Combining Force Vectors
Finding the Resultant When Forces Act Together
A girl applies a 5 N east force on a dresser. Her brother also exerts a 5 N force. Why didn't the dresser move?
When doesn't 100 + 100 = 200?
Vector Addition - The Setup
Skip Right To The Answer
Video Instruction
showmethephysics.com
B. Combining Force Vectors
Resultant - single vector that will have the same effect as two or more vectors
1) Finding Resultants
Case I
Forces acting in same direction (Angle = 0 degrees)
Is resultant different than the one above?
RULE when θ = 0°
Add Forces
Case II
Forces acting in opposite direction (Angle = 180°)
If θ = 180°
Subtract smaller vector from larger
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. What is a resultant?
2. A 5 N force and a 4 N force both act east on a box. What is the resultant?
3. A 50 N force acts east and a 10 N force acts west on an object. What is the resultant?
4. A girl pushes a dresser 5 N east and her brother pushes with 5 N, but it doesn't move. What explains this?
5. What is the angle between two forces acting in opposite directions?
Rocket Engine Test Stand
Engineers test a small rocket that weighs 600. N on Earth. During a test, the engine produces an upward thrust of 900. N. Air resistance is negligible.
Diagram 1: Forces on the Rocket
What is the net force on the rocket?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response.
C2. The engineers throttle the engine so the thrust equals 600. N. Which statement describes the rocket?
C3. The rocket’s mass is 61 kg. Substitute numerical values into a mathematical representation that can be used to determine the magnitude of the rocket’s acceleration with 900. N of thrust. Then calculate the acceleration.
C4. During a later test, the engine is throttled down to 450. N of upward thrust. Choose the direction of the net force on the rocket, and determine its magnitude.
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C5. As the rocket burns fuel, its mass decreases while the thrust stays at 900. N. What happens to the net force?
C6. The engineers consider replacing the engine with a larger one that produces more thrust but is also heavier. Identify one benefit and one drawback of this design change, using net force in your answer.
