An object that is at rest will not begin to move on its own. For an object to begin moving from rest, an external force must be applied to it. The amount of force needed to get the object moving depends on its mass and the friction acting on it. Once enough force is applied to overcome static friction, the object will accelerate in the direction of the applied force. The rate of acceleration depends on the amount of force applied relative to the object’s mass according to Newton’s Second Law (F=ma).
What causes an object at rest to begin moving?
There are four main ways an external force can be applied to cause an object at rest to begin moving:
- Push/Pull – A direct push or pull force is applied to the object. For example, pushing a box across the floor or pulling a wagon.
- Gravity – The force of gravity accelerates objects downward. If an object is held up and then released, gravity applies a downward force causing it to begin accelerating downwards.
- Friction – The force friction supplies can cause objects in contact to move. For example, dragging an object along a surface. The friction force pulls the object in the direction of motion.
- Tension – Forces transmitted through extended objects can cause motion. For example, hitting a golf ball with a club or tugging on a rope attached to an object.
In each case, the applied external force generates an unbalanced net force on the object. This net force causes the object to accelerate from rest according to Newton’s Second Law.
Factors that determine how fast an object will accelerate
Several factors determine how quickly an object at rest will accelerate once an external force is applied:
- Amount of force – The greater the amount of external force applied, the greater the acceleration. Doubling the net force doubles the acceleration.
- Mass – The more massive an object, the lower its acceleration. Doubling the mass halves the acceleration if force is kept constant.
- Friction – Frictional forces act in the opposite direction of motion. More friction means lower net force and acceleration.
- Strength of contact – Better traction and grip allow larger friction forces to be applied.
The magnitude of the acceleration is described quantitatively by Newton’s Second Law (a = Fnet/m). The direction of the acceleration is in the direction of the net applied external force.
How friction affects the motion of objects from rest
Friction is a force that opposes relative motion between two surfaces in contact. Friction acts in a direction opposite the intended motion of an object. Since friction removes some of the applied external force, it reduces the net force and acceleration of an object trying to move from rest.
Two main types of friction are relevant when starting motion from rest:
- Static friction – Acts between surfaces not moving relative to each other. Prevents start of motion.
- Kinetic friction – Acts between surfaces moving relative to each other. Opposes continued motion.
Static friction is typically higher than kinetic friction. This means it is harder to start motion from rest than to continue motion once started. More force must be applied to overcome static friction when initially accelerating from rest.
Table showing coefficients of static and kinetic friction
| Surfaces | Static Friction Coefficient | Kinetic Friction Coefficient |
|---|---|---|
| Rubber on dry concrete | 1.0 | 0.7 |
| Waxed wood on wet ice | 0.03 | 0.02 |
| Steel on steel (dry) | 0.6 | 0.3 |
Higher coefficient = more friction. Rubber on concrete has more friction than surfaces like ice.
How to calculate how much force is needed
The minimum force needed to start an object moving from rest can be calculated using the following steps:
- Determine the normal force (N) pressing the surfaces together.
- Find the static friction coefficient (μs) for the surfaces.
- Calculate maximum static friction force: Fs = μsN
- Add 10-20% to overcome initial inertia and start motion.
For example, calculate the force needed to slide a 100 kg crate across a wood floor if μs=0.45. The normal force is equal to object’s weight = mass x gravity = 100 kg x 9.8 m/s2 = 980 N.
Fs = μsN = (0.45)(980 N) = 441 N. Add 20% so needed force is ~530 N.
How Newton’s Laws apply to objects at rest
Newton’s Laws help explain the motion (or lack of motion) of objects starting from rest:
- First Law – An object at rest remains at rest unless acted on by a net external force.
- Second Law – The acceleration of an object is directly proportional to the net force acting on it.
- Third Law – When one object exerts a force on another, the second object exerts an equal & opposite force back on the first.
The First Law states that without an unbalanced net force, an object will remain at rest. The Second Law gives acceleration as a = Fnet/m. The Third Law indicates friction forces arise between two surfaces in contact.
Table of Newton’s Three Laws of Motion
| Law | Statement |
|---|---|
| First Law | An object at rest stays at rest and an object in motion stays in motion at constant velocity unless acted upon by an unbalanced force. |
| Second Law | Force equals mass times acceleration (F=ma). Acceleration is produced when a force acts on a mass. |
| Third Law | For every action there is an equal and opposite reaction. |
Examples of how forces get objects moving from rest
Here are some examples of how different forces can start objects at rest moving:
- Pushing a box – Your hand exerts a push force transferring kinetic energy to the box.
- Dropping a ball – Gravity exerts a downward force causing downward acceleration.
- Dragging a crate – Friction from the ground exerts a force parallel to the motion.
- Firing a cannon – Gas pressure exerts a large force on the cannonball.
- Hitting a golf ball – The club head exerts an impulse force at impact.
- Towing a car – The tow cable exerts a pulling tension force to accelerate the car.
In all cases, the object at rest begins to move once enough external force is applied to overcome friction and inertia. The greater the net force, the greater the initial acceleration starting from rest.
Experimental techniques for analyzing motion from rest
Some techniques physicists use to study motion of objects starting from rest include:
- Force sensors – Measures push/pull forces applied to move objects.
- Motion sensors – Determines acceleration of objects starting from rest.
- High speed cameras – Records motion starting from rest at high frame rates.
- Inclined planes – Vary friction and force required to start motion.
- Accelerometers – Attached to objects to directly measure accelerations.
- Computer simulations – Models forces and motions mathematically.
Experiments determine how applied force relates to overcoming inertia, static friction thresholds, resulting accelerations, and velocities. Relationships are quantified through equations like Newton’s Second Law.
Conclusion
Objects at rest will not move without an external force acting on them. The required net force to start motion depends on inertia, friction, and contact forces. Greater applied force produces greater acceleration based on F=ma. Understanding how external forces influence objects starting from rest allows predicting and controlling real world motion.
