Force, mass and acceleration
A stationary object remains stationary if the sum of the forces acting upon it - resultant force - is zero. A moving object with a zero resultant force keeps moving at the same speed and in the same direction.
If the resultant force acting on an object is not zero, a stationary object begins to accelerate in the same direction as the force. A moving object speeds up, slows down or changes direction.
Acceleration depends on the force applied to an object and the object's mass.
The resultant force
An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.
When all the forces are balanced, the resultant force is zero. In this case:
- a stationary object remains stationary
- a moving object keeps on moving at the same speed in the same direction
When all the forces are not balanced, the resultant force is not zero. In this case:
- A stationary object begins to move in the direction of the resultant force.
- A moving object speeds up, slows down or changes direction depending on the direction of the resultant force.
Force and acceleration
You should know that objects accelerate when the resultant force is not zero, and understand the factors that affect the size of the acceleration.
Size of the force
An object will accelerate in the direction of the resultant force. The bigger the force, the greater the acceleration.
Doubling the size of the (resultant) force doubles the acceleration.
An object will accelerate in the direction of the resultant force. A force on a large mass will accelerate it less than the same force on a smaller mass.
Doubling the mass halves the acceleration.
Forces and accelerations calculation
resultant force (newton, N) = mass (kg) × acceleration (m/s2)
Write down the equation. To find the acceleration:
- force = mass × acceleration
- Rearrange the equation.
- acceleration = force⁄mass
- Put in the values.
- acceleration = 3,000N⁄ 2,000kg
- Work out the answer and write it down. Acceleration = 1.5m/s2
To find the mass: Mass= force divide by acceleration
To find the acceleration: Acceleration= force divide by mass
Questions and answers
- Air resistance - drag
- When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.
- Contact force
- This happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet even as you stand still. This is the force you feel in your feet. You feel the ground pushing back against your weight pushing down.
- This is the force that resists movement between two surfaces which are in contact.
- This is the force that pulls objects towards the Earth. We call the force of gravity on an object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.