Answers for Chapter 1: Physics in Action
Activity 1: A Running Start and Frames of Reference
1. Possible answers:
- An outfielder diving for a line drive – the outfielder continues in motion, sliding
along the ground
- A slap shot in hockey – the puck continues to move in a constant horizontal motion
once it has been set in motion by the player
- A diver entering the water – the diver continues to fall downwards through the
water after they have entered
4. A hockey puck on ice has either nearly constant speed, or very small uniform
deceleration.
5. A baseball player slides into second or third to decelerate to a stop at the base
because if the base is overrun, the player can be tagged out. A person can overrun
first base without the risk of being tagged out, and the fastest way to get to first is to
continue running and not to slide.
6. It does not seem possible to eliminate friction to arrive at perpetual motion in the
real world.
8. 14.5m/s
9. a) 8.0m/s
b) 3.2m/s
c) (must use Pythagorean Theorem, since velocities are perpendicular)
6.1m/s (v2 = 5.62 + 2.42)
Activity 2: Push or Pull – Adding Vectors
1.
1
2
3
4
5
6
Force (N)
359
800
70
400
-1500
-300
Mass (kg)
70
80
7
80
100
100
Acceleration (m/s2)
5
10
10
5
-15
-30
2. a) The long jumper and the shot put both are cases of free fall, therefore the
acceleration is g, the acceleration due to gravity.
b) The negative sign is used to denote that the force and acceleration are in a
direction opposite to the motion.
c) Since acceleration occurs in the direction of the causal force, yes the force should
be shown as negative.
5. a) A bowling ball has greater inertia (mass) than a baseball, therefore a bowling
ball has a greater tendency to either remain at rest or remain in motion than does a
baseball.
b) More force is required to cause a bowling ball to accelerate than a baseball,
therefore throwing (accelerating) or catching (decelerating) a bowling ball involves
much greater forces than throwing or catching a baseball when equal speeds are
involved.
9. Answers vary.
Calculations:
3. 14 m/s2
4. 0.8N
6. 1N
7. weight in Newtons = body mass (kg) x 10
11. 90N
12. 800N
13. 130N
14. 6403N
15. 179m/s2
16. 125N
17. a) 50N
b) 8.9m/s2
18. a) F = 36N
b) 0.36m/s2
c) 50N
0.5m/s2
Activity 3: Center of Mass
1. If not directed toward the center of mass, part of the force will be used to
make the object rotate, not accelerate along a line.
2. A player having a low center of gravity must be “hit” low, at the level of the
center of mass to have his state of rest or motion changed.
3. The body’s center of mass has no support directly beneath it, so it falls
5. The pushoff force is directed at an angle to the intended path of travel.
Activity 4: Defy Gravity
1. 50J
6. a) 1J
b) 10J
c) 10J
d) 10J
e)10J
7. Same as #6
8. Answers are same as #6
9. 2150J
10. 2084J
11. a) 6m/s2
b) 563J
12. a) 12.5m
b) 2.7m/s2
13. 120J
14. 7200J
17.3m
Activity 5: Run and Jump