A.2 Forces and momentum

• Newton’s three laws of motion.

• Forces as interactions between bodies.

• Forces acting on a body can be represented in a free-body diagram.

• Free-body diagrams can be analysed to find the resultant force on a system.

• Contact forces include normal, friction, tension, elastic restoring force, viscous drag and buoyancy.

• Normal force FN acts perpendicular to the contact surface.

• Friction acts parallel to contact.

• Static: Ff <= μsFN; dynamic: Ff = μdFN.

• Tension.

• Elastic restoring force follows Hooke’s law: FH = -kx.

• Viscous drag on a small sphere: Fd = 6πηrv, opposite motion.

• η is fluid viscosity, r sphere radius, v speed through fluid.

• Buoyancy from displaced fluid: Fb = ρVg.

• Know field forces: gravitational, electric and magnetic.

• Weight is gravitational force: Fg = mg.

• Electric force Fe.

• Magnetic force Fm.

• Linear momentum p=mv is conserved unless a resultant external force acts.

• Impulse from resultant external force: J = FΔt = Δp.

• The applied external impulse equals the change in momentum of the system.

• Use F=ma for constant mass; use F=Δp/Δt when mass changes.

• Elastic and inelastic collisions of two bodies.

• Explosions.

• Compare energy in elastic collisions, inelastic collisions and explosions.

• Centripetal acceleration is radial: a=v^2/r=ω^2r=4π^2r/T^2.

• Direction is radially toward the centre of the circle.

• Circular motion is caused by a centripetal force acting perpendicular to the velocity.

• A centripetal force causes the body to change direction even if its magnitude of velocity may remain constant.

• Circular motion relation: v=2πr/T=ωr.

• Use angular velocity ω and period T to link angular and linear descriptions.