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The net force is in the direction of the applied force.

A sports car accelerates from zero to 30 mph in 1.5 s . How long does it take for it to accelerate from zero to 60 mph, assuming the power of the engine to be independent of velocity and neglecting friction? 1. 2 s 2. 3 s 3. 4.5 s 4. 6 s 5. 9 s 6. 12 s.

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The net force is in the direction of the applied force.

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  1. A sports car accelerates from zero to 30 mph in 1.5 s. How long does it take for it to accelerate from zero to 60 mph, assuming the power of the engine to be independent of velocity and neglecting friction? 1. 2 s 2. 3 s 3. 4.5 s 4. 6 s 5. 9 s 6. 12 s

  2. Example: A 20 kg crate is slid across the floor. It is pushed with a force of 500 N a distance of 20 m in 30 s. Friction acts on the crate with a force of 300 N. • What is the work done by pushing? • What is the work done by friction? • What is the total work done? • How much power was expended in each case? 1 -1 1 a) b) The net force is in the direction of the applied force. c) or d)

  3. CH 7: momentum and collisions

  4. Momentum What is momentum? • Resistance to change in the state of motion of a moving object. • Inertia of a moving object. p – momentum [kg m/s] Momentum is a vector! What do we need in order to change the state of motion (i.e. change the velocity) of an object? A force is required for any acceleration (change in velocity) and therefore also required to change the momentum. Average Force I – Impulse [kg m/s] or Impulse is the force applied over a period of time to change the momentum. • Apply a large force over a short time. • Apply a small force over a long time.

  5. Example: If you were to jump off of a building would you prefer to land on a large air mattress or a concrete slab? Why? (Assume you stop after impact) Concrete slab – a very large force is applied to change your momentum in a very short period of time. Air Mattress – A smaller force is applied over a much longer time. Both situations correspond to the same change in momentum (same change in velocity with a constant mass). This is the definition of the average force. It is typically more convenient to use the average force, especially when the contact time is very small.

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