Worked example
A cart has a 7 N force right and a 3 N force left. What is the net force?
Answer & why
Answer: 4 N right
The opposite forces subtract: 7 − 3 = 4 N toward the larger rightward force.
Grade 6 · Science
Combine forces along a line
Calculate a net force from stated opposite directions and distinguish balance from rest.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
A cart has 6 N left and 6 N right. What is its net horizontal force?
- 12 N left
- 0 N
- 12 N right
- 6 N left
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: 0 N
Equal opposite forces balance.
Question 2
An object is already moving at constant speed in a straight line. Its net force stays zero. Which prediction fits this ideal model?
- It keeps the same velocity.
- It must speed up.
- It must reverse direction.
- It must stop immediately.
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: It keeps the same velocity.
Zero net force means no acceleration in the stated reference frame.
Question 3
A 2 kg cart has a net force of 8 N. Using acceleration = force ÷ mass, what is its acceleration?
- 16 m/s²
- 0.25 m/s²
- 6 m/s²
- 4 m/s²
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: 4 m/s²
8 N ÷ 2 kg = 4 m/s².
Question 4
The same net force acts on a 2 kg cart and a 4 kg cart. Using acceleration = force ÷ mass, which has greater acceleration?
- The 4 kg cart.
- The 2 kg cart.
- They must have equal acceleration.
- Neither can accelerate.
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: The 2 kg cart.
Dividing a fixed force by the smaller mass gives the larger acceleration.
Question 5
A cart has a 7 N force right and a 3 N force left. What is the net force?
- 10 N right
- 4 N right
- 4 N left
- 0 N
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: 4 N right
The opposite forces subtract: 7 − 3 = 4 N toward the larger rightward force.
Question 6
Two forces act right: 4 N and 5 N. No other horizontal force acts. What is the net horizontal force?
- 9 N right
- 0 N
- 9 N left
- 1 N right
A hint
Add forces in the same direction and subtract opposite forces; zero net force does not by itself mean zero speed.
Answer & why
Answer: 9 N right
Forces in the same direction add: 4 + 5 = 9 N.