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Active Learning: Applying Newton’s Laws
rev. 5/12
I Equilibrium
1. (Applying Newton’s 2nd Law I ppt) A 100 kg sign is hanging by two support cables as shown. Draw
a FBD and find the tensions in the cables.
2. (Applying Newton’s 2nd Law I ppt) A 20 kg block is sliding down a 20° incline at constant speed.
Draw a free body diagram and find the normal force and friction force.
3. A 5 kg object is being pulled across a horizontal, rough floor at a constant velocity of 2 m/s by a
horizontal force of 20 N.
A. What is the horizontal force opposing the motion?
B. Assuming the opposing force does not change, what happens if you increase the pull to 40 N?
C. While you are still pulling with 40 N, the object comes to a patch of floor where the opposing
force is 60 N. What happens now?
Active Learning: Applying Newton’s Laws
II 1D Dynamics
1. A 60 kg skydiver is falling with acceleration 7.6 m/s2. What is the force of air resistance acting on
him?
2. What is the apparent weight of a 60 kg man?
A. Traveling at constant velocity downward in an elevator?
B. In an elevator traveling downward and slowing with |a| = 1.5 m/s2?
C. In an elevator traveling upward and slowing with |a| = 1.5 m/s2?
III Friction
1. A 100 N block is sitting on a horizontal, rough surface. The coefficient of static friction is 0.4; the
coefficient of kinetic friction is 0.3.
A. What is the magnitude of static friction if no horizontal forces are acting on the block?
B. What if you push horizontally on the block with 20 N? Will the block start to move? What is the
friction force acting on the block?
C. What if you increase the horizontal push to 50 N. Will the block start to move? What is the
friction force acting on the block?
2. Suppose you wanted to move a heavy crate across a rough floor. Which would require the least force
by you? Why?
a. Push or pull it with a horizontal force
b. Push it with a somewhat downward force
c. Attach a rope and pull it with a somewhat upward force
Active Learning: Applying Newton’s Laws
IV 2D Dynamics
1. Consider the 20 kg block sliding down the incline in I.2.
A. What is the coefficient of kinetic friction?
B. Now suppose you start pushing with a 50 N horizontal force to stop the block. Draw the FBD and
answer the following.
1 What is the normal force now? What is the friction force?
2 What is the deceleration of the block
2. Two blocks (2 kg and 4 kg) are connected by a string. The two are being pulled at constant velocity
across a rough floor by a 2nd string with tension 12 N connected to the 2 kg block.
A. Draw a FBD for each block.
B. Is the tension in the connecting string greater than, less than, or equal to 12 N?
C. Write the 2nd law equations for both blocks (4 equations).
D. What is the coefficient of kinetic friction between the blocks and the floor, assuming it’s the same
for both blocks?
E. Find the tension in the connecting string.
4 kg
2 kg
12 N