Transcript Rockets do accelerate in space.

Newton’s
rd
3
Law
For every force there is an
equal and opposite force
If your feet push down on the floor,
what does the floor do?
A. Nothing
B. Absorbs the push
C. Pushes up on your feet, but with less force
D. Pushes up on your feet with an equal force
E. Pushes up on your feet with greater force
Can the floor push up on
your feet with a force greater
than your weight?
A. Yes, but nothing will happen
B. Yes, you will accelerate up into the air
C. No, it always equals your weight
D. No, it can only equal or be less than your
weight
E. Yes, but only on a planet with stronger
gravity
What is the force from that causes
a car to accelerate down the road?
A. The engine
B. The transmission
C. The pistons
D. The road
E. The tires
What is the force from that causes a
swimmer to accelerate across a pool?
A. The swimmer’s legs
B. The swimmers arms
C. The swimmer’s arms and legs
D. Gravity
E. The water
Swimming
Due to the 3rd Law, when you swim you push the water
(blue), and it pushes you back just as hard (red) in the
forward direction. The water around your body also
produces a drag force (green) on you, pushing you in the
backward direction. If the green and red cancel out, you
don’t accelerate (2nd Law) and maintain a constant velocity.
Note: The blue vector is a force on the water, not the on
swimmer! Only the green and red vectors act on the swimmer.
The Earth pulls on the Moon with
gravity, causing it to orbit. What does
the Moon do to the Earth?
A. Pulls on the Earth with equal gravity,
causing it to orbit
B. Nothing, it is too small
C. Pulls on the Earth with less gravity, having
no effect
D. Pulls on the Earth with less gravity, causing
tides
The Earth pulls on you with the force of
gravity, what is the reaction to this force?
A.
B.
C.
D.
Normal force
You pull on the Earth with an equal force of gravity
You pull on the Earth with a smaller force of gravity
Nothing, only the Earth has gravity
The Forces are the same but the Masses
and Accelerations are different
F
=
a
Earth
F
MPerson
Earth
=
a
Person
A gun shoots a bullet, which
received the greatest force?
A. The bullet
B. The gun
C. Each received an equal force
D. Depends on the size of the bullet
The Forces are the same but the Masses
and Accelerations are different
F
=
a
Gun
F
MBullet
Gun
=
a
Bullet
A bug splatters on a windshield,
which received the greatest force?
A. The windshield
B. The bug
C. Each received an equal force
D. Depends on the size of the bug
The Forces are the same but the Masses
and Accelerations are different
F
=
a
Car
F
MBug
Car
=
a
Bug
In the tug-of-war, what is the
reading on the scale?
1000 N
0
3000 1000
2000
A. 0 N
B. 1000 N
C. 2000 N
D. 3000 N
1000 N
The tree pulls just like the tug-ofwar team.
What will the scale read that is
pulled from both sides by 10 N
weights?
A. 0 N
B. 10 N
C. 20 N
D. 30 N
If a Mack truck and a Volkswagen have a head-on
collision, which vehicle will experience the greater
impact force?
a) the Mack truck
b) the Volkswagen
c) both the same
d) ... it depends on other factors
Does the scale read 100N, 200N, or zero?
For every force there exists an equal and opposite
force. Consider action and reaction forces in the
case of a rock falling under the influence of
gravity. If action is considered to be that of the
earth pulling down on the rock, can you clearly
identify the reaction force?
End of Chapter exercise questions
• For each of the following interactions, identify action and
reaction forces. (a) A hammer hits a nail. (b) Earth gravity
pulls down on you. (c) A helicopter blade pushes air
downward.
• Identify the action-reaction pairs of forces for the following
situations: (a) You step off a curb. (b) You pat your tutor on
the back. (c) A wave hits a rocky shore.
• When you drop a rubber ball on the floor it bounces almost to
its original height. What causes the ball to bounce?
• Suppose two carts, one twice as massive as the other, fly
apart when the compressed spring that joins them is released.
How fast does the heavier cart roll compared with the lighter
cart?
• If a massive truck and small sports car have a head-on
collision, upon which vehicle is the impact force greater?
Which vehicle experiences the greater acceleration?
Explain your answers.
• Which team wins in a tug-of-war: the team that pulls
harder on the rope, or the team that pushes harder
against the ground? Explain.
• A stone is shown at rest on the ground. (a) The vector
shows the weight of the stone. Complete the vector
diagram showing another vector that results in zero net
force on the stone. (b) What is the conventional name of
the vector you have drawn?
• Here a stone is suspended at rest by a string. (a) Draw
force vectors for all the forces that act on the stone. (b)
Should your vectors have a zero resultant? (c) Why, or
why not?
• Here the same stone is being accelerated vertically
upward. (a) Draw force vectors to some suitable scale
showing relative forces acting on the stone. (b) Which is
the longer vector, and why?
Problems
• If you apply a net force of 5 N on a cart with a
mass 5 kg, what is the acceleration?
• If you increase the speed of a 2.0-kg air puck by
3.0 m/s in 4.0 s, what force do you exert on it?
• A boxer punches a sheet of paper in midair, and
brings it from rest up to a speed of 25 m/s in
0.05 s. If the mass of the paper is 0.003 kg, what
force does the boxer exert on it?
• If you stand next to a wall on a frictionless
skateboard and push the wall with a force of 30
N, how hard does the wall push on you? If your
mass is 60 kg, what's your acceleration?