Forces and Momentum

• Newton’s First Law states: Every object continues in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by an external force acting on it.
• A car will maintain a constant speed if the drive force and the drag are balanced.
• Newton's Second Law states: Rate of change of momentum is proportional to the total force acting on a body, and occurs in the direction of the force.
• An object of mass, m, is pushed with a constant force, F, so that its velocity increases from an initial value, u, to a final value, v, in time, t.
• Thus m(v - u ) µ F.
• Force = mass x acceleration.
• Acceleration is always caused by a resultant force, the vector sum of all the forces.
• The acceleration is always, without exception, in the same direction as the resultant force.
• Newton’s Third law states that: If body A exerts a force on body B, body B must exert an equal and opposite force on body A.
• In other words, forces always act in pairs.
• Momentum is the product between mass and velocity.
• Momentum (kg ms-1) = mass (kg) x velocity (ms-1).
• Units are kilogram metres per second (kg ms-1) or newton seconds (Ns).
• When doing momentum calculations, always be careful about the directions you are using.
• The change in momentum is called the impulse and is given the physics symbol Δp.
• Impulse (Ns) = Force (N) x time interval (s).
• If we plot a force time graph, we can see that impulse is the area under the graph.
• Impulse is the physics phenomenon that explains how a ball behaves when kicked or hit with a bat.
• Impulse also has important implications in road safety.
• When a car is involved in a collision, we want the impulse to occur over a longer time interval to reduce the forces involved.