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Force, mass and acceleration

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Here are four typical forces on which you could be asked questions:

1. Air resistance - drag
• When an object moves through the air, the force of air resistanceair resistance: A force of friction produced when an object moves through the air. acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.
2. 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.
3. Friction
• This is the force that resists movement between two surfaces which are in contact.
4. Gravity
• 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.
Question

Look at the animation of the parachutist falling at a steady speed. Name the forces acting on the parachutist and state how they are acting.

There are just two forces acting on the parachutist. Gravity (weight) pulls the parachutist down. Air resistance - drag - pushes up on the canopy of the parachute.

Question

Look at the animation of the car moving at a steady speed. Name the forces acting on the car and state how they are acting.

There are several forces acting on the car. Gravity pulls down on the car. The contact force from the road pushes up on the wheels. And the driving force from the engine pushes the car along. Also, there is friction between the road and the tyres. There is friction in the wheel bearings. And air resistance acts on the front of the car.

## Summary

Watch this version of the 'Forces, mass and acceleration' podcast with drawings to help you out.

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Back to Forces and motion index

BBC iD