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Active Learning: Forces and Newton’s Laws
rev. 5/12
I Gravitational Force
1. A crate has a weight of 420 N. What is the mass of the crate?
2. An astronaut who weighs 600 N on earth is orbiting 100 km above the surface of the earth. What is his
weight in orbit?
a. Zero
b. A lot less than 600 N, but still not zero
c. A little less than 600 N
d. 600 N, same as on the surface of the earth
II. Normal Force
1. (circle all that apply) The normal force
a. may be equal to the weight of the object
b. may be greater than the weight of the object
c. may be less than the weight of the object
d. may point in any direction (with respect to up and down)
2. Consider a crate of weight 300 N sitting on the floor
a. What is the normal force of the floor on the crate?
b. Now suppose you push straight down on the crate with a 100 N force. What is the normal force
on the crate now?
c. Instead of pushing down, suppose you attach a rope to a hook on the top of the crate and pull
straight up with a force of 125 N. What is the normal force on the crate now?
d. Finally, you remove the rope and hook and stack a second 200 N crate on top of the first. What is
the normal force of the floor on the original crate? What is the normal force of the lower crate on
the upper crate?
Active Learning: Forces and Newton’s Laws
II Newton’s Laws
1. In which of the following scenarios is the net force on the object zero? (circle all that apply)
a. car traveling at constant speed in a straight line
b. block sliding down a steep incline at constant speed
c. parachuter drifting down at constant speed after the chute opens
d. falling with acceleration equal the acceleration of gravity
e. car traveling at constant speed on a circular track
2. What is inertia?
3. Blocks A and B are each 5 kg. At a particular instant in time, block A is traveling at v = 10 m/s with
a = 4 m/s2 and block B is traveling at v = 20 m/s with a = 4 m/s2.
A. Which block is experiencing the greater net force?
B. What is the net force on block A? on block B?
4. Identify the “reaction” to each of the following “actions” as required by Newton’s 3rd law?
A. You are pulling on a rope attached to a sled.
B. The normal force on a book sitting on a table
C. The weight of the book sitting on the table
D. Friction force on a book sliding across a table
E. The weight of a ball in free fall
5. Two balls are the same size, but one is steel (heavy) and the other is plastic (light). The two balls are
moving at the same speed toward each other and they collide head-on.
A. Which ball undergoes the greater acceleration (in magnitude)?
B. Which ball experiences the greater force of interaction (in magnitude)?
C. Which is traveling at the greater speed after the collision?
6. A large truck is towing a small car by means of a horizontal tow bar. Which is bigger (or are they the
same): the force the truck exerts on the car or the force the car exerts back on the truck in the
following situations.
A. The truck is towing the car at constant velocity up a hill
B. The truck is speeding up on a level road
C. The truck is slowing down going down a hill