line at a constant speed unless the net force acting on it is not
zero.
An object moving in a circle is accelerating, and therefore a net non-zero force must be acting on
it to produce the acceleration. For an object moving in a circle at a constant speed, the direction
of the acceleration is towards the centre of the circle, so the net force producing the acceleration
is also directed towards the centre of the circle.
12. When a baseball is thrown, its path is a beautiful parabolic arc. The force acting on the
baseball at each moment is directed tangent to the path, pushing the ball along its path.
(Draw a diagram!)
Brief Answer: False. The velocity of the ball is directed along a tangent to the path; the force acts
in the direction of the ball’s acceleration, which is downward. No force is required to keep an
object moving along its path; on the contrary, a force is needed to push or pull an object away
from its line of motion.
13. My friend Bubba, who weighs twice as much as I do, comes up behind me and gives me
a good-natured slap on the back. Unfortunately, I wasn’t expecting this, and I am
knocked down. Clearly, in slapping me on the back, the force that Bubba exerts on me is
greater than the force I exert on Bubba, which is probably zero. If the forces were equal,
Bubba would fall down too.
Brief Answer: False. By Newton’s third law, the force that Bubba exerts on me is equal to the
force that I exert on Bubba. However, the effects on us are different, because Bubba’s force on
me is not balanced by an equal force that the ground might have exerted on me had I braced
myself. Bubba does brace himself, so the net force on him is zero, because my force on him is
balanced by the force that the ground exerts on him.
14. The Earth is much more massive than the Moon, and so the Earth therefore exerts a larger
force on the Moon than the Moon exerts on the Earth.
Brief Answer: False. By Newton’s third law, the two forces are equal.
15. If an object is at rest, it takes a force to get it moving. Therefore, it also takes a force to
keep an object moving. For example, there must be a net force acting on a car moving at
a constant speed, because if you switch the car’s engine off, the car will come to a stop.
Brief Answer: False. It takes a force to keep an object moving only if an opposing force (such as
friction) is present. If an object is moving at a constant speed, such as the car described here, the
net force acting on it is zero. We must be careful not to confuse “a force” with “the net force”
acting on an object.