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What Makes Things Move
Forces, Net Force, and Newton's Laws
Related Labs
Physics explains motion by connecting forces to changes in speed and direction. Every time a car accelerates, a ball rolls to a stop, or a bicycle turns a corner, forces are acting. Learning what makes things move helps students describe everyday events with clear rules instead of guesses. It also builds the foundation for engineering, sports science, transportation, and space travel.
The main idea is that motion changes when forces are unbalanced. A push or pull can start motion, stop motion, speed an object up, slow it down, or bend its path. Newton's laws describe how mass, force, and acceleration are related, while friction and gravity explain many common motions near Earth. By identifying all the forces on an object, students can predict how it will move.
Key Facts
- Force is a push or pull measured in newtons, N.
- Net force is the total of all forces acting on an object: F_net = F1 + F2 + ...
- Newton's second law connects force and motion: F_net = ma.
- If F_net = 0, an object stays at rest or keeps moving at constant velocity.
- Weight is the force of gravity: W = mg.
- Friction acts opposite motion and often reduces speed by opposing the direction of movement.
Vocabulary
- force
- A force is a push or pull that can change an object's motion.
- net force
- Net force is the overall force found by combining all the forces acting on an object.
- acceleration
- Acceleration is the rate at which velocity changes in speed, direction, or both.
- mass
- Mass is the amount of matter in an object and a measure of how hard it is to accelerate.
- friction
- Friction is a force that resists motion between surfaces that touch.
Common Mistakes to Avoid
- Confusing speed with acceleration, because an object can move fast without accelerating if its velocity stays constant. Acceleration only happens when speed or direction changes.
- Forgetting to add forces as vectors, which gives the wrong net force. Forces in opposite directions must be subtracted, not simply added.
- Assuming motion always means a force is needed in the direction of travel, which is wrong when net force is zero. An object can keep moving at constant velocity without a forward net force.
- Mixing up mass and weight, which leads to incorrect calculations. Mass is measured in kilograms, while weight is a force found from W = mg.
Practice Questions
- 1 A 4 kg cart is pushed with a net force of 12 N to the right. What is its acceleration?
- 2 A 10 kg box is pulled right with 50 N while friction pushes left with 20 N. What is the net force and resulting acceleration?
- 3 A ball rolls across a floor and gradually stops. Explain which force is mainly responsible and why the ball does not keep the same velocity forever.