Forces & Statics Unit Review
Physics
Students should:
•Know what a force is and what conditions are necessary to produce a force
•Know common forces and recognize situations where they are at work:
Gravitational Force
Normal Force
Frictional Force
Tension
•Be able to draw an accurate free body diagram for an object in most situations
•Be able to calculate the values for gravitational force, frictional force and forces exerted by springs
given enough information.
•Know that forces are in equilibrium in static situations
•Be able to figure out how big an unknown force is when it is balanced against another force
•Be able to find the components of a force acting at an angle
•Be able to combine the components of a force to find the total force (including angle)
•Be able to approach and solve problems where someone pushes or pulls an object at an angle
•Be able to use tricks of perspective to approach and solve problems that take place on a hill
•Know what Newton’s 3rd Law says and what it means
•Be able to apply Newton’s 3rd Law to solve problems where two objects are interacting with each other.
Conceptual Questions
1. What is a force? When do forces happen?
2. What does it mean for an object to be static? What has to be true for an object to remain static?
3. What is friction? Why does friction happen?
4. What factors control how strong the friction force is? Why do they matter?
5. Draw a free body diagram of a ladder leaning against a wall
6. In a tug–of –war contest between two evenly matched teams, there is no movement as the teams
begin tugging.
a) Why doesn’t the rope move?
b) Why doesn’t either team move?
7. Why is it advantageous to pull up at an angle while trying to slide a heavy object across the floor?
8. a) What does Newton’s 3rd Law say?
b) Give an example of how Newton’s 3rd Law applies to a real life situation.
9. Identify the two forces (often called action force and reaction force) that occur when a swimmer
moves their arm through the water.
10. When you walk into a wall, which experiences a bigger force, you or the wall? Explain.
Sample Calculations:
For each problem, draw a force diagram and use it to answer the question asked.
1. During your grocery shopping, you notice a spring scale hanging in the produce section and decide to
experiment on it. You place a 2 kg bag of carrots on basket and it stretches from 10 cm long to 15 cm
long. What is the spring constant for the spring in the spring scale?
2. How much horizontal force must you exert to slide a 30 kg wooden desk to the right across a wooden
floor?
3. While you are playing with a Frisbee, two young punks come up and try to take the Frisbee from you.
If one pulls with 70 N of force to the north and the other pulls with 85 N of force to the east, what force
do you have to exert to keep them from pulling the Frisbee away?
4. Bobo, the 80 kg chimp, hangs from two rings suspended by ropes from the
ceiling of his cage. If one pulls up and to the right with 500 N at 50° above
horizontal, what is the tension in the other rope?
5. A wild 1000 kg bull is tied in place with two (big) ropes. He is trying to go
30° South of East with as much force as friction will apply to him ( = 0.7) but is held in place by the
ropes. If one rope is pulling with 4780 N at 20° West of North, what is the tension in the other rope?
6. In the thick of the mosh pit, a mosher who is standing still for a moment is hit simultaneously by
three people at the same time. One person pushes them with 220 N at 37° S of E, another pushes them
with 280 N at 40° S of W, and the third pushes with 315 N at 83° N of E. Will the mosher remain
standing still?
7. A 120 kg Sherpa (including gear) stands still on a 42° incline while climbing to the top of Mount
Everest.
a) How much normal force must the slope exert to support the Sherpa?
b) How much frictional force must the slope exert to keep the Sherpa from sliding down?
c) What is the minimum coefficient of friction between the ground and the Sherpa’s feet?
8. In attempting to slide a 5 kg brick across a partially-laid patio, a construction worker pushes it to ther
right with their foot, applying 130 N at an angle of 70° below horizontal. The coefficient of friction
between the brick and the ground is 0.6 N. Will the worker be able to move the brick?
b) Would it be easier or harder for them to move the brick if they pushed at a lower angle? Explain.
9. Dottie (65 kg), Eugene (70 kg) and Trixie (50 kg) are standing on a wooden suspension bridge when
it collapses. Dottie manages to grab onto part of the bridge as it falls apart, Trixie manages to grab onto
Dottie and Eugene grabs onto Trixie. After the collapse is complete, they are all sitting still hanging
from each other and from the bridge. Draw a separate free body diagram for each person and find the
size of all the force acting in each diagram.
10. While biking, Sally ends up agreeing to haul a train of skateboarders – Saul, Serena and Sassafras.
Saul, who has 55 kg of mass, hangs onto Sally’s bike; while Serena, with 60 kg, hangs onto Saul; and 70
kg Sassafras hangs onto Serena. The coefficient of friction against the skateboard’s wheels is 0.1.
a) Draw a free body diagram for each skateboarder at the moment when Sally is pulling nearly hard
enough to get the whole train moving.
b) How hard is Sally pulling?
c) If the skateboarders are all attached with 40 cm long bungee cords – which act as springs with a
spring constant of 800 N/m, how long is each bungee cord?