Net force and acceleration: Class 9 practice
Forces in the same direction add; in opposite directions subtract. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- F = ma
The idea behind net force and acceleration
Forces in the same direction add; in opposite directions subtract. If the net force is zero the forces are balanced and the velocity doesn't change; otherwise a = F_net/m in the direction of the larger force.
Step-by-step method
- Choose a positive direction.
- Add forces with signs to get F_net.
- a = F_net/m.
Worked example 1 (Basic)
Forces of 48 N to the right and 16 N to the left act on a 5 kg block on a smooth floor. Find the net force and acceleration.
- F_net = 48 − 16 = 32 N
- a = 32/5 = 6.4 m/s² right
- Answer: 32 N right, 6.4 m/s²
Worked example 2 (Standard)
Forces of 29 N to the right and 33 N to the left act on a 2 kg block on a smooth floor. Find the net force and acceleration.
- F_net = 29 − 33 = -4 N
- a = -4/2 = -2 m/s² left
- Answer: 4 N left, 2 m/s²
Worked example 3 (Challenge)
Forces of 52 N to the right and 21 N to the left act on a 5 kg block on a smooth floor. Find the net force and acceleration.
- F_net = 52 − 21 = 31 N
- a = 31/5 = 6.2 m/s² right
- Answer: 31 N right, 6.2 m/s²
Common mistakes and misconceptions
- Adding opposite forces
- Expecting motion to stop when forces balance
Practice questions
- (Basic) Forces of 19 N to the right and 54 N to the left act on a 5 kg block on a smooth floor. Find the net force and acceleration.
- (Basic) Forces of 42 N to the right and 58 N to the left act on a 20 kg block on a smooth floor. Find the net force and acceleration.
- (Standard) Forces of 19 N to the right and 27 N to the left act on a 19 kg block on a smooth floor. Find the net force and acceleration.
- (Standard) Forces of 27 N to the right and 5 N to the left act on a 17 kg block on a smooth floor. Find the net force and acceleration.
- (Challenge) Forces of 15 N to the right and 44 N to the left act on a 18 kg block on a smooth floor. Find the net force and acceleration.
- (Challenge) Forces of 58 N to the right and 31 N to the left act on a 9 kg block on a smooth floor. Find the net force and acceleration.
Answer key with full working
- Q1: F_net = 19 − 54 = -35 N → a = -35/5 = -7 m/s² left → Answer: 35 N left, 7 m/s²
- Q2: F_net = 42 − 58 = -16 N → a = -16/20 = -0.8 m/s² left → Answer: 16 N left, 0.8 m/s²
- Q3: F_net = 19 − 27 = -8 N → a = -8/19 = -0.42 m/s² left → Answer: 8 N left, 0.42 m/s²
- Q4: F_net = 27 − 5 = 22 N → a = 22/17 = 1.29 m/s² right → Answer: 22 N right, 1.29 m/s²
- Q5: F_net = 15 − 44 = -29 N → a = -29/18 = -1.61 m/s² left → Answer: 29 N left, 1.61 m/s²
- Q6: F_net = 58 − 31 = 27 N → a = 27/9 = 3 m/s² right → Answer: 27 N right, 3 m/s²
Syllabus note
This skill is listed under "Force and Laws of Motion" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is net force and acceleration in?
"Force and Laws of Motion", Class 9 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.