The net force on an object refers to the total or resultant force acting on the object. For a 1 Newton apple sitting at rest on a table, the net force on the apple is zero. This is because the downward pull of gravity exerts a 1 Newton force, while the upward support force from the table also exerts a 1 Newton force. These two equal and opposite forces cancel out, resulting in zero net force.
Breaking Down the Forces on an Apple
To understand net force, we first need to examine the different forces that can act on an object:
Gravity
Gravity exerts a downward force on all objects proportional to their mass. On Earth, gravitational acceleration is approximately 9.8 m/s2. This means a 1 kg mass experiences a 9.8 N gravitational force. For our 1 N apple, it must have a mass of 0.1 kg. Using Newton’s law of gravitation:
Fgravity = mg
Fgravity = (0.1 kg)(9.8 m/s2) = 1 N
So the force of gravity pulling down on the 1 N apple is 1 Newton.
Normal Force
When an object sits on a surface, the surface exerts an upward force called the normal force that counteracts gravity. If the surface is rigid and non-deformable, this upward normal force will be equal in magnitude to the downward force of gravity.
For our apple sitting on a table, the table exerts a 1 N normal force upward to balance the 1 N gravitational force.
Tension
If a rope, string, or other connector is attached to an object and pulling on it, this force is called tension. The tension force pulls parallel to the length of the string or connector.
If we hung our 1 N apple from a string, the string would need to supply a 1 N tension force upward to balance gravity.
Friction
Frictional forces resist relative motion between surfaces in contact. Static friction acts on non-moving objects, while kinetic friction acts on objects already in motion.
If we gave the apple a gentle push across the table, static friction would briefly oppose the push until the apple began sliding. Kinetic friction would then resist continued motion.
Air Resistance
Any object moving through air experiences a drag force that opposes its motion. This air resistance force depends on the object’s speed, surface area, and aerodynamic shape.
For a slow-moving apple, air resistance would be minimal. But for a fast apple or ball, air drag becomes significant.
Net Force Calculation
Now that we’ve looked at the main forces acting on an object, we can calculate net force. This is done by vector addition, taking into account all forces acting on the object.
For our stationary apple sitting on a table:
| Force | Magnitude | Direction |
|---|---|---|
| Gravity | 1 N | Down |
| Normal Force | 1 N | Up |
The two equal and opposite forces cancel out, giving:
Net Force = 0 N
This zero net force results in the apple remaining stationary on the table.
If we cut the string holding the apple, it would begin accelerating downward at 9.8 m/s2 due to the unbalanced 1 N gravitational force.
If we gave the resting apple a horizontal push, static friction would temporarily create a leftward friction force. This left friction would balance the rightward push force, again giving zero net force initially.
Once static friction is overcome and the apple begins sliding, kinetic friction would be weaker than the push force, resulting in a net force and apple acceleration.
Significance of Net Force
The net force on an object determines its motion and acceleration. Specifically:
- Net force = 0 N implies the object remains at rest or continues moving at constant velocity
- Net force > 0 N causes the object to accelerate in the direction of the net force
- Net force < 0 N causes the object to accelerate against the direction of the net force
This relationship is described by Newton’s Second Law:
Net Force = mass x acceleration
Fnet = ma
For a given mass, larger net forces produce greater acceleration. Zero net forces means zero acceleration.
This shows why net force is so important in physics – it governs whether objects remain still or begin moving, and how quickly they can accelerate.
Factors that Affect Net Force
What factors can influence the net force on an object?
Applied Forces
Pushing, pulling, lifting up, dropping down, or throwing an object all apply external forces that contribute to net force. A thrown ball experiences a sudden net impulse force that gives it launch acceleration.
Mass
Heavier objects require more force to accelerate them. A feather and hammer experience the same gravitational force, but the feather has less mass so it accelerates more slowly.
Friction
Frictional forces from surfaces can reduce the net force. Low friction surfaces like ice or rollers allow larger net forces and higher accelerations.
Buoyancy
When immersed in a fluid like water, buoyant forces act upward to counteract gravity. This buoyancy reduces the apparent weight and net downward force on the object.
Drag
Air resistance drag depends on speed, surface area, and shape. Streamlined objects experience minimal drag and can achieve large net accelerating forces.
Examples of Net Force
Here are some examples of how net force determines an object’s motion:
Car acceleration
When you step on the gas pedal, the engine force propels the car forward. This force overcomes friction and air resistance, resulting in a net forward force that accelerates the car.
Falling object
A falling object experiences only the downward force of gravity. With no forces opposing gravity, this results in a net downward force causing downward acceleration.
Banked curve
Vehicles turning on a steeply banked curve experience a centripetal force pushing them into the curve. This counters gravity, allowing high-speed turns without skidding off the road.
Magnetic force
When turned on, a magnet exerts an attractive or repulsive force on nearby iron objects. This magnetic force can provide the net force to accelerate the objects toward or away from the magnet.
Rocket launch
The thrust from a rocket’s engines produces a large downward net force that accelerates the rocket upwards, allowing it to lift off. Thrust must overcome gravity and air resistance.
Practice Calculating Net Force
Let’s practice calculating net force on some sample objects, applying Newton’s laws:
Example 1
A 5 kg object is being pulled rightward by a 10 N force. Friction exerts a leftward 3 N force opposing the pull.
| Force | Magnitude | Direction |
|---|---|---|
| Pulling force | 10 N | Right |
| Friction | 3 N | Left |
Net force = 10 N right – 3 N left = 7 N right
Using Fnet = ma, the acceleration is a = Fnet/m = 7 N/5 kg = 1.4 m/s2
Example 2
A 15 kg crate rests on the floor. The normal force supports it against gravity.
| Force | Magnitude | Direction |
|---|---|---|
| Gravity | 15*9.8 = 147 N | Down |
| Normal force | 147 N | Up |
The upward and downward forces balance, giving zero net force.
With no net force, the crate’s acceleration is zero and it remains at rest.
Conclusion
In summary, net force tells us how an object will move by combining all forces acting on it. Calculating the vector sum of forces reveals if motion will occur and in what direction. Mastering net force allows predicting acceleration from interactions between gravity, friction, tension, buoyancy, magnetism, and applied pushes or pulls. Whether designing vehicles, sports plays, or amusement park rides, the engineer must understand net forces to control motion.